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Diffstat (limited to 'src/runtime/mgcscavenge.go')
| -rw-r--r-- | src/runtime/mgcscavenge.go | 953 |
1 files changed, 953 insertions, 0 deletions
diff --git a/src/runtime/mgcscavenge.go b/src/runtime/mgcscavenge.go new file mode 100644 index 0000000..a7c5bc4 --- /dev/null +++ b/src/runtime/mgcscavenge.go @@ -0,0 +1,953 @@ +// Copyright 2019 The Go Authors. All rights reserved. +// Use of this source code is governed by a BSD-style +// license that can be found in the LICENSE file. + +// Scavenging free pages. +// +// This file implements scavenging (the release of physical pages backing mapped +// memory) of free and unused pages in the heap as a way to deal with page-level +// fragmentation and reduce the RSS of Go applications. +// +// Scavenging in Go happens on two fronts: there's the background +// (asynchronous) scavenger and the heap-growth (synchronous) scavenger. +// +// The former happens on a goroutine much like the background sweeper which is +// soft-capped at using scavengePercent of the mutator's time, based on +// order-of-magnitude estimates of the costs of scavenging. The background +// scavenger's primary goal is to bring the estimated heap RSS of the +// application down to a goal. +// +// That goal is defined as: +// (retainExtraPercent+100) / 100 * (next_gc / last_next_gc) * last_heap_inuse +// +// Essentially, we wish to have the application's RSS track the heap goal, but +// the heap goal is defined in terms of bytes of objects, rather than pages like +// RSS. As a result, we need to take into account for fragmentation internal to +// spans. next_gc / last_next_gc defines the ratio between the current heap goal +// and the last heap goal, which tells us by how much the heap is growing and +// shrinking. We estimate what the heap will grow to in terms of pages by taking +// this ratio and multiplying it by heap_inuse at the end of the last GC, which +// allows us to account for this additional fragmentation. Note that this +// procedure makes the assumption that the degree of fragmentation won't change +// dramatically over the next GC cycle. Overestimating the amount of +// fragmentation simply results in higher memory use, which will be accounted +// for by the next pacing up date. Underestimating the fragmentation however +// could lead to performance degradation. Handling this case is not within the +// scope of the scavenger. Situations where the amount of fragmentation balloons +// over the course of a single GC cycle should be considered pathologies, +// flagged as bugs, and fixed appropriately. +// +// An additional factor of retainExtraPercent is added as a buffer to help ensure +// that there's more unscavenged memory to allocate out of, since each allocation +// out of scavenged memory incurs a potentially expensive page fault. +// +// The goal is updated after each GC and the scavenger's pacing parameters +// (which live in mheap_) are updated to match. The pacing parameters work much +// like the background sweeping parameters. The parameters define a line whose +// horizontal axis is time and vertical axis is estimated heap RSS, and the +// scavenger attempts to stay below that line at all times. +// +// The synchronous heap-growth scavenging happens whenever the heap grows in +// size, for some definition of heap-growth. The intuition behind this is that +// the application had to grow the heap because existing fragments were +// not sufficiently large to satisfy a page-level memory allocation, so we +// scavenge those fragments eagerly to offset the growth in RSS that results. + +package runtime + +import ( + "runtime/internal/atomic" + "runtime/internal/sys" + "unsafe" +) + +const ( + // The background scavenger is paced according to these parameters. + // + // scavengePercent represents the portion of mutator time we're willing + // to spend on scavenging in percent. + scavengePercent = 1 // 1% + + // retainExtraPercent represents the amount of memory over the heap goal + // that the scavenger should keep as a buffer space for the allocator. + // + // The purpose of maintaining this overhead is to have a greater pool of + // unscavenged memory available for allocation (since using scavenged memory + // incurs an additional cost), to account for heap fragmentation and + // the ever-changing layout of the heap. + retainExtraPercent = 10 + + // maxPagesPerPhysPage is the maximum number of supported runtime pages per + // physical page, based on maxPhysPageSize. + maxPagesPerPhysPage = maxPhysPageSize / pageSize + + // scavengeCostRatio is the approximate ratio between the costs of using previously + // scavenged memory and scavenging memory. + // + // For most systems the cost of scavenging greatly outweighs the costs + // associated with using scavenged memory, making this constant 0. On other systems + // (especially ones where "sysUsed" is not just a no-op) this cost is non-trivial. + // + // This ratio is used as part of multiplicative factor to help the scavenger account + // for the additional costs of using scavenged memory in its pacing. + scavengeCostRatio = 0.7 * (sys.GoosDarwin + sys.GoosIos) + + // scavengeReservationShards determines the amount of memory the scavenger + // should reserve for scavenging at a time. Specifically, the amount of + // memory reserved is (heap size in bytes) / scavengeReservationShards. + scavengeReservationShards = 64 +) + +// heapRetained returns an estimate of the current heap RSS. +func heapRetained() uint64 { + return memstats.heap_sys.load() - atomic.Load64(&memstats.heap_released) +} + +// gcPaceScavenger updates the scavenger's pacing, particularly +// its rate and RSS goal. +// +// The RSS goal is based on the current heap goal with a small overhead +// to accommodate non-determinism in the allocator. +// +// The pacing is based on scavengePageRate, which applies to both regular and +// huge pages. See that constant for more information. +// +// mheap_.lock must be held or the world must be stopped. +func gcPaceScavenger() { + // If we're called before the first GC completed, disable scavenging. + // We never scavenge before the 2nd GC cycle anyway (we don't have enough + // information about the heap yet) so this is fine, and avoids a fault + // or garbage data later. + if memstats.last_next_gc == 0 { + mheap_.scavengeGoal = ^uint64(0) + return + } + // Compute our scavenging goal. + goalRatio := float64(atomic.Load64(&memstats.next_gc)) / float64(memstats.last_next_gc) + retainedGoal := uint64(float64(memstats.last_heap_inuse) * goalRatio) + // Add retainExtraPercent overhead to retainedGoal. This calculation + // looks strange but the purpose is to arrive at an integer division + // (e.g. if retainExtraPercent = 12.5, then we get a divisor of 8) + // that also avoids the overflow from a multiplication. + retainedGoal += retainedGoal / (1.0 / (retainExtraPercent / 100.0)) + // Align it to a physical page boundary to make the following calculations + // a bit more exact. + retainedGoal = (retainedGoal + uint64(physPageSize) - 1) &^ (uint64(physPageSize) - 1) + + // Represents where we are now in the heap's contribution to RSS in bytes. + // + // Guaranteed to always be a multiple of physPageSize on systems where + // physPageSize <= pageSize since we map heap_sys at a rate larger than + // any physPageSize and released memory in multiples of the physPageSize. + // + // However, certain functions recategorize heap_sys as other stats (e.g. + // stack_sys) and this happens in multiples of pageSize, so on systems + // where physPageSize > pageSize the calculations below will not be exact. + // Generally this is OK since we'll be off by at most one regular + // physical page. + retainedNow := heapRetained() + + // If we're already below our goal, or within one page of our goal, then disable + // the background scavenger. We disable the background scavenger if there's + // less than one physical page of work to do because it's not worth it. + if retainedNow <= retainedGoal || retainedNow-retainedGoal < uint64(physPageSize) { + mheap_.scavengeGoal = ^uint64(0) + return + } + mheap_.scavengeGoal = retainedGoal +} + +// Sleep/wait state of the background scavenger. +var scavenge struct { + lock mutex + g *g + parked bool + timer *timer + sysmonWake uint32 // Set atomically. +} + +// readyForScavenger signals sysmon to wake the scavenger because +// there may be new work to do. +// +// There may be a significant delay between when this function runs +// and when the scavenger is kicked awake, but it may be safely invoked +// in contexts where wakeScavenger is unsafe to call directly. +func readyForScavenger() { + atomic.Store(&scavenge.sysmonWake, 1) +} + +// wakeScavenger immediately unparks the scavenger if necessary. +// +// May run without a P, but it may allocate, so it must not be called +// on any allocation path. +// +// mheap_.lock, scavenge.lock, and sched.lock must not be held. +func wakeScavenger() { + lock(&scavenge.lock) + if scavenge.parked { + // Notify sysmon that it shouldn't bother waking up the scavenger. + atomic.Store(&scavenge.sysmonWake, 0) + + // Try to stop the timer