This was useful in debugging the mutator assist behavior for #11911,
and it fits with the other gcpacertrace output.
Change-Id: I1e25590bb4098223a160de796578bd11086309c7
Reviewed-on: https://go-review.googlesource.com/13046
Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Proportional concurrent sweep is currently based on a ratio of spans
to be swept per bytes of object allocation. However, proportional
sweeping is performed during span allocation, not object allocation,
in order to minimize contention and overhead. Since objects are
allocated from spans after those spans are allocated, the system tends
to operate in debt, which means when the next GC cycle starts, there
is often sweep debt remaining, so GC has to finish the sweep, which
delays the start of the cycle and delays enabling mutator assists.
For example, it's quite likely that many Ps will simultaneously refill
their span caches immediately after a GC cycle (because GC flushes the
span caches), but at this point, there has been very little object
allocation since the end of GC, so very little sweeping is done. The
Ps then allocate objects from these cached spans, which drives up the
bytes of object allocation, but since these allocations are coming
from cached spans, nothing considers whether more sweeping has to
happen. If the sweep ratio is high enough (which can happen if the
next GC trigger is very close to the retained heap size), this can
easily represent a sweep debt of thousands of pages.
Fix this by making proportional sweep proportional to the number of
bytes of spans allocated, rather than the number of bytes of objects
allocated. Prior to allocating a span, both the small object path and
the large object path ensure credit for allocating that span, so the
system operates in the black, rather than in the red.
Combined with the previous commit, this should eliminate all sweeping
from GC start up. On the stress test in issue #11911, this reduces the
time spent sweeping during GC (and delaying start up) by several
orders of magnitude:
mean 99%ile max
pre fix 1 ms 11 ms 144 ms
post fix 270 ns 735 ns 916 ns
Updates #11911.
Change-Id: I89223712883954c9d6ec2a7a51ecb97172097df3
Reviewed-on: https://go-review.googlesource.com/13044
Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Currently it's possible for the next_gc heap size trigger computed for
the next GC cycle to be less than the current allocated heap size.
This means the next cycle will start immediately, which means there's
no time to perform the concurrent sweep between GC cycles. This places
responsibility for finishing the sweep on GC itself, which delays GC
start-up and hence delays mutator assist.
Fix this by ensuring that next_gc is always at least a little higher
than the allocated heap size, so we won't trigger the next cycle
instantly.
Updates #11911.
Change-Id: I74f0b887bf187518d5fedffc7989817cbcf30592
Reviewed-on: https://go-review.googlesource.com/13043
Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Currently allocation checks the GC trigger speculatively during
allocation and then triggers the GC without rechecking. As a result,
it's possible for G 1 and G 2 to detect the trigger simultaneously,
both enter startGC, G 1 actually starts GC while G 2 gets preempted
until after the whole GC cycle, then G 2 immediately starts another GC
cycle even though the heap is now well under the trigger.
Fix this by re-checking the GC trigger non-speculatively just before
actually kicking off a new GC cycle.
This contributes to #11911 because when this happens, we definitely
don't finish the background sweep before starting the next GC cycle,
which can significantly delay the start of concurrent scan.
Change-Id: I560ab79ba5684ba435084410a9765d28f5745976
Reviewed-on: https://go-review.googlesource.com/13025
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>
Dangling pointer error. Unlikely to trigger in practice, but still.
Found by running GODEBUG=efence=1 GOGC=1 trace.test.
Change-Id: Ice474dedcf62dd33ab77526287a023ba3b166db9
Reviewed-on: https://go-review.googlesource.com/12991
Reviewed-by: Austin Clements <austin@google.com>
This adds a GCCPUFraction field to MemStats that reports the
cumulative fraction of the program's execution time spent in the
garbage collector. This is equivalent to the utilization percent shown
in the gctrace output and makes this available programmatically.
This does make one small effect on the gctrace output: we now report
the duration of mark termination up to just before the final
start-the-world, rather than up to just after. However, unlike
stop-the-world, I don't believe there's any way that start-the-world
can block, so it should take negligible time.
While there are many statistics one might want to expose via MemStats,
this is one of the few that will undoubtedly remain meaningful
regardless of future changes to the memory system.
The diff for this change is larger than the actual change. Mostly it
lifts the code for computing the GC CPU utilization out of the
debug.gctrace path.
Updates #10323.
Change-Id: I0f7dc3fdcafe95e8d1233ceb79de606b48acd989
Reviewed-on: https://go-review.googlesource.com/12844
Reviewed-by: Russ Cox <rsc@golang.org>
Currently we only capture GC phase transition times if
debug.gctrace>0, but we're about to compute GC CPU utilization
regardless of whether debug.gctrace is set, so we need these
regardless of debug.gctrace.
Change-Id: If3acf16505a43d416e9a99753206f03287180660
Reviewed-on: https://go-review.googlesource.com/12843
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>
The scheduler, work buffer's dispose, and write barriers
can conspire to hide the a pointer from the GC's concurent
mark phase. If this pointer is the only path to a large
amount of marking the STW mark termination phase may take
a lot of time.
Consider the following:
1) dispose places a work buffer on the partial queue
2) the GC is busy so it does not immediately remove and
process the work buffer
3) the scheduler runs a mutator whose write barrier dequeues the
work buffer from the partial queue so the GC won't see it
This repeats until the GC reaches the mark termination
phase where the GC finally discovers the pointer along
with a lot of work to do.
This CL fixes the problem by having the mutator
dispose of the buffer to the full queue instead of
the partial queue. Since the write buffer never asks for full
buffers the conspiracy described above is not possible.
Updates #11694.
Change-Id: I2ce832f9657a7570f800e8ce4459cd9e304ef43b
Reviewed-on: https://go-review.googlesource.com/12840
Reviewed-by: Austin Clements <austin@google.com>
Currently we enter mark 2 by first flushing all existing gcWork caches
and then setting gcBlackenPromptly, which disables further gcWork
caching. However, if a worker or assist pulls a work buffer in to its
gcWork cache after that cache has been flushed but before caching is
disabled, that work may remain in that cache until mark termination.
If that work represents a heap bottleneck (e.g., a single pointer that
is the only way to reach a large amount of the heap), this can force
mark termination to do a large amount of work, resulting in a long
STW.
Fix this by reversing the order of these steps: first disable caching,
then flush all existing caches.
Rick Hudson <rlh> did the hard work of tracking this down. This CL
combined with CL 12672 and CL 12646 distills the critical parts of his
fix from CL 12539.
Fixes#11694.
Change-Id: Ib10d0a21e3f6170a80727d0286f9990df049fed2
Reviewed-on: https://go-review.googlesource.com/12688
Reviewed-by: Rick Hudson <rlh@golang.org>
Currently the GC coordinator enables GC assists at the same time it
enables background mark workers, after the concurrent scan phase is
done. However, this means a rapidly allocating mutator has the entire
scan phase during which to allocate beyond the heap trigger and
potentially beyond the heap goal with no back-pressure from assists.
This prevents the feedback system that's supposed to keep the heap
size under the heap goal from doing its job.
Fix this by enabling mutator assists during the scan phase. This is
safe because the write barrier is already enabled and globally
acknowledged at this point.
