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This change modifies the scavenger to no longer hold the heap lock while actively scavenging pages. To achieve this, the change also: * Reverses the locking behavior of the (*pageAlloc).scavenge API, to only acquire the heap lock when necessary. * Introduces a new lock on the scavenger-related fields in a pageAlloc so that access to those fields doesn't require the heap lock. There are a few places in the scavenge path, notably reservation, that requires synchronization. The heap lock is far too heavy handed for this case. * Changes the scavenger to marks pages that are actively being scavenged as allocated, and "frees" them back to the page allocator the usual way. * Lifts the heap-growth scavenging code out of mheap.grow, where the heap lock is held, and into allocSpan, just after the lock is released. Releasing the lock during mheap.grow is not feasible if we want to ensure that allocation always makes progress (post-growth, another allocator could come in and take all that space, forcing the goroutine that just grew the heap to do so again). This change means that the scavenger now must do more work for each scavenge, but it is also now much more scalable. Although in theory it's not great by always taking the locked paths in the page allocator, it takes advantage of some properties of the allocator: * Most of the time, the scavenger will be working with one page at a time. The page allocator's locked path is optimized for this case. * On the allocation path, it doesn't need to do the find operation at all; it can go straight to setting bits for the range and updating the summary structure. Change-Id: Ie941d5e7c05dcc96476795c63fef74bcafc2a0f1 Reviewed-on: https://go-review.googlesource.com/c/go/+/353974 Trust: Michael Knyszek <mknyszek@google.com> Reviewed-by: Michael Pratt <mpratt@google.com> |
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