89c341c5e9
Currently we track the per-G GC assist balance as two monotonically increasing values: the bytes allocated by the G this cycle (gcalloc) and the scan work performed by the G this cycle (gcscanwork). The assist balance is hence assistRatio*gcalloc - gcscanwork. This works, but has two important downsides: 1) It requires floating-point math to figure out if a G is in debt or not. This makes it inappropriate to check for assist debt in the hot path of mallocgc, so we only do this when a G allocates a new span. As a result, Gs can operate "in the red", leading to under-assist and extended GC cycle length. 2) Revising the assist ratio during a GC cycle can lead to an "assist burst". If you think of plotting the scan work performed versus heaps size, the assist ratio controls the slope of this line. However, in the current system, the target line always passes through 0 at the heap size that triggered GC, so if the runtime increases the assist ratio, there has to be a potentially large assist to jump from the current amount of scan work up to the new target scan work for the current heap size. This commit replaces this approach with directly tracking the GC assist balance in terms of allocation credit bytes. Allocating N bytes simply decreases this by N and assisting raises it by the amount of scan work performed divided by the assist ratio (to get back to bytes). This will make it cheap to figure out if a G is in debt, which will let us efficiently check if an assist is necessary *before* performing an allocation and hence keep Gs "in the black". This also fixes assist bursts because the assist ratio is now in terms of *remaining* work, rather than work from the beginning of the GC cycle. Hence, the plot of scan work versus heap size becomes continuous: we can revise the slope, but this slope always starts from where we are right now, rather than where we were at the beginning of the cycle. Change-Id: Ia821c5f07f8a433e8da7f195b52adfedd58bdf2c Reviewed-on: https://go-review.googlesource.com/15408 Reviewed-by: Rick Hudson <rlh@golang.org> |
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