Two changes to avoid disk driver using up kernel memory and triggering
oom-killer
while creating guest dump to a disk file:
1) Decreased write chunks from 16MB to 2MB;
2) Flush (fsync) after each 2MB write.
To make sure we don't suffer significant write performance penalty, each
of the
following cases have been tested several times to dump a 1GB guest to a
file on
a SAN disk:
With current code (16MB writes, sync once after all writes); ~38s
16MB writes, sync after each write: ~46s
2MB writes, sync after each write: ~49s
Sync-often costs us about 20%, smaller-write-size costs another 8%. The
total is
less than 30% and about 10 seconds more for a 1GB guest.
Signed-off-by: Ben Guthro <bguthro@xxxxxxxxxxxxxx>
Signed-off-by: Daniel Li <dli@xxxxxxxxxxxxxxx>
diff -r 9d263e1fcd02 tools/libxc/xc_core.c
--- a/tools/libxc/xc_core.c Wed Oct 24 16:25:11 2007 -0400
+++ b/tools/libxc/xc_core.c Wed Oct 24 16:26:04 2007 -0400
@@ -56,7 +56,7 @@
#include <unistd.h>
/* number of pages to write at a time */
-#define DUMP_INCREMENT (4 * 1024)
+#define DUMP_INCREMENT (512)
/* Don't yet support cross-address-size core dump */
#define guest_width (sizeof (unsigned long))
@@ -875,7 +875,7 @@ static int local_file_dump(void *args, c
if (length >= DUMP_INCREMENT*PAGE_SIZE) {
// Now dumping pages -- make sure we discard clean pages from
// the cache after each write
- discard_file_cache(da->fd, 0 /* no flush */);
+ discard_file_cache(da->fd, 1 /* flush */);
}
return 0;
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