but we don't really care if we succeed. + // It's possible that either a timer was never started, or that + // we're racing with it. + // In the case that we're racing with there's the low chance that + // we experience a spurious wake-up of the scavenger, but that's + // totally safe. + stopTimer(scavenge.timer) + + // Unpark the goroutine and tell it that there may have been a pacing + // change. Note that we skip the scheduler's runnext slot because we + // want to avoid having the scavenger interfere with the fair + // scheduling of user goroutines. In effect, this schedules the + // scavenger at a "lower priority" but that's OK because it'll + // catch up on the work it missed when it does get scheduled. + scavenge.parked = false + + // Ready the goroutine by injecting it. We use injectglist instead + // of ready or goready in order to allow us to run this function + // without a P. injectglist also avoids placing the goroutine in + // the current P's runnext slot, which is desireable to prevent + // the scavenger from interfering with user goroutine scheduling + // too much. + var list gList + list.push(scavenge.g) + injectglist(&list) + } + unlock(&scavenge.lock) +} + +// scavengeSleep attempts to put the scavenger to sleep for ns. +// +// Note that this function should only be called by the scavenger. +// +// The scavenger may be woken up earlier by a pacing change, and it may not go +// to sleep at all if there's a pending pacing change. +// +// Returns the amount of time actually slept. +func scavengeSleep(ns int64) int64 { + lock(&scavenge.lock) + + // Set the timer. + // + // This must happen here instead of inside gopark + // because we can't close over any variables without + // failing escape analysis. + start := nanotime() + resetTimer(scavenge.timer, start+ns) + + // Mark ourself as asleep and go to sleep. + scavenge.parked = true + goparkunlock(&scavenge.lock, waitReasonSleep, traceEvGoSleep, 2) + + // Return how long we actually slept for. + return nanotime() - start +} + +// Background scavenger. +// +// The background scavenger maintains the RSS of the application below +// the line described by the proportional scavenging statistics in +// the mheap struct. +func bgscavenge(c chan int) { + scavenge.g = getg() + + lockInit(&scavenge.lock, lockRankScavenge) + lock(&scavenge.lock) + scavenge.parked = true + + scavenge.timer = new(timer) + scavenge.timer.f = func(_ interface{}, _ uintptr) { + wakeScavenger() + } + + c <- 1 + goparkunlock(&scavenge.lock, waitReasonGCScavengeWait, traceEvGoBlock, 1) + + // Exponentially-weighted moving average of the fraction of time this + // goroutine spends scavenging (that is, percent of a single CPU). + // It represents a measure of scheduling overheads which might extend + // the sleep or the critical time beyond what's expected. Assume no + // overhead to begin with. + // + // TODO(mknyszek): Consider making this based on total CPU time of the + // application (i.e. scavengePercent * GOMAXPROCS). This isn't really + // feasible now because the scavenger acquires the heap lock over the + // scavenging operation, which means scavenging effectively blocks + // allocators and isn't scalable. However, given a scalable allocator, + // it makes sense to also make the scavenger scale with it; if you're + // allocating more frequently, then presumably you're also generating + // more work for the scavenger. + const idealFraction = scavengePercent / 100.0 + scavengeEWMA := float64(idealFraction) + + for { + released := uintptr(0) + + // Time in scavenging critical section. + crit := float64(0) + + // Run on the system stack since we grab the heap lock, + // and a stack growth with the heap lock means a deadlock. + systemstack(func() { + lock(&mheap_.lock) + + // If background scavenging is disabled or if there's no work to do just park. + retained, goal := heapRetained(), mheap_.scavengeGoal + if retained <= goal { + unlock(&mheap_.lock) + return + } + + // Scavenge one page, and measure the amount of time spent scavenging. + start := nanotime() + released = mheap_.pages.scavenge(physPageSize, true) + mheap_.pages.scav.released += released + crit = float64(nanotime() - start) + + unlock(&mheap_.lock) + }) + + if released == 0 { + lock(&scavenge.lock) + scavenge.parked = true + goparkunlock(&scavenge.lock, waitReasonGCScavengeWait, traceEvGoBlock, 1) + continue + } + + if released < physPageSize { + // If this happens, it means that we may have attempted to release part + // of a physical page, but the likely effect of that is that it released + // the whole physical