There's still a very small window between when the heap size reaches
the heap trigger and when the GC coordinator is able to stop the world
during which the mutator can allocate unabated. This allows *very*
rapidly allocator mutators like TestTraceStress to still occasionally
exceed the heap goal by a small amount (~20 MB at most for
TestTraceStress). However, this seems like a corner case.
Fixes#11677.
Change-Id: I0f80d949ec82341cd31ca1604a626efb7295a819
Reviewed-on: https://go-review.googlesource.com/12674
Reviewed-by: Russ Cox <rsc@golang.org>
Currently we clear both the mark 1 and mark 2 signals at the beginning
of concurrent mark. If either if these is clear, it acts as a signal
to the scheduler that it should start background workers. However,
this means that in the interim *between* mark 1 and mark 2, the
scheduler basically loops starting up new workers only to have them
return with nothing to do. In addition to harming performance and
delaying mutator work, this approach has a race where workers started
for mark 1 can mistakenly signal mark 2, causing it to complete
prematurely. This approach also interferes with starting assists
earlier to fix#11677.
Fix this by initially setting both mark 1 and mark 2 to "signaled".
The scheduler will not start background mark workers, though assists
can still run. When we're ready to enter mark 1, we clear the mark 1
signal and wait for it. Then, when we're ready to enter mark 2, we
clear the mark 2 signal and wait for it.
This structure also lets us deal cleanly with the situation where all
work is drained *prior* to the mark 2 wait, meaning that there may be
no workers to signal completion. Currently we deal with this using a
racy (and possibly incorrect) check for work in the coordinator itself
to skip the mark 2 wait if there's no work. This change makes the
coordinator unconditionally wait for mark completion and makes the
scheduler itself signal completion by slightly extending the logic it
already has to determine that there's no work and hence no use in
starting a new worker.
This is a prerequisite to fixing the remaining component of #11677,
which will require enabling assists during the scan phase. However, we
don't want to enable background workers until the mark phase because
they will compete with the scan. This change lets us use bgMark1 and
bgMark2 to indicate when it's okay to start background workers
independent of assists.
This is also a prerequisite to fixing #11694. It significantly reduces
the occurrence of long mark termination pauses in #11694 (from 64 out
of 1000 to 2 out of 1000 in one experiment).
Coincidentally, this also reduces the final heap size (and hence run
time) of TestTraceStress from ~100 MB and ~1.9 seconds to ~14 MB and
~0.4 seconds because it significantly shortens concurrent mark
duration.
Rick Hudson <rlh> did the hard work of tracking this down.
Change-Id: I12ea9ee2db9a0ae9d3a90dde4944a75fcf408f4c
Reviewed-on: https://go-review.googlesource.com/12672
Reviewed-by: Russ Cox <rsc@golang.org>
Currently it's possible for the GC assist to signal completion of the
mark phase, which puts the GC coordinator goroutine on the current P's
run queue, and then return to mutator code that delays until the next
forced preemption before actually yielding control to the GC
coordinator, dragging out completion of the mark phase. This delay can
be further exacerbated if the mutator makes other goroutines runnable
before yielding control, since this will push the GC coordinator on
the back of the P's run queue.
To fix this, this adds a Gosched to the assist if it completed the
mark phase. This immediately and directly yields control to the GC
coordinator. This already happens implicitly in the background mark
workers because they park immediately after completing the mark.
This is one of the reasons completion of the mark phase is being
dragged out and allowing the mutator to allocate without assisting,
leading to the large heap goal overshoot in issue #11677. This is also
a prerequisite to making the assist block when it can't pay off its
debt.
Change-Id: I586adfbecb3ca042a37966752c1dc757f5c7fc78
Reviewed-on: https://go-review.googlesource.com/12670
Reviewed-by: Russ Cox <rsc@golang.org>
Currently fractional and idle mark workers dispose of their gcWork
cache during mark 2 after incrementing work.nwait and after checking
whether there are any workers or any work available. This creates a
window for two races:
1) If the only remaining work is in this worker's gcWork cache, it
will see that there are no more workers and no more work on the
global lists (since it has not yet flushed its own cache) and
prematurely signal mark 2 completion.
2) After this worker has incremented work.nwait but before it has
flushed its cache, another worker may observe that there are no
more workers and no more work and prematurely signal mark 2
completion.
We can fix both of these by simply moving the cache flush above the
increment of nwait and the test of the completion condition.
This is probably contributing to #11694, though this alone is not
enough to fix it.
Change-Id: Idcf9656e5c460c5ea0d23c19c6c51e951f7716c3
Reviewed-on: https://go-review.googlesource.com/12646
Reviewed-by: Russ Cox <rsc@golang.org>
Currently the gctrace output reports the trigger heap size, rather
than the actual heap size at the beginning of GC. Often these are the
same, or at least very close. However, it's possible for the heap to
already have exceeded this trigger when we first check the trigger and
start GC; in this case, this output is very misleading. We've
encountered this confusion a few times when debugging and this
behavior is difficult to document succinctly.
Change the gctrace output to report the actual heap size when GC
starts, rather than the trigger.
Change-Id: I246b3ccae4c4c7ea44c012e70d24a46878d7601f
Reviewed-on: https://go-review.googlesource.com/12452
Reviewed-by: Russ Cox <rsc@golang.org>
Whenever someone pastes gctrace output into GitHub, it helpfully turns
the GC cycle number into a link to some unrelated issue. Prevent this
by removing the pound before the cycle number. The fact that this is a
cycle number is probably more obvious at a glance than most of the
other numbers.
Change-Id: Ifa5fc7fe6c715eac50e639f25bc36c81a132ffea
Reviewed-on: https://go-review.googlesource.com/12413
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
The runtime.GC documentation was rewritten in df2809f to make it clear
that it blocks until GC is complete, but the re-rewrite in ed9a4c9 and
e28a679 lost this property when clarifying that it may also block the
entire program and not just the caller.
Try to arrive at wording that conveys both of these properties.
Change-Id: I1e255322aa28a21a548556ecf2a44d8d8ac524ef
Reviewed-on: https://go-review.googlesource.com/12392
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
We shouldn't guarantee this behavior, but suggest it's possible.
Change-Id: I4c2afb48b99be4d91537306d3337171a13c9990a
Reviewed-on: https://go-review.googlesource.com/12346
Reviewed-by: David Crawshaw <crawshaw@golang.org>
No code changes. Just make it clear that runtime.GC is not concurrent.
Change-Id: I00a99ebd26402817c665c9a128978cef19f037be
Reviewed-on: https://go-review.googlesource.com/12345
Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
These memstats are currently being computed by gcMark, which was
appropriate in Go 1.4, but gcMark is now just one part of a bigger
picture. In particular, it can't account for the sweep termination
pause time, it can't account for all of the mark termination pause
time, and the reported "pause end" and "last GC" times will be
slightly earlier than they really are.
Lift computing of these statistics into func gc, which has the
appropriate visibility into the process to compute them correctly.
Fixes one of the issues in #10323. This does not add new statistics
appropriate to the concurrent collector; it simply fixes existing
statistics that are being misreported.
Change-Id: I670cb16594a8641f6b27acf4472db15b6e8e086e
Reviewed-on: https://go-review.googlesource.com/11794
Reviewed-by: Russ Cox <rsc@golang.org>
Currently we report MemStats.PauseEnd in nanoseconds, but with no
particular 0 time. On Linux, the 0 time is when the host started. On
Darwin, it's the UNIX epoch. This is also inconsistent with the other
absolute time in MemStats, LastGC, which is always reported in
nanoseconds since 1970.
