src/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 (
	"math/bits"
	"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.
	//
	// scavengePageLatency is a worst-case estimate (order-of-magnitude) of
	// the time it takes to scavenge one (regular-sized) page of memory.
	// scavengeHugePageLatency is the same but for huge pages.
	//
	// scavengePagePeriod is derived from scavengePercent and scavengePageLatency,
	// and represents the average time between scavenging one page that we're
	// aiming for. scavengeHugePagePeriod is the same but for huge pages.
	// These constants are core to the scavenge pacing algorithm.
	scavengePercent         = 1    // 1%
	scavengePageLatency     = 10e3 // 10µs
	scavengeHugePageLatency = 10e3 // 10µs
	scavengePagePeriod      = scavengePageLatency / (scavengePercent / 100.0)
	scavengeHugePagePeriod  = scavengePageLatency / (scavengePercent / 100.0)

	// 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
)

// heapRetained returns an estimate of the current heap RSS.
//
// mheap_.lock must be held or the world must be stopped.
func heapRetained() uint64 {
	return memstats.heap_sys - 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_.scavengeBytesPerNS = 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 there's less the one physical page
	// worth of work to do, publish the goal in case it changed then disable
	// the background scavenger. We disable the background scavenger if there's
	// less than one physical page of work to do to avoid a potential divide-by-zero
	// in the calculations below (totalTime will be zero), and it's not worth
	// turning on the scavenger for less than one page of work.
	if retainedNow <= retainedGoal || retainedNow-retainedGoal < uint64(physPageSize) {
		mheap_.scavengeRetainedGoal = retainedGoal
		mheap_.scavengeBytesPerNS = 0
		return
	}

	// Now we start to compute the total amount of work necessary and the total
	// amount of time we're willing to give the scavenger to complete this work.
	// This will involve calculating how much of the work consists of huge pages
	// and how much consists of regular pages since the former can let us scavenge
	// more memory in the same time.
	totalWork := retainedNow - retainedGoal

	// On systems without huge page support, all work is regular work.
	regularWork := totalWork
	hugeTime := uint64(0)

	// On systems where we have huge pages, we want to do as much of the
	// scavenging work as possible on huge pages, because the costs are the
	// same per page, but we can give back more more memory in a shorter
	// period of time.
	if physHugePageSize != 0 {
		// Start by computing the amount of free memory we have in huge pages
		// in total. Trivially, this is all the huge page work we need to do.
		hugeWork := uint64(mheap_.free.unscavHugePages) << physHugePageShift

		// ...but it could turn out that there's more huge work to do than
		// total work, so cap it at total work. This might happen for very large
		// heaps where the additional factor of retainExtraPercent can make it so
		// that there are free chunks of memory larger than a huge page that we don't want
		// to scavenge.
		if hugeWork >= totalWork {
			hugePages := totalWork >> physHugePageShift
			hugeWork = hugePages << physHugePageShift
		}
		// Everything that's not huge work is regular work. At this point we
		// know huge work so we can calculate how much time that will take
		// based on scavengePageRate (which applies to pages of any size).
		regularWork = totalWork - hugeWork
		hugeTime = (hugeWork >> physHugePageShift) * scavengeHugePagePeriod
	}
	// Finally, we can compute how much time it'll take to do the regular work
	// and the total time to do all the work.
	regularTime := regularWork / uint64(physPageSize) * scavengePagePeriod
	totalTime := hugeTime + regularTime

	now := nanotime()

	// Update all the pacing parameters in mheap with scavenge.lock held,
	// so that scavenge.gen is kept in sync with the updated values.
	mheap_.scavengeRetainedGoal = retainedGoal
	mheap_.scavengeRetainedBasis = retainedNow
	mheap_.scavengeTimeBasis = now
	mheap_.scavengeBytesPerNS = float64(totalWork) / float64(totalTime)
	mheap_.scavengeGen++ // increase scavenge generation
}

// Sleep/wait state of the background scavenger.
var scavenge struct {
	lock   mutex
	g      *g
	parked bool
	timer  *timer

	// Generation counter.
	//
	// It represents the last generation count (as defined by
	// mheap_.scavengeGen) checked by the scavenger and is updated
	// each time the scavenger checks whether it is on-pace.
	//
	// Skew between this field and mheap_.scavengeGen is used to
	// determine whether a new update is available.
	//
	// Protected by mheap_.lock.
	gen uint64
}

// wakeScavenger unparks the scavenger if necessary. It must be called
// after any pacing update.
//
// mheap_.lock and scavenge.lock must not be held.
func wakeScavenger() {
	lock(&scavenge.lock)
	if scavenge.parked {
		// 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.
		scavenge.parked = false
		goready(scavenge.g, 0)
	}
	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 false if awoken early (i.e. true means a complete sleep).
func scavengeSleep(ns int64) bool {
	lock(&scavenge.lock)

