go / gofrontend / 9782e85bef1c16c72a4980856d921cea104b129c / . / libgo / go / runtime / mgcscavenge.go

// 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) { | |

setSystemGoroutine() | |

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 Âµs 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 | |

} |