Fix PauseEnd so it's always reported in nanoseconds since 1970, like
LastGC.
Fixes one of the issues raised in #10323.
Change-Id: Ie2fe3169d45113992363a03b764f4e6c47e5c6a8
Reviewed-on: https://go-review.googlesource.com/11801
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
The one in misc/makerelease/makerelease.go is particularly bad and
probably warrants rotating our keys.
I didn't update old weekly notes, and reverted some changes involving
test code for now, since we're late in the Go 1.5 freeze. Otherwise,
the rest are all auto-generated changes, and all manually reviewed.
Change-Id: Ia2753576ab5d64826a167d259f48a2f50508792d
Reviewed-on: https://go-review.googlesource.com/12048
Reviewed-by: Rob Pike <r@golang.org>
Currently we fail to reset the live heap accounting state before the
checkmark mark and before the gctrace=2 extra mark. As a result, if
either are enabled, at the end of GC it thinks there are 0 bytes of
live heap, which causes the GC controller to initiate a new GC
immediately, regardless of the true heap size.
Fix this by factoring this state reset into a function and calling it
before all three possible marks.
This function should be merged with gcResetGState, but doing so
requires some additional cleanup, so it will wait for after the
freeze. Filed #11427 for this cleanup.
Fixes#10492.
Change-Id: Ibe46348916fc8368fac6f086e142815c970a6f4d
Reviewed-on: https://go-review.googlesource.com/11561
Reviewed-by: Russ Cox <rsc@golang.org>
Memory for stacks is manually managed by the runtime and, currently
(with one exception) we free stack spans immediately when the last
stack on a span is freed. However, the garbage collector assumes that
spans can never transition from non-free to free during scan or mark.
This disagreement makes it possible for the garbage collector to mark
uninitialized objects and is blocking us from re-enabling the bad
pointer test in the garbage collector (issue #9880).
For example, the following sequence will result in marking an
uninitialized object:
1. scanobject loads a pointer slot out of the object it's scanning.
This happens to be one of the special pointers from the heap into a
stack. Call the pointer p and suppose it points into X's stack.
2. X, running on another thread, grows its stack and frees its old
stack.
3. The old stack happens to be large or was the last stack in its
span, so X frees this span, setting it to state _MSpanFree.
4. The span gets reused as a heap span.
5. scanobject calls heapBitsForObject, which loads the span containing
p, which is now in state _MSpanInUse, but doesn't necessarily have
an object at p. The not-object at p gets marked, and at this point
all sorts of things can go wrong.
We already have a partial solution to this. When shrinking a stack, we
put the old stack on a queue to be freed at the end of garbage
collection. This was done to address exactly this problem, but wasn't
a complete solution.
This commit generalizes this solution to both shrinking and growing
stacks. For stacks that fit in the stack pool, we simply don't free
the span, even if its reference count reaches zero. It's fine to reuse
the span for other stacks, and this enables that. At the end of GC, we
sweep for cached stack spans with a zero reference count and free
them. For larger stacks, we simply queue the stack span to be freed at
the end of GC. Ideally, we would reuse these large stack spans the way
we can small stack spans, but that's a more invasive change that will
have to wait until after the freeze.
Fixes#11267.
Change-Id: Ib7f2c5da4845cc0268e8dc098b08465116972a71
Reviewed-on: https://go-review.googlesource.com/11502
Reviewed-by: Russ Cox <rsc@golang.org>
We don't use this state. _GCoff means we're sweeping in the
background. This makes it clear in the next commit that _GCoff and
only _GCoff means sweeping.
Change-Id: I416324a829ba0be3794a6cf3cf1655114cb6e47c
Reviewed-on: https://go-review.googlesource.com/11501
Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Some latency regressions have crept into our system over the past few
weeks. This CL fixes those by having the mark phase more aggressively
blacken objects so that the mark termination phase, a STW phase, has less
work to do. Three approaches were taken when the mark phase believes
it has no more work to do, ie all the work buffers are empty.
If things have gone well the mark phase is correct and there is
in fact little or no work. In that case the following items will
take very little time. If the mark phase is wrong this CL will
ferret that work out and give the mark phase a chance to deal with
it concurrently before mark termination begins.
When the mark phase first appears to be out of work, it does three things:
1) It switches from allocating white to allocating black to reduce the
number of unmarked objects reachable only from stacks.
2) It flushes and disables per-P GC work caches so all work must be in
globally visible work buffers.
3) It rescans the global roots---the BSS and data segments---so there
are fewer objects to blacken during mark termination. We do not rescan
stacks at this point, though that could be done in a later CL.
After these steps, it again drains the global work buffers.
On a lightly loaded machine the garbage benchmark has reduced the
number of GC cycles with latency > 10 ms from 83 out of 4083 cycles
down to 2 out of 3995 cycles. Maximum latency was reduced from
60+ msecs down to 20 ms.
Change-Id: I152285b48a7e56c5083a02e8e4485dd39c990492
Reviewed-on: https://go-review.googlesource.com/10590
Reviewed-by: Austin Clements <austin@google.com>
This fixes a hang during runtime.TestTraceStress.
It also fixes double-scan of stacks, which leads to
stack barrier installation failures.
Both of these have shown up as flaky failures on the dashboard.
Fixes#10941.
Change-Id: Ia2a5991ce2c9f43ba06ae1c7032f7c898dc990e0
Reviewed-on: https://go-review.googlesource.com/11089
Reviewed-by: Austin Clements <austin@google.com>
While we're here, update the documentation and delete variables with no effect.
Change-Id: I4df0d266dff880df61b488ed547c2870205862f0
Reviewed-on: https://go-review.googlesource.com/10790
Reviewed-by: Austin Clements <austin@google.com>
These were found by grepping the comments from the go code and feeding
the output to aspell.
Change-Id: Id734d6c8d1938ec3c36bd94a4dbbad577e3ad395
Reviewed-on: https://go-review.googlesource.com/10941
Reviewed-by: Aamir Khan <syst3m.w0rm@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Currently, write barriers are only enabled after completion of the
concurrent scan phase, as we enter the concurrent mark phase. However,
stack barriers are installed during the scan phase and assume that
write barriers will track changes to frames above the stack
barriers. Since write barriers aren't enabled until after stack
barriers are installed, we may miss modifications to the stack that
happen after installing the stack barriers and before enabling write
barriers.
Fix this by enabling write barriers during the scan phase.
This commit intentionally makes the minimal change to do this (there's
only one line of code change; the rest are comment changes). At the
very least, we should consider eliminating the ragged barrier that's
intended to synchronize the enabling of write barriers, but now just
wastes time. I've included a large comment about extensions and
alternative designs.
Change-Id: Ib20fede794e4fcb91ddf36f99bd97344d7f96421
Reviewed-on: https://go-review.googlesource.com/10795
Reviewed-by: Russ Cox <rsc@golang.org>
Currently checkmarks mode fails to rescan stacks because it sees the
leftover state bits indicating that the stacks haven't changed since
the last scan. As a result, it won't detect lost marks caused by
failing to scan stacks correctly during regular garbage collection.
