• Rick Hudson's avatar
    runtime: reduce thrashing of gs between ps · c4fe5031
    Rick Hudson authored
    One important use case is a pipeline computation that pass values
    from one Goroutine to the next and then exits or is placed in a
    wait state. If GOMAXPROCS > 1 a Goroutine running on P1 will enable
    another Goroutine and then immediately make P1 available to execute
    it. We need to prevent other Ps from stealing the G that P1 is about
    to execute. Otherwise the Gs can thrash between Ps causing unneeded
    synchronization and slowing down throughput.
    
    Fix this by changing the stealing logic so that when a P attempts to
    steal the only G on some other P's run queue, it will pause
    momentarily to allow the victim P to schedule the G.
    
    As part of optimizing stealing we also use a per P victim queue
    move stolen gs. This eliminates the zeroing of a stack local victim
    queue which turned out to be expensive.
    
    This CL is a necessary but not sufficient prerequisite to changing
    the default value of GOMAXPROCS to something > 1 which is another
    CL/discussion.
    
    For highly serialized programs, such as GoroutineRing below this can
    make a large difference. For larger and more parallel programs such
    as the x/benchmarks there is no noticeable detriment.
    
    ~/work/code/src/rsc.io/benchstat/benchstat old.txt new.txt
    name                old mean              new mean              delta
    GoroutineRing       30.2µs × (0.98,1.01)  30.1µs × (0.97,1.04)     ~    (p=0.941)
    GoroutineRing-2      113µs × (0.91,1.07)    30µs × (0.98,1.03)  -73.17% (p=0.004)
    GoroutineRing-4      144µs × (0.98,1.02)    32µs × (0.98,1.01)  -77.69% (p=0.000)
    GoroutineRingBuf    32.7µs × (0.97,1.03)  32.5µs × (0.97,1.02)     ~    (p=0.795)
    GoroutineRingBuf-2   120µs × (0.92,1.08)    33µs × (1.00,1.00)  -72.48% (p=0.004)
    GoroutineRingBuf-4   138µs × (0.92,1.06)    33µs × (1.00,1.00)  -76.21% (p=0.003)
    
    The bench benchmarks show little impact.
        	  	      old  	 new
    garbage	      	      7032879	 7011696
    httpold		        25509	   25301
    splayold	      1022073	 1019499
    jsonold		     28230624   28081433
    
    Change-Id: I228c48fed8d85c9bbef16a7edc53ab7898506f50
    Reviewed-on: https://go-review.googlesource.com/9872Reviewed-by: 's avatarAustin Clements <austin@google.com>
    c4fe5031
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