runtime: new allocation strategy for amd64

Do not reserve virtual address space.
Instead, assume it will be there when we need it,
and crash loudly if that assumption is violated.
Reserving the address space gets charged to
ulimit -v, which exceeds commonly set limits.

http://groups.google.com/group/golang-dev/msg/7c477af5f5a8dd2c

R=r, niemeyer
CC=golang-dev
https://golang.org/cl/4148045

src/pkg/runtime/freebsd/mem.c

@@ -33,6 +33,12 @@ runtime·SysFree(void *v, uintptr n)
 void*
 runtime·SysReserve(void *v, uintptr n)
 {
+	// On 64-bit, people with ulimit -v set complain if we reserve too
+	// much address space.  Instead, assume that the reservation is okay
+	// and check the assumption in SysMap.
+	if(sizeof(void*) == 8)
+		return v;
+	
 	return runtime·mmap(v, n, PROT_NONE, MAP_ANON|MAP_PRIVATE, -1, 0);
 }
 
@@ -42,6 +48,17 @@ runtime·SysMap(void *v, uintptr n)
 	void *p;
 	
 	mstats.sys += n;
+
+	// On 64-bit, we don't actually have v reserved, so tread carefully.
+	if(sizeof(void*) == 8) {
+		p = runtime·mmap(v, n, PROT_READ|PROT_WRITE|PROT_EXEC, MAP_ANON|MAP_PRIVATE, -1, 0);
+		if(p != v) {
+			runtime·printf("runtime: address space conflict: map(%v) = %v\n", v, p);
+			runtime·throw("runtime: address space conflict");
+		}
+		return;
+	}
+
 	p = runtime·mmap(v, n, PROT_READ|PROT_WRITE|PROT_EXEC, MAP_ANON|MAP_FIXED|MAP_PRIVATE, -1, 0);
 	if(p != v)
 		runtime·throw("runtime: cannot map pages in arena address space");

src/pkg/runtime/linux/mem.c

@@ -39,6 +39,12 @@ runtime·SysFree(void *v, uintptr n)
 void*
 runtime·SysReserve(void *v, uintptr n)
 {
+	// On 64-bit, people with ulimit -v set complain if we reserve too
+	// much address space.  Instead, assume that the reservation is okay
+	// and check the assumption in SysMap.
+	if(sizeof(void*) == 8)
+		return v;
+	
 	return runtime·mmap(v, n, PROT_NONE, MAP_ANON|MAP_PRIVATE, -1, 0);
 }
 
@@ -48,6 +54,17 @@ runtime·SysMap(void *v, uintptr n)
 	void *p;
 	
 	mstats.sys += n;
+
+	// On 64-bit, we don't actually have v reserved, so tread carefully.
+	if(sizeof(void*) == 8) {
+		p = runtime·mmap(v, n, PROT_READ|PROT_WRITE|PROT_EXEC, MAP_ANON|MAP_PRIVATE, -1, 0);
+		if(p != v) {
+			runtime·printf("runtime: address space conflict: map(%v) = %v\n", v, p);
+			runtime·throw("runtime: address space conflict");
+		}
+		return;
+	}
+
 	p = runtime·mmap(v, n, PROT_READ|PROT_WRITE|PROT_EXEC, MAP_ANON|MAP_FIXED|MAP_PRIVATE, -1, 0);
 	if(p != v)
 		runtime·throw("runtime: cannot map pages in arena address space");

test/run

@@ -42,9 +42,7 @@ TMP2FILE=/tmp/gotest2-$$-$USER
 
 # don't run the machine out of memory: limit individual processes to 4GB.
 # on thresher, 3GB suffices to run the tests; with 2GB, peano fails.
-# Linux charges reserved but not mapped addresses to ulimit -v
-# so we have to use ulimit -m.
-ulimit -m 4000000
+ulimit -v 4000000
 
 # no core files please
 ulimit -c 0