Fix this by marking all stacks dirty before performing the checkmark
phase.
Change-Id: I1f06882bb8b20257120a4b8e7f95bb3ffc263895
Reviewed-on: https://go-review.googlesource.com/10794
Reviewed-by: Russ Cox <rsc@golang.org>
Currently the GODEBUG=gctrace=1 trace line includes "@n.nnns" to
indicate the time that the GC cycle ended relative to the time the
program started. This was meant to be consistent with the utilization
as of the end of the cycle, which is printed next on the trace line,
but it winds up just being confusing and unexpected.
Change the trace line to include the time that the GC cycle started
relative to the time the program started.
Change-Id: I7d64580cd696eb17540716d3e8a74a9d6ae50650
Reviewed-on: https://go-review.googlesource.com/10634
Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
The stack barrier code will need a bookkeeping structure to keep track
of the overwritten return PCs. This commit introduces and allocates
this structure, but does not yet use the structure.
We don't want to allocate space for this structure during garbage
collection, so this commit allocates it along with the allocation of
the corresponding stack. However, we can't do a regular allocation in
newstack because mallocgc may itself grow the stack (which would lead
to a recursive allocation). Hence, this commit makes the bookkeeping
structure part of the stack allocation itself by stealing the
necessary space from the top of the stack allocation. Since the size
of this bookkeeping structure is logarithmic in the size of the stack,
this has minimal impact on stack behavior.
Change-Id: Ia14408be06aafa9ca4867f4e70bddb3fe0e96665
Reviewed-on: https://go-review.googlesource.com/10313
Reviewed-by: Russ Cox <rsc@golang.org>
Currently we truncate gctrace clock and CPU times to millisecond
precision. As a result, many phases are typically printed as 0, which
is fine for user consumption, but makes gathering statistics and
reports over GC traces difficult.
In 1.4, the gctrace line printed times in microseconds. This was
better for statistics, but not as easy for users to read or interpret,
and it generally made the trace lines longer.
This change strikes a balance between these extremes by printing
milliseconds, but including the decimal part to two significant
figures down to microsecond precision. This remains easy to read and
interpret, but includes more precision when it's useful.
For example, where the code currently prints,
gc #29 @1.629s 0%: 0+2+0+12+0 ms clock, 0+2+0+0/12/0+0 ms cpu, 4->4->2 MB, 4 MB goal, 1 P
this prints,
gc #29 @1.629s 0%: 0.005+2.1+0+12+0.29 ms clock, 0.005+2.1+0+0/12/0+0.29 ms cpu, 4->4->2 MB, 4 MB goal, 1 P
Fixes#10970.
Change-Id: I249624779433927cd8b0947b986df9060c289075
Reviewed-on: https://go-review.googlesource.com/10554
Reviewed-by: Russ Cox <rsc@golang.org>
runtime.GC() is intentionally very weakly specified. However, it is so
weakly specified that it's difficult to know that it's being used
correctly for its one intended use case: to ensure garbage collection
has run in a test that is garbage-sensitive. In particular, it is
unclear whether it is synchronous or asynchronous. In the old STW
collector this was essentially self-evident; short of queuing up a
garbage collection to run later, it had to be synchronous. However,
with the concurrent collector, there's evidence that people are
inferring that it may be asynchronous (e.g., issue #10986), as this is
both unclear in the documentation and possible in the implementation.
In fact, runtime.GC() runs a fully synchronous STW collection. We
probably don't want to commit to this exact behavior. But we can
commit to the essential property that tests rely on: that runtime.GC()
does not return until the GC has finished.
Change-Id: Ifc3045a505e1898ecdbe32c1f7e80e2e9ffacb5b
Reviewed-on: https://go-review.googlesource.com/10488
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>
This is dead code. If you want to quiesce the system the
preferred way is to use forEachP(func(*p){}).
Change-Id: Ic7677a5dd55e3639b99e78ddeb2c71dd1dd091fa
Reviewed-on: https://go-review.googlesource.com/10267
Reviewed-by: Austin Clements <austin@google.com>
Prior to this CL whenever the GC marking was enabled and
a P was looking for work we supplied a G to help
the GC do its marking tasks. Once this G finished all
the marking available it would release the P to find another
available G. In the case where there was no work the P would drop
into findrunnable which would execute the mark helper G which would
immediately return and the P would drop into findrunnable again repeating
the process. Since the P was always given a G to run it never blocks.
This CL first checks if the GC mark helper G has available work and if
not the P immediately falls through to its blocking logic.
Fixes#10901
Change-Id: I94ac9646866ba64b7892af358888bc9950de23b5
Reviewed-on: https://go-review.googlesource.com/10189
Reviewed-by: Austin Clements <austin@google.com>
Currently setGCPercent sets heapminimum to heapminimum*GOGC/100. The
real intent is to set heapminimum to a scaled multiple of a fixed
default heap minimum, not to scale heapminimum based on its current
value. This turns out to be okay because setGCPercent is only called
once and heapminimum is initially set to this default heap minimum.
However, the code as written is confusing, especially since
setGCPercent is otherwise written so it could be called again to
change GOGC. Fix this by introducing a defaultHeapMinimum constant and
using this instead of the current value of heapminimum to compute the
scaled heap minimum.
As part of this, this commit improves the documentation on
heapminimum.
Change-Id: I4eb82c73dc2eb44a6e5a17c780a747a2e73d7493
Reviewed-on: https://go-review.googlesource.com/10181
Reviewed-by: Russ Cox <rsc@golang.org>
Currently, startTheWorld releases worldsema before starting the
world. Since startTheWorld can change gomaxprocs after allowing Ps to
run, this means that gomaxprocs can change while another P holds
worldsema.
Unfortunately, the garbage collector and forEachP assume that holding
worldsema protects against changes in gomaxprocs (which it *almost*
does). In particular, this is causing somewhat frequent "P did not run
fn" crashes in forEachP in the runtime tests because gomaxprocs is
changing between the several loops that forEachP does over all the Ps.
Fix this by only releasing worldsema after the world is started.
This relates to issue #10618. forEachP still fails under stress
testing, but much less frequently.
Change-Id: I085d627b70cca9ebe9af28fe73b9872f1bb224ff
Reviewed-on: https://go-review.googlesource.com/10156
Reviewed-by: Russ Cox <rsc@golang.org>
There are several steps to stopping and starting the world and
currently they're open-coded in several places. The garbage collector
is the only thing that needs to stop and start the world in a
non-trivial pattern. Replace all other uses with calls to higher-level
functions that implement the entire pattern necessary to stop and
start the world.
This is a pure refectoring and should not change any code semantics.
In the following commits, we'll make changes that are easier to do
with this abstraction in place.
This commit renames the old starttheworld to startTheWorldWithSema.
This is a slight misnomer right now because the callers release
worldsema just before calling this. However, a later commit will swap
these and I don't want to think of another name in the mean time.
Change-Id: I5dc97f87b44fb98963c49c777d7053653974c911
Reviewed-on: https://go-review.googlesource.com/10154
Reviewed-by: Russ Cox <rsc@golang.org>
In order to avoid deadlocks, startGC avoids kicking off GC if locks
are held by the calling M. However, it currently fails to check
preemptoff, which is the other way to disable preemption.
Fix this by adding a check for preemptoff.