page, some of which may have still been in-use. + // This could lead to memory corruption. Throw. + throw("released less than one physical page of memory") + } + + // On some platforms we may see crit as zero if the time it takes to scavenge + // memory is less than the minimum granularity of its clock (e.g. Windows). + // In this case, just assume scavenging takes 10 µs per regular physical page + // (determined empirically), and conservatively ignore the impact of huge pages + // on timing. + // + // We shouldn't ever see a crit value less than zero unless there's a bug of + // some kind, either on our side or in the platform we're running on, but be + // defensive in that case as well. + const approxCritNSPerPhysicalPage = 10e3 + if crit <= 0 { + crit = approxCritNSPerPhysicalPage * float64(released/physPageSize) + } + + // Multiply the critical time by 1 + the ratio of the costs of using + // scavenged memory vs. scavenging memory. This forces us to pay down + // the cost of reusing this memory eagerly by sleeping for a longer period + // of time and scavenging less frequently. More concretely, we avoid situations + // where we end up scavenging so often that we hurt allocation performance + // because of the additional overheads of using scavenged memory. + crit *= 1 + scavengeCostRatio + + // If we spent more than 10 ms (for example, if the OS scheduled us away, or someone + // put their machine to sleep) in the critical section, bound the time we use to + // calculate at 10 ms to avoid letting the sleep time get arbitrarily high. + const maxCrit = 10e6 + if crit > maxCrit { + crit = maxCrit + } + + // Compute the amount of time to sleep, assuming we want to use at most + // scavengePercent of CPU time. Take into account scheduling overheads + // that may extend the length of our sleep by multiplying by how far + // off we are from the ideal ratio. For example, if we're sleeping too + // much, then scavengeEMWA < idealFraction, so we'll adjust the sleep time + // down. + adjust := scavengeEWMA / idealFraction + sleepTime := int64(adjust * crit / (scavengePercent / 100.0)) + + // Go to sleep. + slept := scavengeSleep(sleepTime) + + // Compute the new ratio. + fraction := crit / (crit + float64(slept)) + + // Set a lower bound on the fraction. + // Due to OS-related anomalies we may "sleep" for an inordinate amount + // of time. Let's avoid letting the ratio get out of hand by bounding + // the sleep time we use in our EWMA. + const minFraction = 1 / 1000 + if fraction < minFraction { + fraction = minFraction + } + + // Update scavengeEWMA by merging in the new crit/slept ratio. + const alpha = 0.5 + scavengeEWMA = alpha*fraction + (1-alpha)*scavengeEWMA + } +} + +// scavenge scavenges nbytes worth of free pages, starting with the +// highest address first. Successive calls continue from where it left +// off until the heap is exhausted. Call scavengeStartGen to bring it +// back to the top of the heap. +// +// Returns the amount of memory scavenged in bytes. +// +// p.mheapLock must be held, but may be temporarily released if +// mayUnlock == true. +// +// Must run on the system stack because p.mheapLock must be held. +// +//go:systemstack +func (p *pageAlloc) scavenge(nbytes uintptr, mayUnlock bool) uintptr { + assertLockHeld(p.mheapLock) + + var ( + addrs addrRange + gen uint32 + ) + released := uintptr(0) + for released < nbytes { + if addrs.size() == 0 { + if addrs, gen = p.scavengeReserve(); addrs.size() == 0 { + break + } + } + r, a := p.scavengeOne(addrs, nbytes-released, mayUnlock) + released += r + addrs = a + } + // Only unreserve the space which hasn't been scavenged or searched + // to ensure we always make progress. + p.scavengeUnreserve(addrs, gen) + return released +} + +// printScavTrace prints a scavenge trace line to standard error. +// +// released should be the amount of memory released since the last time this +// was called, and forced indicates whether the scavenge was forced by the +// application. +func printScavTrace(gen uint32, released uintptr, forced bool) { + printlock() + print("scav ", gen, " ", + released>>10, " KiB work, ", + atomic.Load64(&memstats.heap_released)>>10, " KiB total, ", + (atomic.Load64(&memstats.heap_inuse)*100)/heapRetained(), "% util", + ) + if forced { + print(" (forced)") + } + println() + printunlock() +} + +// scavengeStartGen starts a new scavenge generation, resetting +// the scavenger's search space to the full in-use address space. +// +// p.mheapLock must be held. +// +// Must run on the system stack because p.mheapLock must be held. +// +//go:systemstack +func (p *pageAlloc) scavengeStartGen() { + assertLockHeld(p.mheapLock) + + if debug.scavtrace > 0 { + printScavTrace(p.scav.gen, p.scav.released, false) + } + p.inUse.cloneInto(&p.scav.inUse) + + // Pick the new starting address for the scavenger cycle. + var startAddr offAddr + if p.scav.scavLWM.lessThan(p.scav.freeHWM) { + // The "free" high watermark exceeds the "scavenged" low watermark, + // so there are free scavengable pages in parts of the address space + // that the scavenger already searched, the high watermark being the + // highest one. Pick that as our new starting point to ensure we + // see those pages. + startAddr = p.scav.freeHWM + } else { + // The "free" high watermark does not exceed the "scavenged" low + // watermark. This means the allocator didn't free any memory in + // the range we scavenged last cycle, so we might as well continue + // scavenging from where we were. + startAddr = p.scav.scavLWM + } + p.scav.inUse.removeGreaterEqual(startAddr.addr()) + + // reservationBytes may be zero if p.inUse.totalBytes is small, or if + // scavengeReservationShards is large. This case is fine as the scavenger + // will simply be turned off, but it does mean that scavengeReservationShards, + // in concert with pallocChunkBytes, dictates the minimum heap size at which + // the scavenger triggers. In practice this minimum is generally less than an + // arena in size, so virtually every heap has the scavenger on. + p.scav.reservationBytes = alignUp(p.inUse.totalBytes, pallocChunkBytes) / scavengeReservationShards + p.scav.gen++ + p.scav.released = 0 + p.scav.freeHWM = minOffAddr + p.scav.scavLWM = maxOffAddr +} + +// scavengeReserve reserves a contiguous range of the address space +// for scavenging. The maximum amount of space it reserves is proportional +// to the size of the heap. The ranges are reserved from the high addresses +// first. +// +// Returns the reserved range and the scavenge generation number for it. +// +// p.mheapLock must be held. +// +// Must run on the system stack because p.mheapLock must be held. +// +//go:systemstack +func (p *pageAlloc) scavengeReserve() (addrRange, uint32) { + assertLockHeld(p.mheapLock) + + // Start by reserving the minimum. + r := p.scav.inUse.removeLast(p.scav.reservationBytes) + + // Return early if the size is zero; we don't want to use + // the bogus address below. + if r.size() == 0 { + return r, p.scav.gen + } + + // The scavenger requires that base be aligned to a + // palloc chunk because that's the unit of operation for + // the scavenger, so align down, potentially extending + // the range. + newBase := alignDown(r.base.addr(), pallocChunkBytes) + + // Remove from inUse however much extra we just pulled out. + p.scav.inUse.removeGreaterEqual(newBase) + r.base = offAddr{newBase} + return r, p.scav.gen +} + +// scavengeUnreserve returns an unscavenged portion of a range that was +// previously reserved with scavengeReserve. +// +// p.mheapLock must be held. +// +// Must run on the system stack because p.mheapLock must be held. +// +//go:systemstack +func (p *pageAlloc) scavengeUnreserve(r addrRange, gen uint32) { + assertLockHeld(p.mheapLock) + + if r.size() == 0 || gen != p.scav.gen { + return + } + if r.base.addr()%pallocChunkBytes != 0 { + throw("unreserving unaligned region") + } + p.scav.inUse.add(r) +} + +// scavengeOne walks over address range work until it finds +// a contiguous run of pages to scavenge. It will try to scavenge +// at most max bytes at once, but may scavenge more to avoid +// breaking huge pages. Once it scavenges some memory it returns +// how much it scavenged in bytes. +// +// Returns the number of bytes scavenged and the part of work +// which was not yet searched. +// +// work's base address must be aligned to pallocChunkBytes. +// +// p.mheapLock must be held, but may be temporarily released if +// mayUnlock == true. +// +// Must run on the system stack because p.mheapLock must be held. +// +//go:systemstack +func (p *pageAlloc) scavengeOne(work addrRange, max uintptr, mayUnlock bool) (uintptr, addrRange) { + assertLockHeld(p.mheapLock) + + // Defensively check if we've received an empty address range. + // If so, just return. + if work.size() == 0 { + // Nothing to do. + return 0, work + } + // Check the prerequisites of work. + if work.base.addr()%pallocChunkBytes != 0 { + throw("scavengeOne called with unaligned work region") + } + // Calculate the maximum number of pages to scavenge. + // + // This should be alignUp(max, pageSize) / pageSize but max can and will + // be ^uintptr(0), so we need to be very