forwprop: Also dce from added statements from gimple_simplify

[official-gcc.git] / libgo / go / runtime / mgcscavenge.go

// Copyright 2019 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Scavenging free pages.
//
// This file implements scavenging (the release of physical pages backing mapped
// memory) of free and unused pages in the heap as a way to deal with page-level
// fragmentation and reduce the RSS of Go applications.
//
// Scavenging in Go happens on two fronts: there's the background
// (asynchronous) scavenger and the heap-growth (synchronous) scavenger.
//
// The former happens on a goroutine much like the background sweeper which is
// soft-capped at using scavengePercent of the mutator's time, based on
// order-of-magnitude estimates of the costs of scavenging. The background
// scavenger's primary goal is to bring the estimated heap RSS of the
// application down to a goal.
//
// That goal is defined as:
//   (retainExtraPercent+100) / 100 * (heapGoal / lastHeapGoal) * 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. heapGoal / lastHeapGoal 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 (
        "internal/goos"
        "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 * (goos.IsDarwin + goos.IsIos)

        // scavengeReservationShards determines the amount of memory the scavenger
        // should reserve for scavenging at a time. Specifically, the amount of
        // memory reserved is (heap size in bytes) / scavengeReservationShards.
        scavengeReservationShards = 64
)

// heapRetained returns an estimate of the current heap RSS.
func heapRetained() uint64 {
        return memstats.heap_sys.load() - atomic.Load64(&memstats.heap_released)
}

// gcPaceScavenger updates the scavenger's pacing, particularly
// its rate and RSS goal. For this, it requires the current heapGoal,
// and the heapGoal for the previous GC cycle.
//
// 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.
//
// Must be called whenever GC pacing is updated.
//
// mheap_.lock must be held or the world must be stopped.
func gcPaceScavenger(heapGoal, lastHeapGoal uint64) {
        assertWorldStoppedOrLockHeld(&mheap_.lock)

        // 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 lastHeapGoal == 0 {
                atomic.Store64(&mheap_.scavengeGoal, ^uint64(0))
                return
        }
        // Compute our scavenging goal.
        goalRatio := float64(heapGoal) / float64(lastHeapGoal)
        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) {
                atomic.Store64(&mheap_.scavengeGoal, ^uint64(0))
                return
        }
        atomic.Store64(&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.
        printControllerReset bool   // Whether the scavenger is in cooldown.
}

// 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 desirable 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(_ any, _ uintptr) {
                wakeScavenger()
        }

        c <- 1
        goparkunlock(&scavenge.lock, waitReasonGCScavengeWait, traceEvGoBlock, 1)

        // idealFraction is the ideal % of overall application CPU time that we
        // spend scavenging.
        idealFraction := float64(scavengePercent) / 100.0

        // Input: fraction of CPU time used.
        // Setpoint: idealFraction.
        // Output: ratio of critical time to sleep time (determines sleep time).
        //
        // The output of this controller is somewhat indirect to what we actually
        // want to achieve: how much time to sleep for. The reason for this definition
        // is to ensure that the controller's outputs have a direct relationship with
        // its inputs (as opposed to an inverse relationship), making it somewhat
        // easier to reason about for tuning purposes.
        critSleepController := piController{
                // Tuned loosely via Ziegler-Nichols process.
                kp: 0.3375,
                ti: 3.2e6,
                tt: 1e9, // 1 second reset time.