	// First check if there's a pending update.
	// If there is one, don't bother sleeping.
	var hasUpdate bool
	systemstack(func() {
		lock(&mheap_.lock)
		hasUpdate = mheap_.scavengeGen != scavenge.gen
		unlock(&mheap_.lock)
	})
	if hasUpdate {
		unlock(&scavenge.lock)
		return false
	}

	// Set the timer.
	//
	// This must happen here instead of inside gopark
	// because we can't close over any variables without
	// failing escape analysis.
	now := nanotime()
	resetTimer(scavenge.timer, now+ns)

	// Mark ourself as asleep and go to sleep.
	scavenge.parked = true
	goparkunlock(&scavenge.lock, waitReasonSleep, traceEvGoSleep, 2)

	// Return true if we completed the full sleep.
	return (nanotime() - now) >= ns
}

// 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()

	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)

	// Parameters for sleeping.
	//
	// If we end up doing more work than we need, we should avoid spinning
	// until we have more work to do: instead, we know exactly how much time
	// until more work will need to be done, so we sleep.
	//
	// We should avoid sleeping for less than minSleepNS because Gosched()
	// overheads among other things will work out better in that case.
	//
	// There's no reason to set a maximum on sleep time because we'll always
	// get woken up earlier if there's any kind of update that could change
	// the scavenger's pacing.
	//
	// retryDelayNS tracks how much to sleep next time we fail to do any
	// useful work.
	const minSleepNS = int64(100 * 1000) // 100 µs

	retryDelayNS := minSleepNS

	for {
		released := uintptr(0)
		park := false
		ttnext := int64(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)

			// Update the last generation count that the scavenger has handled.
			scavenge.gen = mheap_.scavengeGen

			// If background scavenging is disabled or if there's no work to do just park.
			retained := heapRetained()
			if mheap_.scavengeBytesPerNS == 0 || retained <= mheap_.scavengeRetainedGoal {
				unlock(&mheap_.lock)
				park = true
				return
			}

			// Calculate how big we want the retained heap to be
			// at this point in time.
			//
			// The formula is for that of a line, y = b - mx
			// We want y (want),
			//   m = scavengeBytesPerNS (> 0)
			//   x = time between scavengeTimeBasis and now
			//   b = scavengeRetainedBasis
			rate := mheap_.scavengeBytesPerNS
			tdist := nanotime() - mheap_.scavengeTimeBasis
			rdist := uint64(rate * float64(tdist))
			want := mheap_.scavengeRetainedBasis - rdist

			// If we're above the line, scavenge to get below the
			// line.
			if retained > want {
				released = mheap_.scavengeLocked(uintptr(retained - want))
			}
			unlock(&mheap_.lock)

			// If we over-scavenged a bit, calculate how much time it'll
			// take at the current rate for us to make that up. We definitely
			// won't have any work to do until at least that amount of time
			// passes.
			if released > uintptr(retained-want) {
				extra := released - uintptr(retained-want)
				ttnext = int64(float64(extra) / rate)
			}
		})

		if park {
			lock(&scavenge.lock)
			scavenge.parked = true
			goparkunlock(&scavenge.lock, waitReasonGCScavengeWait, traceEvGoBlock, 1)
			continue
		}

		if debug.gctrace > 0 {
			if released > 0 {
				print("scvg: ", released>>20, " MB released\n")
			}
			print("scvg: inuse: ", memstats.heap_inuse>>20, ", idle: ", memstats.heap_idle>>20, ", sys: ", memstats.heap_sys>>20, ", released: ", memstats.heap_released>>20, ", consumed: ", (memstats.heap_sys-memstats.heap_released)>>20, " (MB)\n")
		}

		if released == 0 {
			// If we were unable to release anything this may be because there's
			// no free memory available to scavenge. Go to sleep and try again.
			if scavengeSleep(retryDelayNS) {
				// If we successfully slept through the delay, back off exponentially.
				retryDelayNS *= 2
			}
			continue
		}
		retryDelayNS = minSleepNS

		if ttnext > 0 && ttnext > minSleepNS {
			// If there's an appreciable amount of time until the next scavenging
			// goal, just sleep. We'll get woken up if anything changes and this
			// way we avoid spinning.
			scavengeSleep(ttnext)
			continue
		}

		// Give something else a chance to run, no locks are held.
		Gosched()
	}
}

// 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 resetScavengeAddr to bring it
// back to the top of the heap.
//
// Returns the amount of memory scavenged in bytes.
//
// If locked == false, s.mheapLock must not be locked. If locked == true,
// s.mheapLock must be locked.
//
// Must run on the system stack because scavengeOne must run on the
// system stack.
//
//go:systemstack
func (s *pageAlloc) scavenge(nbytes uintptr, locked bool) uintptr {
	released := uintptr(0)
	for released < nbytes {
		r := s.scavengeOne(nbytes-released, locked)
		if r == 0 {
			// Nothing left to scavenge! Give up.
			break
		}
		released += r
	}
	return released
}

