From 27af7d6ea5015e5ef1f7985eab94a8a218267a2b Mon Sep 17 00:00:00 2001 From: ChenLiang Date: Mon, 24 Nov 2014 19:55:47 +0800 Subject: xbzrle: optimize XBZRLE to decrease the cache misses Avoid hot pages being replaced by others to remarkably decrease cache misses Sample results with the test program which quote from xbzrle.txt ran in vm:(migrate bandwidth:1GE and xbzrle cache size 8MB) the test program: include include int main() { char *buf = (char *) calloc(4096, 4096); while (1) { int i; for (i = 0; i < 4096 * 4; i++) { buf[i * 4096 / 4]++; } printf("."); } } before this patch: virsh qemu-monitor-command test_vm '{"execute": "query-migrate"}' {"return":{"expected-downtime":1020,"xbzrle-cache":{"bytes":1108284, "cache-size":8388608,"cache-miss-rate":0.987013,"pages":18297,"overflow":8, "cache-miss":1228737},"status":"active","setup-time":10,"total-time":52398, "ram":{"total":12466991104,"remaining":1695744,"mbps":935.559472, "transferred":5780760580,"dirty-sync-counter":271,"duplicate":2878530, "dirty-pages-rate":29130,"skipped":0,"normal-bytes":5748592640, "normal":1403465}},"id":"libvirt-706"} 18k pages sent compressed in 52 seconds. cache-miss-rate is 98.7%, totally miss. after optimizing: virsh qemu-monitor-command test_vm '{"execute": "query-migrate"}' {"return":{"expected-downtime":2054,"xbzrle-cache":{"bytes":5066763, "cache-size":8388608,"cache-miss-rate":0.485924,"pages":194823,"overflow":0, "cache-miss":210653},"status":"active","setup-time":11,"total-time":18729, "ram":{"total":12466991104,"remaining":3895296,"mbps":937.663549, "transferred":1615042219,"dirty-sync-counter":98,"duplicate":2869840, "dirty-pages-rate":58781,"skipped":0,"normal-bytes":1588404224, "normal":387794}},"id":"libvirt-266"} 194k pages sent compressed in 18 seconds. The value of cache-miss-rate decrease to 48.59%. Signed-off-by: ChenLiang Signed-off-by: Gonglei Reviewed-by: Eric Blake Signed-off-by: Amit Shah --- docs/xbzrle.txt | 8 ++++++++ 1 file changed, 8 insertions(+) (limited to 'docs') diff --git a/docs/xbzrle.txt b/docs/xbzrle.txt index cc3a26a..52c8511 100644 --- a/docs/xbzrle.txt +++ b/docs/xbzrle.txt @@ -71,6 +71,14 @@ encoded buffer: encoded length 24 e9 07 0f 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 03 01 67 01 01 69 +Cache update strategy +===================== +Keeping the hot pages in the cache is effective for decreased cache +misses. XBZRLE uses a counter as the age of each page. The counter will +increase after each ram dirty bitmap sync. When a cache conflict is +detected, XBZRLE will only evict pages in the cache that are older than +a threshold. + Usage ====================== 1. Verify the destination QEMU version is able to decode the new format. -- cgit v1.1