                // These ranges seem wide, but we want to give the controller plenty of
                // room to hunt for the optimal value.
                min: 0.001,  // 1:1000
                max: 1000.0, // 1000:1
        }
        // It doesn't really matter what value we start at, but we can't be zero, because
        // that'll cause divide-by-zero issues. Pick something conservative which we'll
        // also use as a fallback.
        const startingCritSleepRatio = 0.001
        critSleepRatio := startingCritSleepRatio
        // Duration left in nanoseconds during which we avoid using the controller and
        // we hold critSleepRatio at a conservative value. Used if the controller's
        // assumptions fail to hold.
        controllerCooldown := int64(0)
        for {
                released := uintptr(0)
                crit := float64(0)

                // Spend at least 1 ms scavenging, otherwise the corresponding
                // sleep time to maintain our desired utilization is too low to
                // be reliable.
                const minCritTime = 1e6
                for crit < minCritTime {
                        // If background scavenging is disabled or if there's no work to do just park.
                        retained, goal := heapRetained(), atomic.Load64(&mheap_.scavengeGoal)
                        if retained <= goal {
                                break
                        }

                        // scavengeQuantum is the amount of memory we try to scavenge
                        // in one go. A smaller value means the scavenger is more responsive
                        // to the scheduler in case of e.g. preemption. A larger value means
                        // that the overheads of scavenging are better amortized, so better
                        // scavenging throughput.
                        //
                        // The current value is chosen assuming a cost of ~10µs/physical page
                        // (this is somewhat pessimistic), which implies a worst-case latency of
                        // about 160µs for 4 KiB physical pages. The current value is biased
                        // toward latency over throughput.
                        const scavengeQuantum = 64 << 10

                        // Accumulate the amount of time spent scavenging.
                        start := nanotime()
                        r := mheap_.pages.scavenge(scavengeQuantum)
                        atomic.Xadduintptr(&mheap_.pages.scav.released, r)
                        end := nanotime()

                        // On some platforms we may see end >= start if the time it takes to scavenge
                        // memory is less than the minimum granularity of its clock (e.g. Windows) or
                        // due to clock bugs.
                        //
                        // 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.
                        const approxCritNSPerPhysicalPage = 10e3
                        if end <= start {
                                crit += approxCritNSPerPhysicalPage * float64(r/physPageSize)
                        } else {
                                crit += float64(end - start)
                        }
                        released += r

                        // When using fake time just do one loop.
                        if faketime != 0 {
                                break
                        }
                }

                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")
                }

                if crit < minCritTime {
                        // This means there wasn't enough work to actually fill up minCritTime.
                        // That's fine; we shouldn't try to do anything with this information
                        // because it's going result in a short enough sleep request that things
                        // will get messy. Just assume we did at least this much work.
                        // All this means is that we'll sleep longer than we otherwise would have.
                        crit = minCritTime
                }

                // 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

                // Go to sleep based on how much time we spent doing work.
                slept := scavengeSleep(int64(crit / critSleepRatio))

                // Stop here if we're cooling down from the controller.
                if controllerCooldown > 0 {
                        // crit and slept aren't exact measures of time, but it's OK to be a bit
                        // sloppy here. We're just hoping we're avoiding some transient bad behavior.
                        t := slept + int64(crit)
                        if t > controllerCooldown {
                                controllerCooldown = 0
                        } else {
                                controllerCooldown -= t
                        }
                        continue
                }

                // Calculate the CPU time spent.
                //
                // This may be slightly inaccurate with respect to GOMAXPROCS, but we're
                // recomputing this often enough relative to GOMAXPROCS changes in general
                // (it only changes when the world is stopped, and not during a GC) that
                // that small inaccuracy is in the noise.
                cpuFraction := float64(crit) / ((float64(slept) + crit) * float64(gomaxprocs))

                // Update the critSleepRatio, adjusting until we reach our ideal fraction.
                var ok bool
                critSleepRatio, ok = critSleepController.next(cpuFraction, idealFraction, float64(slept)+crit)
                if !ok {
                        // The core assumption of the controller, that we can get a proportional
                        // response, broke down. This may be transient, so temporarily switch to
                        // sleeping a fixed, conservative amount.
                        critSleepRatio = startingCritSleepRatio
                        controllerCooldown = 5e9 // 5 seconds.