Change-Id: Ie1083166e5ba4af5c9d6c5a42efdfaaef41ca997
Reviewed-on: https://go-review.googlesource.com/10153
Reviewed-by: Russ Cox <rsc@golang.org>
Small types record the location of pointers in their memory layout
by using a simple bitmap. In Go 1.4 the bitmap held 4-bit entries,
and in Go 1.5 the bitmap holds 1-bit entries, but in both cases using
a bitmap for a large type containing arrays does not make sense:
if someone refers to the type [1<<28]*byte in a program in such
a way that the type information makes it into the binary, it would be
a waste of space to write a 128 MB (for 4-bit entries) or even 32 MB
(for 1-bit entries) bitmap full of 1s into the binary or even to keep
one in memory during the execution of the program.
For large types containing arrays, it is much more compact to describe
the locations of pointers using a notation that can express repetition
than to lay out a bitmap of pointers. Go 1.4 included such a notation,
called ``GC programs'' but it was complex, required recursion during
decoding, and was generally slow. Dmitriy measured the execution of
these programs writing directly to the heap bitmap as being 7x slower
than copying from a preunrolled 4-bit mask (and frankly that code was
not terribly fast either). For some tests, unrollgcprog1 was seen costing
as much as 3x more than the rest of malloc combined.
This CL introduces a different form for the GC programs. They use a
simple Lempel-Ziv-style encoding of the 1-bit pointer information,
in which the only operations are (1) emit the following n bits
and (2) repeat the last n bits c more times. This encoding can be
generated directly from the Go type information (using repetition
only for arrays or large runs of non-pointer data) and it can be decoded
very efficiently. In particular the decoding requires little state and
no recursion, so that the entire decoding can run without any memory
accesses other than the reads of the encoding and the writes of the
decoded form to the heap bitmap. For recursive types like arrays of
arrays of arrays, the inner instructions are only executed once, not
n times, so that large repetitions run at full speed. (In contrast, large
repetitions in the old programs repeated the individual bit-level layout
of the inner data over and over.) The result is as much as 25x faster
decoding compared to the old form.
Because the old decoder was so slow, Go 1.4 had three (or so) cases
for how to set the heap bitmap bits for an allocation of a given type:
(1) If the type had an even number of words up to 32 words, then
the 4-bit pointer mask for the type fit in no more than 16 bytes;
store the 4-bit pointer mask directly in the binary and copy from it.
(1b) If the type had an odd number of words up to 15 words, then
the 4-bit pointer mask for the type, doubled to end on a byte boundary,
fit in no more than 16 bytes; store that doubled mask directly in the
binary and copy from it.
(2) If the type had an even number of words up to 128 words,
or an odd number of words up to 63 words (again due to doubling),
then the 4-bit pointer mask would fit in a 64-byte unrolled mask.
Store a GC program in the binary, but leave space in the BSS for
the unrolled mask. Execute the GC program to construct the mask the
first time it is needed, and thereafter copy from the mask.
(3) Otherwise, store a GC program and execute it to write directly to
the heap bitmap each time an object of that type is allocated.
(This is the case that was 7x slower than the other two.)
Because the new pointer masks store 1-bit entries instead of 4-bit
entries and because using the decoder no longer carries a significant
overhead, after this CL (that is, for Go 1.5) there are only two cases:
(1) If the type is 128 words or less (no condition about odd or even),
store the 1-bit pointer mask directly in the binary and use it to
initialize the heap bitmap during malloc. (Implemented in CL 9702.)
(2) There is no case 2 anymore.
(3) Otherwise, store a GC program and execute it to write directly to
the heap bitmap each time an object of that type is allocated.
Executing the GC program directly into the heap bitmap (case (3) above)
was disabled for the Go 1.5 dev cycle, both to avoid needing to use
GC programs for typedmemmove and to avoid updating that code as
the heap bitmap format changed. Typedmemmove no longer uses this
type information; as of CL 9886 it uses the heap bitmap directly.
Now that the heap bitmap format is stable, we reintroduce GC programs
and their space savings.
Benchmarks for heapBitsSetType, before this CL vs this CL:
name old mean new mean delta
SetTypePtr 7.59ns × (0.99,1.02) 5.16ns × (1.00,1.00) -32.05% (p=0.000)
SetTypePtr8 21.0ns × (0.98,1.05) 21.4ns × (1.00,1.00) ~ (p=0.179)
SetTypePtr16 24.1ns × (0.99,1.01) 24.6ns × (1.00,1.00) +2.41% (p=0.001)
SetTypePtr32 31.2ns × (0.99,1.01) 32.4ns × (0.99,1.02) +3.72% (p=0.001)
SetTypePtr64 45.2ns × (1.00,1.00) 47.2ns × (1.00,1.00) +4.42% (p=0.000)
SetTypePtr126 75.8ns × (0.99,1.01) 79.1ns × (1.00,1.00) +4.25% (p=0.000)
SetTypePtr128 74.3ns × (0.99,1.01) 77.6ns × (1.00,1.01) +4.55% (p=0.000)
SetTypePtrSlice 726ns × (1.00,1.01) 712ns × (1.00,1.00) -1.95% (p=0.001)
SetTypeNode1 20.0ns × (0.99,1.01) 20.7ns × (1.00,1.00) +3.71% (p=0.000)
SetTypeNode1Slice 112ns × (1.00,1.00) 113ns × (0.99,1.00) ~ (p=0.070)
SetTypeNode8 23.9ns × (1.00,1.00) 24.7ns × (1.00,1.01) +3.18% (p=0.000)
SetTypeNode8Slice 294ns × (0.99,1.02) 287ns × (0.99,1.01) -2.38% (p=0.015)
SetTypeNode64 52.8ns × (0.99,1.03) 51.8ns × (0.99,1.01) ~ (p=0.069)
SetTypeNode64Slice 1.13µs × (0.99,1.05) 1.14µs × (0.99,1.00) ~ (p=0.767)
SetTypeNode64Dead 36.0ns × (1.00,1.01) 32.5ns × (0.99,1.00) -9.67% (p=0.000)
SetTypeNode64DeadSlice 1.43µs × (0.99,1.01) 1.40µs × (1.00,1.00) -2.39% (p=0.001)
SetTypeNode124 75.7ns × (1.00,1.01) 79.0ns × (1.00,1.00) +4.44% (p=0.000)
SetTypeNode124Slice 1.94µs × (1.00,1.01) 2.04µs × (0.99,1.01) +4.98% (p=0.000)
SetTypeNode126 75.4ns × (1.00,1.01) 77.7ns × (0.99,1.01) +3.11% (p=0.000)
SetTypeNode126Slice 1.95µs × (0.99,1.01) 2.03µs × (1.00,1.00) +3.74% (p=0.000)
SetTypeNode128 85.4ns × (0.99,1.01) 122.0ns × (1.00,1.00) +42.89% (p=0.000)
SetTypeNode128Slice 2.20µs × (1.00,1.01) 2.36µs × (0.98,1.02) +7.48% (p=0.001)
SetTypeNode130 83.3ns × (1.00,1.00) 123.0ns × (1.00,1.00) +47.61% (p=0.000)
SetTypeNode130Slice 2.30µs × (0.99,1.01) 2.40µs × (0.98,1.01) +4.37% (p=0.000)
SetTypeNode1024 498ns × (1.00,1.00) 537ns × (1.00,1.00) +7.96% (p=0.000)
SetTypeNode1024Slice 15.5µs × (0.99,1.01) 17.8µs × (1.00,1.00) +15.27% (p=0.000)
The above compares always using a cached pointer mask (and the
corresponding waste of memory) against using the programs directly.
