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// 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 (
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
// 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 atomic.Load64(&memstats.heap_sys) - 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)
// Compute our scavenging goal.
goalRatio := float64(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)
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() {
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.
// 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
// 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 {
// 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)
scavenge.parked = true
scavenge.timer = new(timer)
scavenge.timer.f = func(_ interface{}, _ uintptr) {
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() {
// If background scavenging is disabled or if there's no work to do just park.
retained, goal := heapRetained(), mheap_.scavengeGoal
if retained <= goal {
// 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)
if released == 0 {
scavenge.parked = true
goparkunlock(&scavenge.lock, waitReasonGCScavengeWait, traceEvGoBlock, 1)
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 ┬Ás 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.
// s.mheapLock must be held, but may be temporarily released if
// mayUnlock == true.
// Must run on the system stack because s.mheapLock must be held.
func (s *pageAlloc) scavenge(nbytes uintptr, mayUnlock bool) uintptr {
var (
addrs addrRange
gen uint32
released := uintptr(0)
for released < nbytes {
if addrs.size() == 0 {
if addrs, gen = s.scavengeReserve(); addrs.size() == 0 {
r, a := s.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.
s.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) {
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)")
// scavengeStartGen starts a new scavenge generation, resetting
// the scavenger's search space to the full in-use address space.
// s.mheapLock must be held.
// Must run on the system stack because s.mheapLock must be held.
func (s *pageAlloc) scavengeStartGen() {
if debug.scavtrace > 0 {
printScavTrace(s.scav.gen, s.scav.released, false)
// Pick the new starting address for the scavenger cycle.
var startAddr offAddr
if s.scav.scavLWM.lessThan(s.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 = s.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 = s.scav.scavLWM
// reservationBytes may be zero if s.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.
s.scav.reservationBytes = alignUp(s.inUse.totalBytes, pallocChunkBytes) / scavengeReservationShards
s.scav.released = 0
s.scav.freeHWM = minOffAddr
s.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.
// s.mheapLock must be held.
// Must run on the system stack because s.mheapLock must be held.
func (s *pageAlloc) scavengeReserve() (addrRange, uint32) {
// Start by reserving the minimum.
r := s.scav.inUse.removeLast(s.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, s.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.
r.base = offAddr{newBase}
return r, s.scav.gen
// scavengeUnreserve returns an unscavenged portion of a range that was
// previously reserved with scavengeReserve.
// s.mheapLock must be held.
// Must run on the system stack because s.mheapLock must be held.
func (s *pageAlloc) scavengeUnreserve(r addrRange, gen uint32) {
if r.size() == 0 || gen != s.scav.gen {
if r.base.addr()%pallocChunkBytes != 0 {
throw("unreserving unaligned region")
// 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.
// s.mheapLock must be held, but may be temporarily released if
// mayUnlock == true.
// Must run on the system stack because s.mheapLock must be held.
func (s *pageAlloc) scavengeOne(work addrRange, max uintptr, mayUnlock bool) (uintptr, addrRange) {
// Defensively check if we've recieved 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 {
// 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 {
unlockHeap := func() {
if mayUnlock {
// 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 s.summary[len(s.summary)-1][maxChunk].max() >= uint(minPages) {
// We only bother looking for a candidate if there at least
// minPages free pages at all.
base, npages := s.chunkOf(maxChunk).findScavengeCandidate(chunkPageIndex(maxAddr), minPages, maxPages)
// If we found something, scavenge it and return!
if npages != 0 {
work.limit = offAddr{s.scavengeRangeLocked(maxChunk, base, npages)}
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 s.summary[len(s.summary)-1][i].max() < uint(minPages) {
// 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(&s.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 {
// 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.
if !ok {
// We didn't find a candidate, so we're done.
work.limit = work.base
// Find, verify, and scavenge if we can.
chunk := s.chunkOf(candidateChunkIdx)
base, npages := chunk.findScavengeCandidate(pallocChunkPages-1, minPages, maxPages)
if npages > 0 {
work.limit = offAddr{s.scavengeRangeLocked(candidateChunkIdx, base, npages)}
return uintptr(npages) * pageSize, work
// We were fooled, so let's continue from where we left off.
work.limit = offAddr{chunkBase(candidateChunkIdx)}
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.
// s.mheapLock must be held.
func (s *pageAlloc) scavengeRangeLocked(ci chunkIdx, base, npages uint) uintptr {
s.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(s.scav.scavLWM) {
s.scav.scavLWM = oAddr
// Only perform the actual scavenging if we're not in a test.
// It's dangerous to do so otherwise.
if s.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.
mSysStatInc(&memstats.heap_released, uintptr(npages)*pageSize)
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
// 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)
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) {
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.
// 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