runtime: increase GC concurrency.

run GC in its own background goroutine making the
caller runnable if resources are available. This is
critical in single goroutine applications.
Allow goroutines that allocate a lot to help out
the GC and in doing so throttle their own allocation.
Adjust test so that it only detects that a GC is run
during init calls and not whether the GC is memory
efficient. Memory efficiency work will happen later
in 1.5.

Change-Id: I4306f5e377bb47c69bda1aedba66164f12b20c2b
Reviewed-on: https://go-review.googlesource.com/2349
Reviewed-by: Russ Cox <[email protected]>
Reviewed-by: Austin Clements <[email protected]>

Author: RLH

src/runtime/malloc.go

@@ -39,10 +39,27 @@ type pageID uintptr
 // base address for all 0-byte allocations
 var zerobase uintptr
 
+// Determine whether to initiate a GC.
+// Currently the primitive heuristic we use will start a new
+// concurrent GC when approximately half the available space
+// made available by the last GC cycle has been used.
+// If the GC is already working no need to trigger another one.
+// This should establish a feedback loop where if the GC does not
+// have sufficient time to complete then more memory will be
+// requested from the OS increasing heap size thus allow future
+// GCs more time to complete.
+// memstat.heap_alloc and memstat.next_gc reads have benign races
+// A false negative simple does not start a GC, a false positive
+// will start a GC needlessly. Neither have correctness issues.
+func shouldtriggergc() bool {
+	return memstats.heap_alloc+memstats.heap_alloc*3/4 >= memstats.next_gc && atomicloaduint(&bggc.working) == 0
+}
+
 // Allocate an object of size bytes.
 // Small objects are allocated from the per-P cache's free lists.
 // Large objects (> 32 kB) are allocated straight from the heap.
 func mallocgc(size uintptr, typ *_type, flags uint32) unsafe.Pointer {
+	shouldhelpgc := false
 	if size == 0 {
 		return unsafe.Pointer(&zerobase)
 	}
@@ -144,6 +161,7 @@ func mallocgc(size uintptr, typ *_type, flags uint32) unsafe.Pointer {
 				systemstack(func() {
 					mCache_Refill(c, tinySizeClass)
 				})
+				shouldhelpgc = true
 				s = c.alloc[tinySizeClass]
 				v = s.freelist
 			}
@@ -174,6 +192,7 @@ func mallocgc(size uintptr, typ *_type, flags uint32) unsafe.Pointer {
 				systemstack(func() {
 					mCache_Refill(c, int32(sizeclass))
 				})
+				shouldhelpgc = true
 				s = c.alloc[sizeclass]
 				v = s.freelist
 			}
@@ -191,6 +210,7 @@ func mallocgc(size uintptr, typ *_type, flags uint32) unsafe.Pointer {
 		c.local_cachealloc += intptr(size)
 	} else {
 		var s *mspan
+		shouldhelpgc = true
 		systemstack(func() {
 			s = largeAlloc(size, uint32(flags))
 		})
@@ -345,8 +365,15 @@ marked:
 		}
 	}
 
-	if memstats.heap_alloc >= memstats.next_gc/2 {
+	if shouldtriggergc() {
 		gogc(0)
+	} else if shouldhelpgc && atomicloaduint(&bggc.working) == 1 {
+		// bggc.lock not taken since race on bggc.working is benign.
+		// At worse we don't call gchelpwork.
+		// Delay the gchelpwork until the epilogue so that it doesn't
+		// interfere with the inner working of malloc such as
+		// mcache refills that might happen while doing the gchelpwork
+		systemstack(gchelpwork)
 	}
 
 	return x
@@ -466,14 +493,25 @@ func gogc(force int32) {
 	releasem(mp)
 	mp = nil
 
-	semacquire(&worldsema, false)
-
-	if force == 0 && memstats.heap_alloc < memstats.next_gc {
-		// typically threads which lost the race to grab
-		// worldsema exit here when gc is done.
-		semrelease(&worldsema)
-		return
+	if force == 0 {
+		lock(&bggc.lock)
+		if !bggc.started {
+			bggc.working = 1
+			bggc.started = true
+			go backgroundgc()
+		} else if bggc.working == 0 {
+			bggc.working = 1
+			ready(bggc.g)
+		}
+		unlock(&bggc.lock)
+	} else {
+		gcwork(force)
 	}
+}
+
+func gcwork(force int32) {
+
+	semacquire(&worldsema, false)
 
 	// Pick up the remaining unswept/not being swept spans concurrently
 	for gosweepone() != ^uintptr(0) {
@@ -482,14 +520,17 @@ func gogc(force int32) {
 