                        // Signal the scav trace printer to output this.
                        lock(&scavenge.lock)
                        scavenge.printControllerReset = true
                        unlock(&scavenge.lock)
                }
        }
}

// 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.
func (p *pageAlloc) scavenge(nbytes uintptr) uintptr {
        var (
                addrs addrRange
                gen   uint32
        )
        released := uintptr(0)
        for released < nbytes {
                if addrs.size() == 0 {
                        if addrs, gen = p.scavengeReserve(); addrs.size() == 0 {
                                break
                        }
                }
                systemstack(func() {
                        r, a := p.scavengeOne(addrs, nbytes-released)
                        released += r
                        addrs = a
                })
        }
        // Only unreserve the space which hasn't been scavenged or searched
        // to ensure we always make progress.
        p.scavengeUnreserve(addrs, gen)
        return released
}

// printScavTrace prints a scavenge trace line to standard error.
//
// released should be the amount of memory released since the last time this
// was called, and forced indicates whether the scavenge was forced by the
// application.
//
// scavenge.lock must be held.
func printScavTrace(gen uint32, released uintptr, forced bool) {
        assertLockHeld(&scavenge.lock)

        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)")
        } else if scavenge.printControllerReset {
                print(" [controller reset]")
                scavenge.printControllerReset = false
        }
        println()
        printunlock()
}

// scavengeStartGen starts a new scavenge generation, resetting
// the scavenger's search space to the full in-use address space.
//
// p.mheapLock must be held.
//
// Must run on the system stack because p.mheapLock must be held.
//
//go:systemstack
func (p *pageAlloc) scavengeStartGen() {
        assertLockHeld(p.mheapLock)

        lock(&p.scav.lock)
        if debug.scavtrace > 0 {
                printScavTrace(p.scav.gen, atomic.Loaduintptr(&p.scav.released), false)
        }
        p.inUse.cloneInto(&p.scav.inUse)

        // Pick the new starting address for the scavenger cycle.
        var startAddr offAddr
        if p.scav.scavLWM.lessThan(p.scav.freeHWM) {
                // The "free" high watermark exceeds the "scavenged" low watermark,
                // so there are free scavengable pages in parts of the address space
                // that the scavenger already searched, the high watermark being the
                // highest one. Pick that as our new starting point to ensure we
                // see those pages.
                startAddr = p.scav.freeHWM
        } else {
                // The "free" high watermark does not exceed the "scavenged" low
                // watermark. This means the allocator didn't free any memory in
                // the range we scavenged last cycle, so we might as well continue
                // scavenging from where we were.
                startAddr = p.scav.scavLWM
        }
        p.scav.inUse.removeGreaterEqual(startAddr.addr())

        // reservationBytes may be zero if p.inUse.totalBytes is small, or if
        // scavengeReservationShards is large. This case is fine as the scavenger
        // will simply be turned off, but it does mean that scavengeReservationShards,
        // in concert with pallocChunkBytes, dictates the minimum heap size at which
        // the scavenger triggers. In practice this minimum is generally less than an
        // arena in size, so virtually every heap has the scavenger on.
        p.scav.reservationBytes = alignUp(p.inUse.totalBytes, pallocChunkBytes) / scavengeReservationShards
        p.scav.gen++
        atomic.Storeuintptr(&p.scav.released, 0)
        p.scav.freeHWM = minOffAddr
        p.scav.scavLWM = maxOffAddr
        unlock(&p.scav.lock)
}

// 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.
func (p *pageAlloc) scavengeReserve() (addrRange, uint32) {
        lock(&p.scav.lock)
        gen := p.scav.gen

        // Start by reserving the minimum.
        r := p.scav.inUse.removeLast(p.scav.reservationBytes)

        // Return early if the size is zero; we don't want to use
        // the bogus address below.
        if r.size() == 0 {
                unlock(&p.scav.lock)
                return r, gen
        }

        // The scavenger requires that base be aligned to a
        // palloc chunk because that's the unit of operation for
        // the scavenger, so align down, potentially extending
        // the range.
        newBase := alignDown(r.base.addr(), pallocChunkBytes)

