runtime: set minimum heap size to 4Mbytes

Set the minimum heap size to 4Mbytes except when the hash
table code wants to force a GC. In an unrelated change when a
mutator is asked to assist the GC by marking pointer workbufs
it will keep working until the requested number of pointers
are processed even if it means asking for additional workbufs.

Change-Id: I661cfc0a7f2efcf6286b5d37d73e593d9ecd04d5
Reviewed-on: https://go-review.googlesource.com/3392Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/mgc.go

@@ -502,13 +502,16 @@ func drainworkbuf(wbuf *workbuf, drainallwbufs bool) {
 	}
 }
 
-// Scan at most count objects in the wbuf.
+// Scan count objects starting with those in wbuf.
 //go:nowritebarrier
 func drainobjects(wbuf *workbuf, count uintptr) {
 	for i := uintptr(0); i < count; i++ {
 		if wbuf.nobj == 0 {
 			putempty(wbuf)
-			return
+			wbuf = trygetfull()
+			if wbuf == nil {
+				return
+			}
 		}
 
 		// This might be a good place to add prefetch code...
@@ -1104,7 +1107,12 @@ func mSpan_Sweep(s *mspan, preserve bool) bool {
 			}
 			c.local_nlargefree++
 			c.local_largefree += size
-			xadd64(&memstats.next_gc, -int64(size)*int64(gcpercent+100)/100)
+			reduction := int64(size) * int64(gcpercent+100) / 100
+			if int64(memstats.next_gc)-reduction > int64(heapminimum) {
+				xadd64(&memstats.next_gc, -reduction)
+			} else {
+				atomicstore64(&memstats.next_gc, heapminimum)
+			}
 			res = true
 		} else {
 			// Free small object.
@@ -1141,7 +1149,12 @@ func mSpan_Sweep(s *mspan, preserve bool) bool {
 	if nfree > 0 {
 		c.local_nsmallfree[cl] += uintptr(nfree)
 		c.local_cachealloc -= intptr(uintptr(nfree) * size)
-		xadd64(&memstats.next_gc, -int64(nfree)*int64(size)*int64(gcpercent+100)/100)
+		reduction := int64(nfree) * int64(size) * int64(gcpercent+100) / 100
+		if int64(memstats.next_gc)-reduction > int64(heapminimum) {
+			xadd64(&memstats.next_gc, -reduction)
+		} else {
+			atomicstore64(&memstats.next_gc, heapminimum)
+		}
 		res = mCentral_FreeSpan(&mheap_.central[cl].mcentral, s, int32(nfree), head, end, preserve)
 		// MCentral_FreeSpan updates sweepgen
 	}
@@ -1360,6 +1373,11 @@ func updatememstats(stats *gcstats) {
 	memstats.heap_objects = memstats.nmalloc - memstats.nfree
 }
 
+// heapminimum is the minimum number of bytes in the heap.
+// This cleans up the corner case of where we have a very small live set but a lot
+// of allocations and collecting every GOGC * live set is expensive.
+var heapminimum = uint64(4 << 20)
+
 func gcinit() {
 	if unsafe.Sizeof(workbuf{}) != _WorkbufSize {
 		throw("runtime: size of Workbuf is suboptimal")
@@ -1369,7 +1387,7 @@ func gcinit() {
 	gcpercent = readgogc()
 	gcdatamask = unrollglobgcprog((*byte)(unsafe.Pointer(&gcdata)), uintptr(unsafe.Pointer(&edata))-uintptr(unsafe.Pointer(&data)))
 	gcbssmask = unrollglobgcprog((*byte)(unsafe.Pointer(&gcbss)), uintptr(unsafe.Pointer(&ebss))-uintptr(unsafe.Pointer(&bss)))
-	memstats.next_gc = 4 << 20 // 4 megs to start with
+	memstats.next_gc = heapminimum
 }
 
 // Called from malloc.go using onM, stopping and starting the world handled in caller.
@@ -1615,6 +1633,10 @@ func gc(start_time int64, eagersweep bool) {
 	// conservatively set next_gc to high value assuming that everything is live
 	// concurrent/lazy sweep will reduce this number while discovering new garbage
 	memstats.next_gc = memstats.heap_alloc + memstats.heap_alloc*uint64(gcpercent)/100
+	if memstats.next_gc < heapminimum {
+		memstats.next_gc = heapminimum
+	}
+
 	if trace.enabled {
 		traceNextGC()
 	}