 	// Ok, we're doing it!  Stop everybody else
 
-	startTime := nanotime()
-	mp = acquirem()
+	mp := acquirem()
 	mp.gcing = 1
 	releasem(mp)
 	gctimer.count++
 	if force == 0 {
 		gctimer.cycle.sweepterm = nanotime()
 	}
+	// Pick up the remaining unswept/not being swept spans before we STW
+	for gosweepone() != ^uintptr(0) {
+		sweep.nbgsweep++
+	}
 	systemstack(stoptheworld)
 	systemstack(finishsweep_m) // finish sweep before we start concurrent scan.
 	if force == 0 {            // Do as much work concurrently as possible
@@ -500,7 +541,7 @@ func gogc(force int32) {
 		systemstack(gcscan_m)
 		gctimer.cycle.installmarkwb = nanotime()
 		systemstack(stoptheworld)
-		gcinstallmarkwb()
+		systemstack(gcinstallmarkwb)
 		systemstack(starttheworld)
 		gctimer.cycle.mark = nanotime()
 		systemstack(gcmark_m)
@@ -509,6 +550,7 @@ func gogc(force int32) {
 		systemstack(gcinstalloffwb_m)
 	}
 
+	startTime := nanotime()
 	if mp != acquirem() {
 		throw("gogc: rescheduled")
 	}
@@ -527,6 +569,7 @@ func gogc(force int32) {
 	eagersweep := force >= 2
 	for i := 0; i < n; i++ {
 		if i > 0 {
+			// refresh start time if doing a second GC
 			startTime = nanotime()
 		}
 		// switch to g0, call gc, then switch back
@@ -579,8 +622,8 @@ func GCcheckmarkdisable() {
 // gctimes records the time in nanoseconds of each phase of the concurrent GC.
 type gctimes struct {
 	sweepterm     int64 // stw
-	scan          int64 // stw
-	installmarkwb int64
+	scan          int64
+	installmarkwb int64 // stw
 	mark          int64
 	markterm      int64 // stw
 	sweep         int64
@@ -601,7 +644,7 @@ type gcchronograph struct {
 
 var gctimer gcchronograph
 
-// GCstarttimes initializes the gc timess. All previous timess are lost.
+// GCstarttimes initializes the gc times. All previous times are lost.
 func GCstarttimes(verbose int64) {
 	gctimer = gcchronograph{verbose: verbose}
 }
@@ -655,6 +698,11 @@ func calctimes() gctimes {
 // the information from the most recent Concurent GC cycle. Calls from the
 // application to runtime.GC() are ignored.
 func GCprinttimes() {
+	if gctimer.verbose == 0 {
+		println("GC timers not enabled")
+		return
+	}
+
 	// Explicitly put times on the heap so printPhase can use it.
 	times := new(gctimes)
 	*times = calctimes()

src/runtime/mgc.go

@@ -123,7 +123,7 @@ const (
 	_DebugGCPtrs     = false // if true, print trace of every pointer load during GC
 	_ConcurrentSweep = true
 
-	_WorkbufSize     = 4 * 1024
+	_WorkbufSize     = 4 * 256
 	_FinBlockSize    = 4 * 1024
 	_RootData        = 0
 	_RootBss         = 1
@@ -191,9 +191,9 @@ var badblock [1024]uintptr
 var nbadblock int32
 
 type workdata struct {
-	full    uint64                // lock-free list of full blocks
-	empty   uint64                // lock-free list of empty blocks
-	partial uint64                // lock-free list of partially filled blocks
+	full    uint64                // lock-free list of full blocks workbuf
+	empty   uint64                // lock-free list of empty blocks workbuf
+	partial uint64                // lock-free list of partially filled blocks workbuf
 	pad0    [_CacheLineSize]uint8 // prevents false-sharing between full/empty and nproc/nwait
 	nproc   uint32
 	tstart  int64
@@ -587,6 +587,11 @@ func scanblock(b0, n0 uintptr, ptrmask *uint8) {
 	// base and extent.
 	b := b0
 	n := n0
+
+	// ptrmask can have 2 possible values:
+	// 1. nil - obtain pointer mask from GC bitmap.
+	// 2. pointer to a compact mask (for stacks and data).
+
 	wbuf := getpartialorempty()
 	if b != 0 {
 		wbuf = scanobject(b, n, ptrmask, wbuf)
@@ -600,23 +605,23 @@ func scanblock(b0, n0 uintptr, ptrmask *uint8) {
 			return
 		}
 	}
-	if gcphase == _GCscan {
-		throw("scanblock: In GCscan phase but no b passed in.")
-	}
 
-	keepworking := b == 0
+	drainallwbufs := b == 0
+	drainworkbuf(wbuf, drainallwbufs)
+}
 
+// Scan objects in wbuf until wbuf is empty.
+// If drainallwbufs is true find all other available workbufs and repeat the process.
+//go:nowritebarrier
+func drainworkbuf(wbuf *workbuf, drainallwbufs bool) {
 	if gcphase != _GCmark && gcphase != _GCmarktermination {
 		println("gcphase", gcphase)
 		throw("scanblock phase")
 	}
 