Some slowdown is expected, in exchange for having a better general algorithm.
The GC programs kick in for SetTypeNode128, SetTypeNode130, SetTypeNode1024,
along with the slice variants of those.
It is possible that the cutoff of 128 words (bits) should be raised
in a followup CL, but even with this low cutoff the GC programs are
faster than Go 1.4's "fast path" non-GC program case.
Benchmarks for heapBitsSetType, Go 1.4 vs this CL:
name old mean new mean delta
SetTypePtr 6.89ns × (1.00,1.00) 5.17ns × (1.00,1.00) -25.02% (p=0.000)
SetTypePtr8 25.8ns × (0.97,1.05) 21.5ns × (1.00,1.00) -16.70% (p=0.000)
SetTypePtr16 39.8ns × (0.97,1.02) 24.7ns × (0.99,1.01) -37.81% (p=0.000)
SetTypePtr32 68.8ns × (0.98,1.01) 32.2ns × (1.00,1.01) -53.18% (p=0.000)
SetTypePtr64 130ns × (1.00,1.00) 47ns × (1.00,1.00) -63.67% (p=0.000)
SetTypePtr126 241ns × (0.99,1.01) 79ns × (1.00,1.01) -67.25% (p=0.000)
SetTypePtr128 2.07µs × (1.00,1.00) 0.08µs × (1.00,1.00) -96.27% (p=0.000)
SetTypePtrSlice 1.05µs × (0.99,1.01) 0.72µs × (0.99,1.02) -31.70% (p=0.000)
SetTypeNode1 16.0ns × (0.99,1.01) 20.8ns × (0.99,1.03) +29.91% (p=0.000)
SetTypeNode1Slice 184ns × (0.99,1.01) 112ns × (0.99,1.01) -39.26% (p=0.000)
SetTypeNode8 29.5ns × (0.97,1.02) 24.6ns × (1.00,1.00) -16.50% (p=0.000)
SetTypeNode8Slice 624ns × (0.98,1.02) 285ns × (1.00,1.00) -54.31% (p=0.000)
SetTypeNode64 135ns × (0.96,1.08) 52ns × (0.99,1.02) -61.32% (p=0.000)
SetTypeNode64Slice 3.83µs × (1.00,1.00) 1.14µs × (0.99,1.01) -70.16% (p=0.000)
SetTypeNode64Dead 134ns × (0.99,1.01) 32ns × (1.00,1.01) -75.74% (p=0.000)
SetTypeNode64DeadSlice 3.83µs × (0.99,1.00) 1.40µs × (1.00,1.01) -63.42% (p=0.000)
SetTypeNode124 240ns × (0.99,1.01) 79ns × (1.00,1.01) -67.05% (p=0.000)
SetTypeNode124Slice 7.27µs × (1.00,1.00) 2.04µs × (1.00,1.00) -71.95% (p=0.000)
SetTypeNode126 2.06µs × (0.99,1.01) 0.08µs × (0.99,1.01) -96.23% (p=0.000)
SetTypeNode126Slice 64.4µs × (1.00,1.00) 2.0µs × (1.00,1.00) -96.85% (p=0.000)
SetTypeNode128 2.09µs × (1.00,1.01) 0.12µs × (1.00,1.00) -94.15% (p=0.000)
SetTypeNode128Slice 65.4µs × (1.00,1.00) 2.4µs × (0.99,1.03) -96.39% (p=0.000)
SetTypeNode130 2.11µs × (1.00,1.00) 0.12µs × (1.00,1.00) -94.18% (p=0.000)
SetTypeNode130Slice 66.3µs × (1.00,1.00) 2.4µs × (0.97,1.08) -96.34% (p=0.000)
SetTypeNode1024 16.0µs × (1.00,1.01) 0.5µs × (1.00,1.00) -96.65% (p=0.000)
SetTypeNode1024Slice 512µs × (1.00,1.00) 18µs × (0.98,1.04) -96.45% (p=0.000)
SetTypeNode124 uses a 124 data + 2 ptr = 126-word allocation.
Both Go 1.4 and this CL are using pointer bitmaps for this case,
so that's an overall 3x speedup for using pointer bitmaps.
SetTypeNode128 uses a 128 data + 2 ptr = 130-word allocation.
Both Go 1.4 and this CL are running the GC program for this case,
so that's an overall 17x speedup when using GC programs (and
I've seen >20x on other systems).
Comparing Go 1.4's SetTypeNode124 (pointer bitmap) against
this CL's SetTypeNode128 (GC program), the slow path in the
code in this CL is 2x faster than the fast path in Go 1.4.
The Go 1 benchmarks are basically unaffected compared to just before this CL.
Go 1 benchmarks, before this CL vs this CL:
name old mean new mean delta
BinaryTree17 5.87s × (0.97,1.04) 5.91s × (0.96,1.04) ~ (p=0.306)
Fannkuch11 4.38s × (1.00,1.00) 4.37s × (1.00,1.01) -0.22% (p=0.006)
FmtFprintfEmpty 90.7ns × (0.97,1.10) 89.3ns × (0.96,1.09) ~ (p=0.280)
FmtFprintfString 282ns × (0.98,1.04) 287ns × (0.98,1.07) +1.72% (p=0.039)
FmtFprintfInt 269ns × (0.99,1.03) 282ns × (0.97,1.04) +4.87% (p=0.000)
FmtFprintfIntInt 478ns × (0.99,1.02) 481ns × (0.99,1.02) +0.61% (p=0.048)
FmtFprintfPrefixedInt 399ns × (0.98,1.03) 400ns × (0.98,1.05) ~ (p=0.533)
FmtFprintfFloat 563ns × (0.99,1.01) 570ns × (1.00,1.01) +1.37% (p=0.000)
FmtManyArgs 1.89µs × (0.99,1.01) 1.92µs × (0.99,1.02) +1.88% (p=0.000)
GobDecode 15.2ms × (0.99,1.01) 15.2ms × (0.98,1.05) ~ (p=0.609)
GobEncode 11.6ms × (0.98,1.03) 11.9ms × (0.98,1.04) +2.17% (p=0.000)
Gzip 648ms × (0.99,1.01) 648ms × (1.00,1.01) ~ (p=0.835)
Gunzip 142ms × (1.00,1.00) 143ms × (1.00,1.01) ~ (p=0.169)
HTTPClientServer 90.5µs × (0.98,1.03) 91.5µs × (0.98,1.04) +1.04% (p=0.045)
JSONEncode 31.5ms × (0.98,1.03) 31.4ms × (0.98,1.03) ~ (p=0.549)
JSONDecode 111ms × (0.99,1.01) 107ms × (0.99,1.01) -3.21% (p=0.000)
Mandelbrot200 6.01ms × (1.00,1.00) 6.01ms × (1.00,1.00) ~ (p=0.878)