careful not to overflow here. + // Rather than use alignUp, calculate the number of pages rounded down + // first, then add back one if necessary. + maxPages := max / pageSize + if max%pageSize != 0 { + maxPages++ + } + + // Calculate the minimum number of pages we can scavenge. + // + // Because we can only scavenge whole physical pages, we must + // ensure that we scavenge at least minPages each time, aligned + // to minPages*pageSize. + minPages := physPageSize / pageSize + if minPages < 1 { + minPages = 1 + } + + // Helpers for locking and unlocking only if mayUnlock == true. + lockHeap := func() { + if mayUnlock { + lock(p.mheapLock) + } + } + unlockHeap := func() { + if mayUnlock { + unlock(p.mheapLock) + } + } + + // Fast path: check the chunk containing the top-most address in work, + // starting at that address's page index in the chunk. + // + // Note that work.end() is exclusive, so get the chunk we care about + // by subtracting 1. + maxAddr := work.limit.addr() - 1 + maxChunk := chunkIndex(maxAddr) + if p.summary[len(p.summary)-1][maxChunk].max() >= uint(minPages) { + // We only bother looking for a candidate if there at least + // minPages free pages at all. + base, npages := p.chunkOf(maxChunk).findScavengeCandidate(chunkPageIndex(maxAddr), minPages, maxPages) + + // If we found something, scavenge it and return! + if npages != 0 { + work.limit = offAddr{p.scavengeRangeLocked(maxChunk, base, npages)} + + assertLockHeld(p.mheapLock) // Must be locked on return. + return uintptr(npages) * pageSize, work + } + } + // Update the limit to reflect the fact that we checked maxChunk already. + work.limit = offAddr{chunkBase(maxChunk)} + + // findCandidate finds the next scavenge candidate in work optimistically. + // + // Returns the candidate chunk index and true on success, and false on failure. + // + // The heap need not be locked. + findCandidate := func(work addrRange) (chunkIdx, bool) { + // Iterate over this work's chunks. + for i := chunkIndex(work.limit.addr() - 1); i >= chunkIndex(work.base.addr()); i-- { + // If this chunk is totally in-use or has no unscavenged pages, don't bother + // doing a more sophisticated check. + // + // Note we're accessing the summary and the chunks without a lock, but + // that's fine. We're being optimistic anyway. + + // Check quickly if there are enough free pages at all. + if p.summary[len(p.summary)-1][i].max() < uint(minPages) { + continue + } + + // Run over the chunk looking harder for a candidate. Again, we could + // race with a lot of different pieces of code, but we're just being + // optimistic. Make sure we load the l2 pointer atomically though, to + // avoid races with heap growth. It may or may not be possible to also + // see a nil pointer in this case if we do race with heap growth, but + // just defensively ignore the nils. This operation is optimistic anyway. + l2 := (*[1 << pallocChunksL2Bits]pallocData)(atomic.Loadp(unsafe.Pointer(&p.chunks[i.l1()]))) + if l2 != nil && l2[i.l2()].hasScavengeCandidate(minPages) { + return i, true + } + } + return 0, false + } + + // Slow path: iterate optimistically over the in-use address space + // looking for any free and unscavenged page. If we think we see something, + // lock and verify it! + for work.size() != 0 { + unlockHeap() + + // Search for the candidate. + candidateChunkIdx, ok := findCandidate(work) + + // Lock the heap. We need to do this now if we found a candidate or not. + // If we did, we'll verify it. If not, we need to lock before returning + // anyway. + lockHeap() + + if !ok { + // We didn't find a candidate, so we're done. + work.limit = work.base + break + } + + // Find, verify, and scavenge if we can. + chunk := p.chunkOf(candidateChunkIdx) + base, npages := chunk.findScavengeCandidate(pallocChunkPages-1, minPages, maxPages) + if npages > 0 { + work.limit = offAddr{p.scavengeRangeLocked(candidateChunkIdx, base, npages)} + + assertLockHeld(p.mheapLock) // Must be locked on return. + return uintptr(npages) * pageSize, work + } + + // We were fooled, so let's continue from where we left off. + work.limit = offAddr{chunkBase(candidateChunkIdx)} + } + + assertLockHeld(p.mheapLock) // Must be locked on return. + return 0, work +} + +// scavengeRangeLocked scavenges the given region of memory. +// The region of memory is described by its chunk index (ci), +// the starting page index of the region relative to that +// chunk (base), and the length of the region in pages (npages). +// +// Returns the base address of the scavenged region. +// +// p.mheapLock must be held. +func (p *pageAlloc) scavengeRangeLocked(ci chunkIdx, base, npages uint) uintptr { + assertLockHeld(p.mheapLock) + + p.chunkOf(ci).scavenged.setRange(base, npages) + + // Compute the full address for the start of the range. + addr := chunkBase(ci) + uintptr(base)*pageSize + + // Update the scavenge low watermark. + if oAddr := (offAddr{addr}); oAddr.lessThan(p.scav.scavLWM) { + p.scav.scavLWM = oAddr + } + + // Only perform the actual scavenging if we're not in a test. + // It's dangerous to do so otherwise. + if p.test { + return addr + } + sysUnused(unsafe.Pointer(addr), uintptr(npages)*pageSize) + + // Update global accounting only when not in test, otherwise + // the runtime's accounting will be wrong. + nbytes := int64(npages) * pageSize + atomic.Xadd64(&memstats.heap_released, nbytes) + + // Update consistent accounting too. + stats := memstats.heapStats.acquire() + atomic.Xaddint64(&stats.committed, -nbytes) + atomic.Xaddint64(&stats.released, nbytes) + memstats.heapStats.release() + + return addr +} + +// fillAligned returns x but with all zeroes in m-aligned +// groups of m bits set to 1 if any bit in the group is non-zero. +// +// For example, fillAligned(0x0100a3, 8) == 0xff00ff. +// +// Note that if m == 1, this is a no-op. +// +// m must be a power of 2 <= maxPagesPerPhysPage. +func fillAligned(x uint64, m uint) uint64 { + apply := func(x uint64, c uint64) uint64 { + // The technique used it here is derived from + // https://graphics.stanford.edu/~seander/bithacks.html#ZeroInWord + // and extended for more than just bytes (like nibbles + // and uint16s) by using an appropriate constant. + // + // To summarize the technique, quoting from that page: + // "[It] works by first zeroing the high bits of the [8] + // bytes in the word. Subsequently, it adds a number that + // will result in an overflow to the high bit of a byte if + // any of the low bits were initially set. Next the high + // bits of the original word are ORed with these values; + // thus, the high bit of a byte is set iff any bit in the + // byte was set. Finally, we determine if any of these high + // bits are zero by ORing with ones everywhere except the + // high bits and inverting the result." + return ^((((x & c) + c) | x) | c) + } + // Transform x to contain a 1 bit at the top of each m-aligned + // group of m zero bits. + switch m { + case 1: + return x + case 2: + x = apply(x, 0x5555555555555555) + case 4: + x = apply(x, 0x7777777777777777) + case 8: + x = apply(x, 0x7f7f7f7f7f7f7f7f) + case 16: + x = apply(x, 0x7fff7fff7fff7fff) + case 32: + x = apply(x, 0x7fffffff7fffffff) + case 64: // == maxPagesPerPhysPage + x = apply(x, 0x7fffffffffffffff) + default: + throw("bad m value") + } + // Now, the top bit of each m-aligned group in x is set + // that group was all zero in the original x. + + // From each group of m bits subtract 1. + // Because we know only the top bits of each + // m-aligned group are set, we know this will + // set each group to have all the bits set except + // the top bit, so just OR with the original + // result to set all the bits. + return ^((x - (x >> (m - 1))) | x) +} + +// hasScavengeCandidate returns true if there's any min-page-aligned groups of +// min pages of free-and-unscavenged memory in the region represented by this +// pallocData. +// +// min must be a non-zero power of 2 <= maxPagesPerPhysPage. +func (m *pallocData) hasScavengeCandidate(min uintptr) bool { + if min&(min-1) != 0 || min == 0 { + print("runtime: min = ", min, "\n") + throw("min must be a non-zero power of 2") + } else if min > maxPagesPerPhysPage { + print("runtime: min = ", min, "\n") + throw("min too large") + } + + // The goal of this search is to see if the chunk contains any free and unscavenged memory. + for i := len(m.scavenged) - 1; i >= 0; i-- { + // 1s are scavenged OR non-free => 0s are unscavenged AND free + // + // TODO(mknyszek): Consider splitting up fillAligned into two + // functions, since here we technically could get by with just + // the first half of its computation. It'll save a few instructions + // but adds some additional code complexity. + x := fillAligned(m.scavenged[i]|m.pallocBits[i], uint(min)) + + // Quickly skip over chunks of non-free or scavenged pages. + if x != ^uint64(0) { + return true + } + } + return false +} + +// findScavengeCandidate returns a start index and a size for this pallocData +// segment which represents a contiguous region of free and unscavenged memory. +// +// searchIdx indicates the page index within this chunk to start the search, but +// note that findScavengeCandidate searches backwards through