runtime: add GODEBUG=madvdontneed=1

Fixes #28466

Change-Id: I05b2e0da09394d111913963b60f2ec865c9b4744
Reviewed-on: https://go-review.googlesource.com/c/155931
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/extern.go

@@ -89,6 +89,11 @@ It is a comma-separated list of name=val pairs setting these named variables:
 		released: #  MB released to the system
 		consumed: #  MB allocated from the system
 
+	madvdontneed: setting madvdontneed=1 will use MADV_DONTNEED
+	instead of MADV_FREE on Linux when returning memory to the
+	kernel. This is less efficient, but causes RSS numbers to drop
+	more quickly.
+
 	memprofilerate: setting memprofilerate=X will update the value of runtime.MemProfileRate.
 	When set to 0 memory profiling is disabled.  Refer to the description of
 	MemProfileRate for the default value.

src/runtime/mem_linux.go

@@ -105,7 +105,12 @@ func sysUnused(v unsafe.Pointer, n uintptr) {
 		throw("unaligned sysUnused")
 	}
 
-	advise := atomic.Load(&adviseUnused)
+	var advise uint32
+	if debug.madvdontneed != 0 {
+		advise = _MADV_DONTNEED
+	} else {
+		advise = atomic.Load(&adviseUnused)
+	}
 	if errno := madvise(v, n, int32(advise)); advise == _MADV_FREE && errno != 0 {
 		// MADV_FREE was added in Linux 4.5. Fall back to MADV_DONTNEED if it is
 		// not supported.

src/runtime/runtime1.go

@@ -308,6 +308,7 @@ var debug struct {
 	gcstoptheworld     int32
 	gctrace            int32
 	invalidptr         int32
+	madvdontneed       int32 // for Linux; issue 28466
 	sbrk               int32
 	scavenge           int32
 	scheddetail        int32
@@ -325,6 +326,7 @@ var dbgvars = []dbgVar{
 	{"gcstoptheworld", &debug.gcstoptheworld},
 	{"gctrace", &debug.gctrace},
 	{"invalidptr", &debug.invalidptr},
+	{"madvdontneed", &debug.madvdontneed},
 	{"sbrk", &debug.sbrk},
 	{"scavenge", &debug.scavenge},
 	{"scheddetail", &debug.scheddetail},