GoParse 6.54ms × (0.99,1.02) 6.61ms × (0.99,1.03) +1.08% (p=0.004)
RegexpMatchEasy0_32 160ns × (1.00,1.01) 161ns × (1.00,1.00) +0.40% (p=0.000)
RegexpMatchEasy0_1K 560ns × (0.99,1.01) 559ns × (0.99,1.01) ~ (p=0.088)
RegexpMatchEasy1_32 138ns × (0.99,1.01) 138ns × (1.00,1.00) ~ (p=0.380)
RegexpMatchEasy1_1K 877ns × (1.00,1.00) 878ns × (1.00,1.00) ~ (p=0.157)
RegexpMatchMedium_32 251ns × (0.99,1.00) 251ns × (1.00,1.01) +0.28% (p=0.021)
RegexpMatchMedium_1K 72.6µs × (1.00,1.00) 72.6µs × (1.00,1.00) ~ (p=0.539)
RegexpMatchHard_32 3.84µs × (1.00,1.00) 3.84µs × (1.00,1.00) ~ (p=0.378)
RegexpMatchHard_1K 117µs × (1.00,1.00) 117µs × (1.00,1.00) ~ (p=0.067)
Revcomp 904ms × (0.99,1.02) 904ms × (0.99,1.01) ~ (p=0.943)
Template 125ms × (0.99,1.02) 127ms × (0.99,1.01) +1.79% (p=0.000)
TimeParse 627ns × (0.99,1.01) 622ns × (0.99,1.01) -0.88% (p=0.000)
TimeFormat 655ns × (0.99,1.02) 655ns × (0.99,1.02) ~ (p=0.976)
For the record, Go 1 benchmarks, Go 1.4 vs this CL:
name old mean new mean delta
BinaryTree17 4.61s × (0.97,1.05) 5.91s × (0.98,1.03) +28.35% (p=0.000)
Fannkuch11 4.40s × (0.99,1.03) 4.41s × (0.99,1.01) ~ (p=0.212)
FmtFprintfEmpty 102ns × (0.99,1.01) 84ns × (0.99,1.02) -18.38% (p=0.000)
FmtFprintfString 302ns × (0.98,1.01) 303ns × (0.99,1.02) ~ (p=0.203)
FmtFprintfInt 313ns × (0.97,1.05) 270ns × (0.99,1.01) -13.69% (p=0.000)
FmtFprintfIntInt 524ns × (0.98,1.02) 477ns × (0.99,1.00) -8.87% (p=0.000)
FmtFprintfPrefixedInt 424ns × (0.98,1.02) 386ns × (0.99,1.01) -8.96% (p=0.000)
FmtFprintfFloat 652ns × (0.98,1.02) 594ns × (0.97,1.05) -8.97% (p=0.000)
FmtManyArgs 2.13µs × (0.99,1.02) 1.94µs × (0.99,1.01) -8.92% (p=0.000)
GobDecode 17.1ms × (0.99,1.02) 14.9ms × (0.98,1.03) -13.07% (p=0.000)
GobEncode 13.5ms × (0.98,1.03) 11.5ms × (0.98,1.03) -15.25% (p=0.000)
Gzip 656ms × (0.99,1.02) 647ms × (0.99,1.01) -1.29% (p=0.000)
Gunzip 143ms × (0.99,1.02) 144ms × (0.99,1.01) ~ (p=0.204)
HTTPClientServer 88.2µs × (0.98,1.02) 90.8µs × (0.98,1.01) +2.93% (p=0.000)
JSONEncode 32.2ms × (0.98,1.02) 30.9ms × (0.97,1.04) -4.06% (p=0.001)
JSONDecode 121ms × (0.98,1.02) 110ms × (0.98,1.05) -8.95% (p=0.000)
Mandelbrot200 6.06ms × (0.99,1.01) 6.11ms × (0.98,1.04) ~ (p=0.184)
GoParse 6.76ms × (0.97,1.04) 6.58ms × (0.98,1.05) -2.63% (p=0.003)
RegexpMatchEasy0_32 195ns × (1.00,1.01) 155ns × (0.99,1.01) -20.43% (p=0.000)
RegexpMatchEasy0_1K 479ns × (0.98,1.03) 535ns × (0.99,1.02) +11.59% (p=0.000)
RegexpMatchEasy1_32 169ns × (0.99,1.02) 131ns × (0.99,1.03) -22.44% (p=0.000)
RegexpMatchEasy1_1K 1.53µs × (0.99,1.01) 0.87µs × (0.99,1.02) -43.07% (p=0.000)
RegexpMatchMedium_32 334ns × (0.99,1.01) 242ns × (0.99,1.01) -27.53% (p=0.000)
RegexpMatchMedium_1K 125µs × (1.00,1.01) 72µs × (0.99,1.03) -42.53% (p=0.000)
RegexpMatchHard_32 6.03µs × (0.99,1.01) 3.79µs × (0.99,1.01) -37.12% (p=0.000)
RegexpMatchHard_1K 189µs × (0.99,1.02) 115µs × (0.99,1.01) -39.20% (p=0.000)
Revcomp 935ms × (0.96,1.03) 926ms × (0.98,1.02) ~ (p=0.083)
Template 146ms × (0.97,1.05) 119ms × (0.99,1.01) -18.37% (p=0.000)
TimeParse 660ns × (0.99,1.01) 624ns × (0.99,1.02) -5.43% (p=0.000)
TimeFormat 670ns × (0.98,1.02) 710ns × (1.00,1.01) +5.97% (p=0.000)
This CL is a bit larger than I would like, but the compiler, linker, runtime,
and package reflect all need to be in sync about the format of these programs,
so there is no easy way to split this into independent changes (at least
while keeping the build working at each change).
Fixes#9625.
Fixes#10524.
Change-Id: I9e3e20d6097099d0f8532d1cb5b1af528804989a
Reviewed-on: https://go-review.googlesource.com/9888
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Russ Cox <rsc@golang.org>
Currently the heap minimum is set to 4MB which prevents our ability to
collect at every allocation by setting GOGC=0. This adjust the
heap minimum to 4MB*GOGC/100 thus reenabling collecting at every allocation.
Fixes#10681
Change-Id: I912d027dac4b14ae535597e8beefa9ac3fb8ad94
Reviewed-on: https://go-review.googlesource.com/9814
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Currently, the GC uses a moving average of recent scan work ratios to
estimate the total scan work required by this cycle. This is in turn
used to compute how much scan work should be done by mutators when
they allocate in order to perform all expected scan work by the time
the allocated heap reaches the heap goal.
