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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> |
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