        // Remove from inUse however much extra we just pulled out.
        p.scav.inUse.removeGreaterEqual(newBase)
        unlock(&p.scav.lock)

        r.base = offAddr{newBase}
        return r, gen
}

// scavengeUnreserve returns an unscavenged portion of a range that was
// previously reserved with scavengeReserve.
func (p *pageAlloc) scavengeUnreserve(r addrRange, gen uint32) {
        if r.size() == 0 {
                return
        }
        if r.base.addr()%pallocChunkBytes != 0 {
                throw("unreserving unaligned region")
        }
        lock(&p.scav.lock)
        if gen == p.scav.gen {
                p.scav.inUse.add(r)
        }
        unlock(&p.scav.lock)
}

// 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.
//
// Must run on the systemstack because it acquires p.mheapLock.
//
//go:systemstack
func (p *pageAlloc) scavengeOne(work addrRange, max uintptr) (uintptr, addrRange) {
        // Defensively check if we've received an empty address range.
        // If so, just return.
        if work.size() == 0 {
                // Nothing to do.
                return 0, work
        }
        // Check the prerequisites of work.
        if work.base.addr()%pallocChunkBytes != 0 {
                throw("scavengeOne called with unaligned work region")
        }
        // Calculate the maximum number of pages to scavenge.
        //
        // This should be alignUp(max, pageSize) / pageSize but max can and will
        // be ^uintptr(0), so we need to be very careful not to overflow here.
        // Rather than use alignUp, calculate the number of pages rounded down
        // first, then add back one if necessary.
        maxPages := max / pageSize
        if max%pageSize != 0 {
                maxPages++
        }

        // Calculate the minimum number of pages we can scavenge.
        //
        // Because we can only scavenge whole physical pages, we must
        // ensure that we scavenge at least minPages each time, aligned
        // to minPages*pageSize.
        minPages := physPageSize / pageSize
        if minPages < 1 {
                minPages = 1
        }

        // Fast path: check the chunk containing the top-most address in work.
        if r, w := p.scavengeOneFast(work, minPages, maxPages); r != 0 {
                return r, w
        } else {
                work = w
        }

        // findCandidate finds the next scavenge candidate in work optimistically.
        //
        // Returns the candidate chunk index and true on success, and false on failure.
        //
        // The heap need not be locked.
        findCandidate := func(work addrRange) (chunkIdx, bool) {
                // Iterate over this work's chunks.
                for i := chunkIndex(work.limit.addr() - 1); i >= chunkIndex(work.base.addr()); i-- {
                        // If this chunk is totally in-use or has no unscavenged pages, don't bother
                        // doing a more sophisticated check.
                        //
                        // Note we're accessing the summary and the chunks without a lock, but
                        // that's fine. We're being optimistic anyway.

                        // Check quickly if there are enough free pages at all.
                        if p.summary[len(p.summary)-1][i].max() < uint(minPages) {
                                continue
                        }

                        // Run over the chunk looking harder for a candidate. Again, we could
                        // race with a lot of different pieces of code, but we're just being
                        // optimistic. Make sure we load the l2 pointer atomically though, to
                        // avoid races with heap growth. It may or may not be possible to also
                        // see a nil pointer in this case if we do race with heap growth, but
                        // just defensively ignore the nils. This operation is optimistic anyway.
                        l2 := (*[1 << pallocChunksL2Bits]pallocData)(atomic.Loadp(unsafe.Pointer(&p.chunks[i.l1()])))
                        if l2 != nil && l2[i.l2()].hasScavengeCandidate(minPages) {
                                return i, true
                        }
                }
                return 0, false
        }

        // Slow path: iterate optimistically over the in-use address space
        // looking for any free and unscavenged page. If we think we see something,
        // lock and verify it!
        for work.size() != 0 {