-	// ptrmask can have 2 possible values:
-	// 1. nil - obtain pointer mask from GC bitmap.
-	// 2. pointer to a compact mask (for stacks and data).
 	for {
 		if wbuf.nobj == 0 {
-			if !keepworking {
+			if !drainallwbufs {
 				putempty(wbuf)
 				return
 			}
@@ -641,9 +646,30 @@ func scanblock(b0, n0 uintptr, ptrmask *uint8) {
 		//         PREFETCH(wbuf->obj[wbuf->nobj - 3];
 		//  }
 		wbuf.nobj--
-		b = wbuf.obj[wbuf.nobj]
+		b := wbuf.obj[wbuf.nobj]
+		wbuf = scanobject(b, mheap_.arena_used-b, nil, wbuf)
+	}
+}
+
+// Scan at most count objects in the wbuf.
+//go:nowritebarrier
+func drainobjects(wbuf *workbuf, count uintptr) {
+	for i := uintptr(0); i < count; i++ {
+		if wbuf.nobj == 0 {
+			putempty(wbuf)
+			return
+		}
+
+		// This might be a good place to add prefetch code...
+		// if(wbuf->nobj > 4) {
+		//         PREFETCH(wbuf->obj[wbuf->nobj - 3];
+		//  }
+		wbuf.nobj--
+		b := wbuf.obj[wbuf.nobj]
 		wbuf = scanobject(b, mheap_.arena_used-b, nil, wbuf)
 	}
+	putpartial(wbuf)
+	return
 }
 
 //go:nowritebarrier
@@ -809,6 +835,17 @@ func putpartial(b *workbuf) {
 	}
 }
 
+// trygetfull tries to get a full or partially empty workbuffer.
+// if one is not immediately available return nil
+//go:nowritebarrier
+func trygetfull() *workbuf {
+	wbuf := (*workbuf)(lfstackpop(&work.full))
+	if wbuf == nil {
+		wbuf = (*workbuf)(lfstackpop(&work.partial))
+	}
+	return wbuf
+}
+
 // Get a full work buffer off the work.full or a partially
 // filled one off the work.partial list. If nothing is available
 // wait until all the other gc helpers have finished and then
@@ -1090,6 +1127,38 @@ func gcmarkwb_m(slot *uintptr, ptr uintptr) {
 	}
 }
 
+// gchelpwork does a small bounded amount of gc work. The purpose is to
+// shorten the time (as measured by allocations) spent doing a concurrent GC.
+// The number of mutator calls is roughly propotional to the number of allocations
+// made by that mutator. This slows down the allocation while speeding up the GC.
+//go:nowritebarrier
+func gchelpwork() {
+	switch gcphase {
+	default:
+		throw("gcphasework in bad gcphase")
+	case _GCoff, _GCquiesce, _GCstw:
+		// No work.
+	case _GCsweep:
+		// We could help by calling sweepone to sweep a single span.
+		// _ = sweepone()
+	case _GCscan:
+		// scan the stack, mark the objects, put pointers in work buffers
+		// hanging off the P where this is being run.
+		// scanstack(gp)
+	case _GCmark:
+		// Get a full work buffer and empty it.
+		var wbuf *workbuf
+		wbuf = trygetfull()
+		if wbuf != nil {
+			drainobjects(wbuf, uintptr(len(wbuf.obj))) // drain upto one buffer's worth of objects
+		}
+	case _GCmarktermination:
+		// We should never be here since the world is stopped.
+		// All available mark work will be emptied before returning.
+		throw("gcphasework in bad gcphase")
+	}
+}
+
 // The gp has been moved to a GC safepoint. GC phase specific
 // work is done here.
 //go:nowritebarrier
@@ -1425,6 +1494,14 @@ type sweepdata struct {
 
 var sweep sweepdata
 
+// State of the background concurrent GC goroutine.
+var bggc struct {
+	lock    mutex
+	g       *g
+	working uint
+	started bool
+}
+
 // sweeps one span
 // returns number of pages returned to heap, or ^uintptr(0) if there is nothing to sweep
 //go:nowritebarrier

src/runtime/mgc0.go

@@ -78,6 +78,19 @@ func clearpools() {
 	}
 }
 
+// backgroundgc is running in a goroutine and does the concurrent GC work.
+// bggc holds the state of the backgroundgc.
+func backgroundgc() {
+	bggc.g = getg()
+	bggc.g.issystem = true
+	for {
+		gcwork(0)
+		lock(&bggc.lock)
+		bggc.working = 0
+		goparkunlock(&bggc.lock, "Concurrent GC wait")
+	}
+}
+
 func bgsweep() {
 	sweep.g = getg()
 	getg().issystem = true

test/init1.go

@@ -31,7 +31,7 @@ func init() {
 	}
 	runtime.ReadMemStats(memstats)
 	sys1 := memstats.Sys
-	if sys1-sys > chunk*50 {
+	if sys1-sys > chunk*500 {
 		println("allocated 1000 chunks of", chunk, "and used ", sys1-sys, "memory")
 		panic("init1")
 	}