the pallocData. As a +// a result, it will return the highest scavenge candidate in address order. +// +// min indicates a hard minimum size and alignment for runs of pages. That is, +// findScavengeCandidate will not return a region smaller than min pages in size, +// or that is min pages or greater in size but not aligned to min. min must be +// a non-zero power of 2 <= maxPagesPerPhysPage. +// +// max is a hint for how big of a region is desired. If max >= pallocChunkPages, then +// findScavengeCandidate effectively returns entire free and unscavenged regions. +// If max < pallocChunkPages, it may truncate the returned region such that size is +// max. However, findScavengeCandidate may still return a larger region if, for +// example, it chooses to preserve huge pages, or if max is not aligned to min (it +// will round up). That is, even if max is small, the returned size is not guaranteed +// to be equal to max. max is allowed to be less than min, in which case it is as if +// max == min. +func (m *pallocData) findScavengeCandidate(searchIdx uint, min, max uintptr) (uint, uint) { + if min&(min-1) != 0 || min == 0 { + print("runtime: min = ", min, "\n") + throw("min must be a non-zero power of 2") + } else if min > maxPagesPerPhysPage { + print("runtime: min = ", min, "\n") + throw("min too large") + } + // max may not be min-aligned, so we might accidentally truncate to + // a max value which causes us to return a non-min-aligned value. + // To prevent this, align max up to a multiple of min (which is always + // a power of 2). This also prevents max from ever being less than + // min, unless it's zero, so handle that explicitly. + if max == 0 { + max = min + } else { + max = alignUp(max, min) + } + + i := int(searchIdx / 64) + // Start by quickly skipping over blocks of non-free or scavenged pages. + for ; i >= 0; i-- { + // 1s are scavenged OR non-free => 0s are unscavenged AND free + x := fillAligned(m.scavenged[i]|m.pallocBits[i], uint(min)) + if x != ^uint64(0) { + break + } + } + if i < 0 { + // Failed to find any free/unscavenged pages. + return 0, 0 + } + // We have something in the 64-bit chunk at i, but it could + // extend further. Loop until we find the extent of it. + + // 1s are scavenged OR non-free => 0s are unscavenged AND free + x := fillAligned(m.scavenged[i]|m.pallocBits[i], uint(min)) + z1 := uint(sys.LeadingZeros64(^x)) + run, end := uint(0), uint(i)*64+(64-z1) + if x<<z1 != 0 { + // After shifting out z1 bits, we still have 1s, + // so the run ends inside this word. + run = uint(sys.LeadingZeros64(x << z1)) + } else { + // After shifting out z1 bits, we have no more 1s. + // This means the run extends to the bottom of the + // word so it may extend into further words. + run = 64 - z1 + for j := i - 1; j >= 0; j-- { + x := fillAligned(m.scavenged[j]|m.pallocBits[j], uint(min)) + run += uint(sys.LeadingZeros64(x)) + if x != 0 { + // The run stopped in this word. + break + } + } + } + + // Split the run we found if it's larger than max but hold on to + // our original length, since we may need it later. + size := run + if size > uint(max) { + size = uint(max) + } + start := end - size + + // Each huge page is guaranteed to fit in a single palloc chunk. + // + // TODO(mknyszek): Support larger huge page sizes. + // TODO(mknyszek): Consider taking pages-per-huge-page as a parameter + // so we can write tests for this. + if physHugePageSize > pageSize && physHugePageSize > physPageSize { + // We have huge pages, so let's ensure we don't break one by scavenging + // over a huge page boundary. If the range [start, start+size) overlaps with + // a free-and-unscavenged huge page, we want to grow the region we scavenge + // to include that huge page. + + // Compute the huge page boundary above our candidate. + pagesPerHugePage := uintptr(physHugePageSize / pageSize) + hugePageAbove := uint(alignUp(uintptr(start), pagesPerHugePage)) + + // If that boundary is within our current candidate, then we may be breaking + // a huge page. + if hugePageAbove <= end { + // Compute the huge page boundary below our candidate. + hugePageBelow := uint(alignDown(uintptr(start), pagesPerHugePage)) + + if hugePageBelow >= end-run { + // We're in danger of breaking apart a huge page since start+size crosses + // a huge page boundary and rounding down start to the nearest huge + // page boundary is included in the full run we found. Include the entire + // huge page in the bound by rounding down to the huge page size. + size = size + (start - hugePageBelow) + start = hugePageBelow + } + } + } + return start, size +} |