// resetScavengeAddr sets the scavenge start address to the top of the heap's
// address space. This should be called each time the scavenger's pacing
// changes.
//
// s.mheapLock must be held.
func (s *pageAlloc) resetScavengeAddr() {
	s.scavAddr = chunkBase(s.end) - 1
}

// scavengeOne starts from s.scavAddr and walks down the heap 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 and updates s.scavAddr
// appropriately. s.scavAddr must be reset manually and externally.
//
// Should it exhaust the heap, it will return 0 and set s.scavAddr to minScavAddr.
//
// If locked == false, s.mheapLock must not be locked.
// If locked == true, s.mheapLock must be locked.
//
// Must be run on the system stack because it either acquires the heap lock
// or executes with the heap lock acquired.
//
//go:systemstack
func (s *pageAlloc) scavengeOne(max uintptr, locked bool) uintptr {
	// 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 locked == false.
	lockHeap := func() {
		if !locked {
			lock(s.mheapLock)
		}
	}
	unlockHeap := func() {
		if !locked {
			unlock(s.mheapLock)
		}
	}

	lockHeap()
	top := chunkIndex(s.scavAddr)
	if top < s.start {
		unlockHeap()
		return 0
	}

	// Check the chunk containing the scav addr, starting at the addr
	// and see if there are any free and unscavenged pages.
	ci := chunkIndex(s.scavAddr)
	base, npages := s.chunks[ci].findScavengeCandidate(chunkPageIndex(s.scavAddr), minPages, maxPages)

	// If we found something, scavenge it and return!
	if npages != 0 {
		s.scavengeRangeLocked(ci, base, npages)
		unlockHeap()
		return uintptr(npages) * pageSize
	}
	unlockHeap()

	// Slow path: iterate optimistically looking for any free and unscavenged page.
	// If we think we see something, stop and verify it!
	for i := top - 1; i >= s.start; 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 if there are enough free pages at all. It's imperative that we
		// check this before the chunk itself so that we quickly skip over
		// unused parts of the address space, which may have a cleared bitmap
		// but a zero'd summary which indicates not to allocate from there.
		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.
		if !s.chunks[i].hasScavengeCandidate(minPages) {
			continue
		}

		// We found a candidate, so let's lock and verify it.
		lockHeap()

		// Find, verify, and scavenge if we can.
		chunk := &s.chunks[i]
		base, npages := chunk.findScavengeCandidate(pallocChunkPages-1, minPages, maxPages)
		if npages > 0 {
			// We found memory to scavenge! Mark the bits and report that up.
			s.scavengeRangeLocked(i, base, npages)
			unlockHeap()
			return uintptr(npages) * pageSize
		}

		// We were fooled, let's take this opportunity to move the scavAddr
		// all the way down to where we searched as scavenged for future calls
		// and keep iterating.
		s.scavAddr = chunkBase(i-1) + pallocChunkPages*pageSize - 1
		unlockHeap()
	}

	lockHeap()
	// We couldn't find anything, so signal that there's nothing left
	// to scavenge.
	s.scavAddr = minScavAddr
	unlockHeap()

	return 0
}

// scavengeRangeLocked scavenges the given region of memory.
//
// s.mheapLock must be held.
func (s *pageAlloc) scavengeRangeLocked(ci chunkIdx, base, npages uint) {
	s.chunks[ci].scavenged.setRange(base, npages)

	// Compute the full address for the start of the range.
	addr := chunkBase(ci) + uintptr(base)*pageSize

	// Update the scav pointer.
	s.scavAddr = addr - 1

	// Only perform the actual scavenging if we're not in a test.
	// It's dangerous to do so otherwise.
	if s.test {
		return
	}
	sysUnused(unsafe.Pointer(addr), uintptr(npages)*pageSize)

	// Update global accounting only when not in test, otherwise
	// the runtime's accounting will be wrong.
	memstats.heap_released += uint64(npages) * pageSize
}

// 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 <= 64.
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 initialy 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:
		x = apply(x, 0x7fffffffffffffff)
	}
	// 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 <= 64.
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 > 64 {
		print("runtime: min = ", min, "\n")
		throw("physical page sizes > 512 KiB are not supported")
	}

	// 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 <= 64.
//
// 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. That is, even if max is small,
// 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 > 64 {
		print("runtime: min = ", min, "\n")
		throw("physical page sizes > 512 KiB are not supported")
	}
	// max is allowed to be less than min, but we need to ensure
	// we never truncate further than min.
	if max < min {
		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(bits.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(bits.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(bits.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

	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
}