text: // 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
  
  	// 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)
  		return
  	}
  	// 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)
  		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 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.
  //
  // 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.
  //
  //go:systemstack
  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 {
  				break
  			}
  		}
  		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) {
  	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.
  //
  // s.mheapLock must be held.
  //
  // Must run on the system stack because s.mheapLock must be held.
  //
  //go:systemstack
  func (s *pageAlloc) scavengeStartGen() {
  	if debug.scavtrace > 0 {
  		printScavTrace(s.scav.gen, s.scav.released, false)
  	}
  	s.inUse.cloneInto(&s.scav.inUse)
  
  	// 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
  	}
  	s.scav.inUse.removeGreaterEqual(startAddr.addr())
  
  	// 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.gen++
  	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.
  //
  //go:systemstack
  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.
  	s.scav.inUse.removeGreaterEqual(newBase)
  	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.
  //
  //go:systemstack
  func (s *pageAlloc) scavengeUnreserve(r addrRange, gen uint32) {
  	if r.size() == 0 || gen != s.scav.gen {
  		return
  	}
  	if r.base.addr()%pallocChunkBytes != 0 {
  		throw("unreserving unaligned region")
  	}
  	s.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.
  //
  // 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.
  //
  //go:systemstack
  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 {
  		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(s.mheapLock)
  		}
  	}
  	unlockHeap := func() {
  		if mayUnlock {
  			unlock(s.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 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) {
  				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(&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 {
  		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 := 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
  		// 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
  }
  