                // Search for the candidate.
                candidateChunkIdx, ok := findCandidate(work)
                if !ok {
                        // We didn't find a candidate, so we're done.
                        work.limit = work.base
                        break
                }

                // Lock, so we can verify what we found.
                lock(p.mheapLock)

                // Find, verify, and scavenge if we can.
                chunk := p.chunkOf(candidateChunkIdx)
                base, npages := chunk.findScavengeCandidate(pallocChunkPages-1, minPages, maxPages)
                if npages > 0 {
                        work.limit = offAddr{p.scavengeRangeLocked(candidateChunkIdx, base, npages)}
                        unlock(p.mheapLock)
                        return uintptr(npages) * pageSize, work
                }
                unlock(p.mheapLock)

                // We were fooled, so let's continue from where we left off.
                work.limit = offAddr{chunkBase(candidateChunkIdx)}
        }
        return 0, work
}

// scavengeOneFast is the fast path for scavengeOne, which just checks the top
// chunk of work for some pages to scavenge.
//
// Must run on the system stack because it acquires the heap lock.
//
//go:systemstack
func (p *pageAlloc) scavengeOneFast(work addrRange, minPages, maxPages uintptr) (uintptr, addrRange) {
        maxAddr := work.limit.addr() - 1
        maxChunk := chunkIndex(maxAddr)

        lock(p.mheapLock)
        if p.summary[len(p.summary)-1][maxChunk].max() >= uint(minPages) {
                // We only bother looking for a candidate if there at least
                // minPages free pages at all.
                base, npages := p.chunkOf(maxChunk).findScavengeCandidate(chunkPageIndex(maxAddr), minPages, maxPages)

                // If we found something, scavenge it and return!
                if npages != 0 {
                        work.limit = offAddr{p.scavengeRangeLocked(maxChunk, base, npages)}
                        unlock(p.mheapLock)
                        return uintptr(npages) * pageSize, work
                }
        }
        unlock(p.mheapLock)

        // Update the limit to reflect the fact that we checked maxChunk already.
        work.limit = offAddr{chunkBase(maxChunk)}
        return 0, work
}

// scavengeRangeLocked scavenges the given region of memory.
// The region of memory is described by its chunk index (ci),
// the starting page index of the region relative to that
// chunk (base), and the length of the region in pages (npages).
//
// Returns the base address of the scavenged region.
//
// p.mheapLock must be held. Unlocks p.mheapLock but reacquires
// it before returning. Must be run on the systemstack as a result.
//
//go:systemstack
func (p *pageAlloc) scavengeRangeLocked(ci chunkIdx, base, npages uint) uintptr {
        assertLockHeld(p.mheapLock)

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

        // Mark the range we're about to scavenge as allocated, because
        // we don't want any allocating goroutines to grab it while
        // the scavenging is in progress.
        if scav := p.allocRange(addr, uintptr(npages)); scav != 0 {
                throw("double scavenge")
        }

        // With that done, it's safe to unlock.
        unlock(p.mheapLock)

        // Update the scavenge low watermark.
        lock(&p.scav.lock)
        if oAddr := (offAddr{addr}); oAddr.lessThan(p.scav.scavLWM) {
                p.scav.scavLWM = oAddr
        }
        unlock(&p.scav.lock)

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

                // Update global accounting only when not in test, otherwise
                // the runtime's accounting will be wrong.
                nbytes := int64(npages) * pageSize
                atomic.Xadd64(&memstats.heap_released, nbytes)

                // Update consistent accounting too.
                stats := memstats.heapStats.acquire()
                atomic.Xaddint64(&stats.committed, -nbytes)
                atomic.Xaddint64(&stats.released, nbytes)
                memstats.heapStats.release()
        }

        // Relock the heap, because now we need to make these pages
        // available allocation. Free them back to the page allocator.
        lock(p.mheapLock)
        p.free(addr, uintptr(npages), true)

        // Mark the range as scavenged.
        p.chunkOf(ci).scavenged.setRange(base, npages)
        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
}