This CL introduces a new mSpanList type to replace the empty mspan
variables that were previously used as list heads.
To be type safe, the previous circular linked list data structure is
now a tail queue instead. One complication of this is
mSpanList_Remove needs to know the list a span is being removed from,
but this appears to be computable in all circumstances.
As a temporary sanity check, mSpanList_Insert and mSpanList_InsertBack
record the list that an mspan has been inserted into so that
mSpanList_Remove can verify that the correct list was specified.
Whereas mspan is 112 bytes on amd64, mSpanList is only 16 bytes. This
shrinks the size of mheap from 50216 bytes to 12584 bytes.
Change-Id: I8146364753dbc3b4ab120afbb9c7b8740653c216
Reviewed-on: https://go-review.googlesource.com/15906
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Austin Clements <austin@google.com>
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, the concurrent sweep follows a 1:1 rule: when allocation
needs a span, it sweeps a span (likewise, when a large allocation
needs N pages, it sweeps until it frees N pages). This rule worked
well for the STW collector (especially when GOGC==100) because it did
no more sweeping than necessary to keep the heap from growing, would
generally finish sweeping just before GC, and ensured good temporal
locality between sweeping a page and allocating from it.
It doesn't work well with concurrent GC. Since concurrent GC requires
starting GC earlier (sometimes much earlier), the sweep often won't be
done when GC starts. Unfortunately, the first thing GC has to do is
finish the sweep. In the mean time, the mutator can continue
allocating, pushing the heap size even closer to the goal size. This
worked okay with the 7/8ths trigger, but it gets into a vicious cycle
with the GC trigger controller: if the mutator is allocating quickly
and driving the trigger lower, more and more sweep work will be left
to GC; this both causes GC to take longer (allowing the mutator to
allocate more during GC) and delays the start of the concurrent mark
phase, which throws off the GC controller's statistics and generally
causes it to push the trigger even lower.
As an example of a particularly bad case, the garbage benchmark with
GOMAXPROCS=4 and -benchmem 512 (MB) spends the first 0.4-0.8 seconds
of each GC cycle sweeping, during which the heap grows by between
109MB and 252MB.
To fix this, this change replaces the 1:1 sweep rule with a
proportional sweep rule. At the end of GC, GC knows exactly how much
heap allocation will occur before the next concurrent GC as well as
how many span pages must be swept. This change computes this "sweep
ratio" and when the mallocgc asks for a span, the mcentral sweeps
enough spans to bring the swept span count into ratio with the
allocated byte count.
On the benchmark from above, this entirely eliminates sweeping at the
beginning of GC, which reduces the time between startGC readying the
GC goroutine and GC stopping the world for sweep termination to ~100µs
during which the heap grows at most 134KB.
Change-Id: I35422d6bba0c2310d48bb1f8f30a72d29e98c1af
Reviewed-on: https://go-review.googlesource.com/8921
Reviewed-by: Rick Hudson <rlh@golang.org>
Everything has moved to Go, but comments still refer to .c/.h files.
Fix all of those up, at least for these three directories.
Fixes#10138
Change-Id: Ie5efe89b247841e0b3f82aac5256b2c606ef67dc
Reviewed-on: https://go-review.googlesource.com/7431
Reviewed-by: Russ Cox <rsc@golang.org>
This is an experiment to see if removing the boundary bit logic will
lead to fewer cache misses and improved performance. Instead of using
boundary bits we use the span information to get element size and use
some bit whacking to get the boundary without having to touch the
random heap bits which cause cache misses.
Furthermore once the boundary bit is removed we can either use that
bit for a simpler checkmark routine or we can reduce the number of
bits in the GC bitmap to 2 bits per pointer sized work. For example
the 2 bits at the boundary can be used for marking and pointer/scalar
differentiation. Since we don't need the mark bit except at the
boundary nibble of the object other nibbles can use this bit
as a noscan bit to indicate that there are no more pointers in
the object.
Currently the changed included in this CL slows down the garbage
benchmark. With the boundary bits garbage gives 5.78 and without
(this CL) it gives 5.88 which is a 2% slowdown.
Change-Id: Id68f831ad668176f7dc9f7b57b339e4ebb6dc4c2
Reviewed-on: https://go-review.googlesource.com/6665
Reviewed-by: Austin Clements <austin@google.com>
Move code from malloc1.go, malloc2.go, mem.go, mgc0.go into
appropriate locations.
Factor mgc.go into mgc.go, mgcmark.go, mgcsweep.go, mstats.go.
A lot of this code was in certain files because the right place was in
a C file but it was written in Go, or vice versa. This is one step toward
making things actually well-organized again.
Change-Id: I6741deb88a7cfb1c17ffe0bcca3989e10207968f
Reviewed-on: https://go-review.googlesource.com/5300
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Rick Hudson <rlh@golang.org>
The code in mfinal.go is moved from malloc*.go and mgc*.go
and substantially unchanged.
The code in mbitmap.go is also moved from those files, but
cleaned up so that it can be called from those files (in most cases
the code being moved was not already a standalone function).
I also renamed the constants and wrote comments describing
the format. The result is a significant cleanup and isolation of
the bitmap code, but, roughly speaking, it should be treated
and reviewed as new code.
The other files changed only as much as necessary to support
this code movement.
This CL does NOT change the semantics of the heap or type
bitmaps at all, although there are now some obvious opportunities
to do so in followup CLs.
Change-Id: I41b8d5de87ad1d3cd322709931ab25e659dbb21d
Reviewed-on: https://go-review.googlesource.com/2991
Reviewed-by: Keith Randall <khr@golang.org>
Rename "gothrow" to "throw" now that the C version of "throw"
is no longer needed.
This change is purely mechanical except in panic.go where the
old version of "throw" has been deleted.
sed -i "" 's/[[:<:]]gothrow[[:>:]]/throw/g' runtime/*.go
Change-Id: Icf0752299c35958b92870a97111c67bcd9159dc3
Reviewed-on: https://go-review.googlesource.com/2150
Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>
The conversion was done with an automated tool and then
modified only as necessary to make it compile and run.
[This CL is part of the removal of C code from package runtime.
See golang.org/s/dev.cc for an overview.]
LGTM=r
R=r
CC=austin, dvyukov, golang-codereviews, iant, khr
https://golang.org/cl/167540043
