• Austin Clements's avatar
    runtime: multi-threaded, utilization-scheduled background mark · 8d03acce
    Austin Clements authored
    Currently, the concurrent mark phase is performed by the main GC
    goroutine. Prior to the previous commit enabling preemption, this
    caused marking to always consume 1/GOMAXPROCS of the available CPU
    time. If GOMAXPROCS=1, this meant background GC would consume 100% of
    the CPU (effectively a STW). If GOMAXPROCS>4, background GC would use
    less than the goal of 25%. If GOMAXPROCS=4, background GC would use
    the goal 25%, but if the mutator wasn't using the remaining 75%,
    background marking wouldn't take advantage of the idle time. Enabling
    preemption in the previous commit made GC miss CPU targets in
    completely different ways, but set us up to bring everything back in
    line.
    
    This change replaces the fixed GC goroutine with per-P background mark
    goroutines. Once started, these goroutines don't go in the standard
    run queues; instead, they are scheduled specially such that the time
    spent in mutator assists and the background mark goroutines totals 25%
    of the CPU time available to the program. Furthermore, this lets
    background marking take advantage of idle Ps, which significantly
    boosts GC performance for applications that under-utilize the CPU.
    
    This requires also changing how time is reported for gctrace, so this
    change splits the concurrent mark CPU time into assist/background/idle
    scanning.
    
    This also requires increasing the size of the StackRecord slice used
    in a GoroutineProfile test.
    
    Change-Id: I0936ff907d2cee6cb687a208f2df47e8988e3157
    Reviewed-on: https://go-review.googlesource.com/8850Reviewed-by: 's avatarRick Hudson <rlh@golang.org>
    8d03acce
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