runtime: Do not reserve huge amount of swap on 32 bit architectures.

The mmap() call which reserves the arena should have MAP_NORESERVE
flag as in typical cases this memory will never be (fully) needed.
This matters in environments which do not do Linux style memory
overcommit, such as OpenIndiana/OpenSolaris/Solaris.

The MAP_NORESERVE flag does not exist on all operating systems
(for example FreeBSD). Therefore we define it to zero value in
case it does not exist.

Fixes issue 21.

From-SVN: r196088

diff --git a/libgo/runtime/mem.c b/libgo/runtime/mem.c
index e70694cf0ed5b00fd80be250423bcb1af13cb751..e606bdd4d2452e8667aef22e1b8dcd70d777e59c 100644 (file)
--- a/libgo/runtime/mem.c
+++ b/libgo/runtime/mem.c
@@ -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 @@ runtime_SysReserve(void *v, uintptr n)
                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;