runtime: add background scavenger

This change adds a background scavenging goroutine whose pacing is
determined when the heap goal changes. The scavenger is paced to use
at most 1% of the mutator's time for most systems. Furthermore, the
scavenger's pacing is computed based on the estimated number of
scavengable huge pages to take advantage of optimizations provided by
the OS.

The purpose of this scavenger is to deal with a shrinking heap: if the
heap goal is falling over time, the scavenger should kick in and start
returning free pages from the heap to the OS.

Also, now that we have a pacing system, the credit system used by
scavengeLocked has become redundant. Replace it with a mechanism which
only scavenges on the allocation path if it makes sense to do so with
respect to the new pacing system.

Fixes #30333.

Change-Id: I6203f8dc84affb26c3ab04528889dd9663530edc
Reviewed-on: https://go-review.googlesource.com/c/go/+/142960
Run-TryBot: Michael Knyszek <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Austin Clements <[email protected]>

Author: mknyszek

src/runtime/mgc.go

@@ -202,10 +202,14 @@ func readgogc() int32 {
 
 // gcenable is called after the bulk of the runtime initialization,
 // just before we're about to start letting user code run.
-// It kicks off the background sweeper goroutine and enables GC.
+// It kicks off the background sweeper goroutine, the background
+// scavenger goroutine, and enables GC.
 func gcenable() {
-	c := make(chan int, 1)
+	// Kick off sweeping and scavenging.
+	c := make(chan int, 2)
 	go bgsweep(c)
+	go bgscavenge(c)
+	<-c
 	<-c
 	memstats.enablegc = true // now that runtime is initialized, GC is okay
 }
@@ -850,6 +854,8 @@ func gcSetTriggerRatio(triggerRatio float64) {
 			atomic.Store64(&mheap_.pagesSweptBasis, pagesSwept)
 		}
 	}
+
+	gcPaceScavenger()
 }
 
 // gcEffectiveGrowthRatio returns the current effective heap growth

src/runtime/mgcscavenge.go

@@ -0,0 +1,365 @@
+// 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) {
+	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()
+	}
+}