libgo/go/runtime/mgcscavenge.go - gofrontend - Git at Google

 // 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.
 //
 // 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

 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 accomodate 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() {
 	// Compute our scavenging goal and align it to a physical page boundary
 	// to make the following calculations more exact.
 	retainedGoal := memstats.next_gc
 	// 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))
 	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, publish the goal in case it changed
 	// then disable the background scavenger.
 	if retainedNow <= retainedGoal {
 		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 * physHugePageSize)

 		// ...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 / uint64(physHugePageSize)
 			hugeWork = hugePages * uint64(physHugePageSize)
 		}
 		// 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 / uint64(physHugePageSize) * 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()

 	lock(&scavenge.lock)

 	// 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)
 	scavenge.gen++ // increase scavenge generation

 	// Wake up background scavenger if needed, since the pacing was just updated.
 	wakeScavengerLocked()

 	unlock(&scavenge.lock)
 }

 // State of the background scavenger.
 var scavenge struct {
 	lock   mutex
 	g      *g
 	parked bool
 	timer  *timer
 	gen    uint32 // read with either lock or mheap_.lock, write with both
 }

 // wakeScavengerLocked unparks the scavenger if necessary. It must be called
 // after any pacing update.
 //
 // scavenge.lock must be held.
 func wakeScavengerLocked() {
 	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
 		ready(scavenge.g, 0, true)
 	}
 }

 // scavengeSleep attempts to put the scavenger to sleep for ns.
 // It also checks to see if gen != scavenge.gen before going to sleep,
 // and aborts if true (meaning an update had occurred).
 //
 // 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(gen uint32, ns int64) bool {
 	lock(&scavenge.lock)

 	// If there was an update, just abort the sleep.
 	if scavenge.gen != gen {
 		unlock(&scavenge.lock)
 		return false
 	}

 	// Set the timer.
 	now := nanotime()
 	scavenge.timer.when = now + ns
 	startTimer(scavenge.timer)

 	// Park the goroutine. It's fine that we don't publish the
 	// fact that the timer was set; even if the timer wakes up
 	// and fire scavengeReady before we park, it'll block on
 	// scavenge.lock.
 	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) {
 	setSystemGoroutine()

 	scavenge.g = getg()

 	lock(&scavenge.lock)
 	scavenge.parked = true

 	scavenge.timer = new(timer)
 	scavenge.timer.f = func(_ interface{}, _ uintptr) {
 		lock(&scavenge.lock)
 		wakeScavengerLocked()
 		unlock(&scavenge.lock)
 	}

 	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)
 		gen := uint32(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)

 			gen = scavenge.gen

 			// 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(gen, 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(gen, ttnext)
 			continue
 		}

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

	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 accomodate 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() {
	// Compute our scavenging goal and align it to a physical page boundary
	// to make the following calculations more exact.
	retainedGoal := memstats.next_gc
	// 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))
	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, publish the goal in case it changed
	// then disable the background scavenger.
	if retainedNow <= retainedGoal {
	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 * physHugePageSize)

	// ...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 / uint64(physHugePageSize)
	hugeWork = hugePages * uint64(physHugePageSize)
	}
	// 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 / uint64(physHugePageSize) * 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()

	lock(&scavenge.lock)

	// 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)
	scavenge.gen++ // increase scavenge generation

	// Wake up background scavenger if needed, since the pacing was just updated.
	wakeScavengerLocked()

	unlock(&scavenge.lock)
	}

	// State of the background scavenger.
	var scavenge struct {
	lock mutex
	g *g
	parked bool
	timer *timer
	gen uint32 // read with either lock or mheap_.lock, write with both
	}

	// wakeScavengerLocked unparks the scavenger if necessary. It must be called
	// after any pacing update.
	//
	// scavenge.lock must be held.
	func wakeScavengerLocked() {
	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
	ready(scavenge.g, 0, true)
	}
	}

	// scavengeSleep attempts to put the scavenger to sleep for ns.
	// It also checks to see if gen != scavenge.gen before going to sleep,
	// and aborts if true (meaning an update had occurred).
	//
	// 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(gen uint32, ns int64) bool {
	lock(&scavenge.lock)

	// If there was an update, just abort the sleep.
	if scavenge.gen != gen {
	unlock(&scavenge.lock)
	return false
	}

	// Set the timer.
	now := nanotime()
	scavenge.timer.when = now + ns
	startTimer(scavenge.timer)

	// Park the goroutine. It's fine that we don't publish the
	// fact that the timer was set; even if the timer wakes up
	// and fire scavengeReady before we park, it'll block on
	// scavenge.lock.
	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) {
	setSystemGoroutine()

	scavenge.g = getg()

	lock(&scavenge.lock)
	scavenge.parked = true

	scavenge.timer = new(timer)
	scavenge.timer.f = func(_ interface{}, _ uintptr) {
	lock(&scavenge.lock)
	wakeScavengerLocked()
	unlock(&scavenge.lock)
	}

	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)
	gen := uint32(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)

	gen = scavenge.gen

	// 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(gen, 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(gen, ttnext)
	continue
	}

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