libgo patch committed: Use MAP_NORESERVE

libgo patch committed: Use MAP_NORESERVE

[Ian Lance Taylor]

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This patch from Janne Snabb changes libgo to use MAP_NORESERVE when
allocating the large memory arena.  This makes no difference on
GNU/Linux but reportedly helps on OpenIndiana and OpenSolaris.
Bootstrapped and ran Go testsuite on x86_64-unknown-linux-gnu.
Committed to mainline.

Ian


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diff -r 38275c614957 libgo/runtime/mem.c
--- a/libgo/runtime/mem.c	Fri Feb 15 10:48:31 2013 -0800
+++ b/libgo/runtime/mem.c	Fri Feb 15 10:49:37 2013 -0800
@@ -18,6 +18,10 @@
 #endif
 #endif
 
+#ifndef MAP_NORESERVE
+#define MAP_NORESERVE 0
+#endif
+
 #ifdef USE_DEV_ZERO
 static int dev_zero = -1;
 #endif
@@ -134,7 +138,11 @@
 		return v;
 	}
 	
-	p = runtime_mmap(v, n, PROT_NONE, MAP_ANON|MAP_PRIVATE, fd, 0);
+	// Use the MAP_NORESERVE mmap() flag here because typically most of
+	// this reservation will never be used. It does not make sense
+	// reserve a huge amount of unneeded swap space. This is important on
+	// systems which do not overcommit memory by default.
+	p = runtime_mmap(v, n, PROT_NONE, MAP_ANON|MAP_PRIVATE|MAP_NORESERVE, fd, 0);
 	if(p == MAP_FAILED)
 		return nil;
 	return p;