However, our current scan work estimate can be arbitrarily wrong if
the heap topography changes significantly from one cycle to the
next. For example, in the go1 benchmarks, at the beginning of each
benchmark, the heap is dominated by a 256MB no-scan object, so the GC
learns that the scan density of the heap is very low. In benchmarks
that then rapidly allocate pointer-dense objects, by the time of the
next GC cycle, our estimate of the scan work can be too low by a large
factor. This in turn lets the mutator allocate faster than the GC can
collect, allowing it to get arbitrarily far ahead of the scan work
estimate, which leads to very long GC cycles with very little mutator
assist that can overshoot the heap goal by large margins. This is
particularly easy to demonstrate with BinaryTree17:
$ GODEBUG=gctrace=1 ./go1.test -test.bench BinaryTree17
gc #1 @0.017s 2%: 0+0+0+0+0 ms clock, 0+0+0+0/0/0+0 ms cpu, 4->262->262 MB, 4 MB goal, 1 P
gc #2 @0.026s 3%: 0+0+0+0+0 ms clock, 0+0+0+0/0/0+0 ms cpu, 262->262->262 MB, 524 MB goal, 1 P
testing: warning: no tests to run
PASS
BenchmarkBinaryTree17 gc #3 @1.906s 0%: 0+0+0+0+7 ms clock, 0+0+0+0/0/0+7 ms cpu, 325->325->287 MB, 325 MB goal, 1 P (forced)
gc #4 @12.203s 20%: 0+0+0+10067+10 ms clock, 0+0+0+0/2523/852+10 ms cpu, 430->2092->1950 MB, 574 MB goal, 1 P
1 9150447353 ns/op
Change this estimate to instead use the *current* scannable heap
size. This has the advantage of being based solely on the current
state of the heap, not on past densities or reachable heap sizes, so
it isn't susceptible to falling behind during these sorts of phase
changes. This is strictly an over-estimate, but it's better to
over-estimate and get more assist than necessary than it is to
under-estimate and potentially spiral out of control. Experiments with
scaling this estimate back showed no obvious benefit for mutator
utilization, heap size, or assist time.
This new estimate has little effect for most benchmarks, including
most go1 benchmarks, x/benchmarks, and the 6g benchmark. It has a huge
effect for benchmarks that triggered the bad pacer behavior:
name old mean new mean delta
BinaryTree17 10.0s × (1.00,1.00) 3.5s × (0.98,1.01) -64.93% (p=0.000)
Fannkuch11 2.74s × (1.00,1.01) 2.65s × (1.00,1.00) -3.52% (p=0.000)
FmtFprintfEmpty 56.4ns × (0.99,1.00) 57.8ns × (1.00,1.01) +2.43% (p=0.000)
FmtFprintfString 187ns × (0.99,1.00) 185ns × (0.99,1.01) -1.19% (p=0.010)
FmtFprintfInt 184ns × (1.00,1.00) 183ns × (1.00,1.00) (no variance)
FmtFprintfIntInt 321ns × (1.00,1.00) 315ns × (1.00,1.00) -1.80% (p=0.000)
FmtFprintfPrefixedInt 266ns × (1.00,1.00) 263ns × (1.00,1.00) -1.22% (p=0.000)
FmtFprintfFloat 353ns × (1.00,1.00) 353ns × (1.00,1.00) -0.13% (p=0.035)
FmtManyArgs 1.21µs × (1.00,1.00) 1.19µs × (1.00,1.00) -1.33% (p=0.000)
GobDecode 9.69ms × (1.00,1.00) 9.59ms × (1.00,1.00) -1.07% (p=0.000)
GobEncode 7.89ms × (0.99,1.01) 7.74ms × (1.00,1.00) -1.92% (p=0.000)
Gzip 391ms × (1.00,1.00) 392ms × (1.00,1.00) ~ (p=0.522)
Gunzip 97.1ms × (1.00,1.00) 97.0ms × (1.00,1.00) -0.10% (p=0.000)
HTTPClientServer 55.7µs × (0.99,1.01) 56.7µs × (0.99,1.01) +1.81% (p=0.001)
JSONEncode 19.1ms × (1.00,1.00) 19.0ms × (1.00,1.00) -0.85% (p=0.000)
JSONDecode 66.8ms × (1.00,1.00) 66.9ms × (1.00,1.00) ~ (p=0.288)
Mandelbrot200 4.13ms × (1.00,1.00) 4.12ms × (1.00,1.00) -0.08% (p=0.000)
GoParse 3.97ms × (1.00,1.01) 4.01ms × (1.00,1.00) +0.99% (p=0.000)
RegexpMatchEasy0_32 114ns × (1.00,1.00) 115ns × (0.99,1.00) ~ (p=0.070)
RegexpMatchEasy0_1K 376ns × (1.00,1.00) 376ns × (1.00,1.00) ~ (p=0.900)
RegexpMatchEasy1_32 94.9ns × (1.00,1.00) 96.3ns × (1.00,1.01) +1.53% (p=0.001)
RegexpMatchEasy1_1K 568ns × (1.00,1.00) 567ns × (1.00,1.00) -0.22% (p=0.001)
RegexpMatchMedium_32 159ns × (1.00,1.00) 159ns × (1.00,1.00) ~ (p=0.178)
RegexpMatchMedium_1K 46.4µs × (1.00,1.00) 46.6µs × (1.00,1.00) +0.29% (p=0.000)
RegexpMatchHard_32 2.37µs × (1.00,1.00) 2.37µs × (1.00,1.00) ~ (p=0.722)
RegexpMatchHard_1K 71.1µs × (1.00,1.00) 71.2µs × (1.00,1.00) ~ (p=0.229)
Revcomp 565ms × (1.00,1.00) 562ms × (1.00,1.00) -0.52% (p=0.000)
Template 81.0ms × (1.00,1.00) 80.2ms × (1.00,1.00) -0.97% (p=0.000)
TimeParse 380ns × (1.00,1.00) 380ns × (1.00,1.00) ~ (p=0.148)
TimeFormat 405ns × (0.99,1.00) 385ns × (0.99,1.00) -5.00% (p=0.000)
Change-Id: I11274158bf3affaf62662e02de7af12d5fb789e4
Reviewed-on: https://go-review.googlesource.com/9696
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
This tracks the number of scannable bytes in the allocated heap. That
is, bytes that the garbage collector must scan before reaching the
last pointer field in each object.
This will be used to compute a more robust estimate of the GC scan
work.
Change-Id: I1eecd45ef9cdd65b69d2afb5db5da885c80086bb
Reviewed-on: https://go-review.googlesource.com/9695
Reviewed-by: Russ Cox <rsc@golang.org>
Currently, we only flush the per-P gcWork caches in gcMark, at the
beginning of mark termination. This is necessary to ensure that no
work is held up in these caches.
However, this flush happens after we update the GC controller state,
which depends on statistics about marked heap size and scan work that
are only updated by this flush. Hence, the controller is missing the
bulk of heap marking and scan work. This bug was introduced in commit
1b4025f, which introduced the per-P gcWork caches.
Fix this by flushing these caches before we update the GC controller
state. We continue to flush them at the beginning of mark termination
as well to be robust in case any write barriers happened between the
previous flush and entering mark termination, but this should be a
no-op.
Change-Id: I8f0f91024df967ebf0c616d1c4f0c339c304ebaa
Reviewed-on: https://go-review.googlesource.com/9646
Reviewed-by: Russ Cox <rsc@golang.org>
During development some tracing routines were added that are not
needed in the release. These included GCstarttimes, GCendtimes, and
GCprinttimes.
Fixes#10462
Change-Id: I0788e6409d61038571a5ae0cbbab793102df0a65
Reviewed-on: https://go-review.googlesource.com/9689
Reviewed-by: Austin Clements <austin@google.com>
This adds a detailed debug dump of the state of the GC controller and
a GODEBUG flag to enable it.
Change-Id: I562fed7981691a84ddf0f9e6fcd9f089f497ac13
Reviewed-on: https://go-review.googlesource.com/9640
Reviewed-by: Russ Cox <rsc@golang.org>
