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xbzrle: optimize XBZRLE to decrease the cache misses

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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 <stdlib.h>
include <stdio.h>
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 <chenliang88@huawei.com>
Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
This commit is contained in:
ChenLiang 2014-11-24 19:55:47 +08:00 committed by Amit Shah
parent df58887b20
commit 27af7d6ea5
4 changed files with 39 additions and 10 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

8
arch_init.c
View File

@ -346,7 +346,8 @@ static void xbzrle_cache_zero_page(ram_addr_t current_addr)
/* We don't care if this fails to allocate a new cache page
* as long as it updated an old one */
cache_insert(XBZRLE.cache, current_addr, ZERO_TARGET_PAGE);
cache_insert(XBZRLE.cache, current_addr, ZERO_TARGET_PAGE,
bitmap_sync_count);
}
#define ENCODING_FLAG_XBZRLE 0x1
@ -358,10 +359,11 @@ static int save_xbzrle_page(QEMUFile *f, uint8_t **current_data,
int encoded_len = 0, bytes_sent = -1;
uint8_t *prev_cached_page;
if (!cache_is_cached(XBZRLE.cache, current_addr)) {
if (!cache_is_cached(XBZRLE.cache, current_addr, bitmap_sync_count)) {
acct_info.xbzrle_cache_miss++;
if (!last_stage) {
if (cache_insert(XBZRLE.cache, current_addr, *current_data) == -1) {
if (cache_insert(XBZRLE.cache, current_addr, *current_data,
bitmap_sync_count) == -1) {
return -1;
} else {
/* update *current_data when the page has been

8
docs/xbzrle.txt
View File

@ -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.

10
include/migration/page_cache.h
View File

@ -43,8 +43,10 @@ void cache_fini(PageCache *cache);
*
* @cache pointer to the PageCache struct
* @addr: page addr
* @current_age: current bitmap generation
*/
bool cache_is_cached(const PageCache *cache, uint64_t addr);
bool cache_is_cached(const PageCache *cache, uint64_t addr,
uint64_t current_age);
/**
* get_cached_data: Get the data cached for an addr
@ -60,13 +62,15 @@ uint8_t *get_cached_data(const PageCache *cache, uint64_t addr);
* cache_insert: insert the page into the cache. the page cache
* will dup the data on insert. the previous value will be overwritten
*
* Returns -1 on error
* Returns -1 when the page isn't inserted into cache
*
* @cache pointer to the PageCache struct
* @addr: page address
* @pdata: pointer to the page
* @current_age: current bitmap generation
*/
int cache_insert(PageCache *cache, uint64_t addr, const uint8_t *pdata);
int cache_insert(PageCache *cache, uint64_t addr, const uint8_t *pdata,
uint64_t current_age);
/**
* cache_resize: resize the page cache. In case of size reduction the extra

23
page_cache.c
View File

@ -33,6 +33,9 @@
do { } while (0)
#endif
/* the page in cache will not be replaced in two cycles */
#define CACHED_PAGE_LIFETIME 2
typedef struct CacheItem CacheItem;
struct CacheItem {
@ -122,7 +125,8 @@ static size_t cache_get_cache_pos(const PageCache *cache,
return pos;
}
bool cache_is_cached(const PageCache *cache, uint64_t addr)
bool cache_is_cached(const PageCache *cache, uint64_t addr,
uint64_t current_age)
{
size_t pos;
@ -131,7 +135,12 @@ bool cache_is_cached(const PageCache *cache, uint64_t addr)
pos = cache_get_cache_pos(cache, addr);
return (cache->page_cache[pos].it_addr == addr);
if (cache->page_cache[pos].it_addr == addr) {
/* update the it_age when the cache hit */
cache->page_cache[pos].it_age = current_age;
return true;
}
return false;
}
static CacheItem *cache_get_by_addr(const PageCache *cache, uint64_t addr)
@ -151,7 +160,8 @@ uint8_t *get_cached_data(const PageCache *cache, uint64_t addr)
return cache_get_by_addr(cache, addr)->it_data;
}
int cache_insert(PageCache *cache, uint64_t addr, const uint8_t *pdata)
int cache_insert(PageCache *cache, uint64_t addr, const uint8_t *pdata,
uint64_t current_age)
{
CacheItem *it = NULL;
@ -162,6 +172,11 @@ int cache_insert(PageCache *cache, uint64_t addr, const uint8_t *pdata)
/* actual update of entry */
it = cache_get_by_addr(cache, addr);
if (it->it_data && it->it_addr != addr &&
it->it_age + CACHED_PAGE_LIFETIME > current_age) {
/* the cache page is fresh, don't replace it */
return -1;
}
/* allocate page */
if (!it->it_data) {
it->it_data = g_try_malloc(cache->page_size);
@ -174,7 +189,7 @@ int cache_insert(PageCache *cache, uint64_t addr, const uint8_t *pdata)
memcpy(it->it_data, pdata, cache->page_size);
it->it_age = ++cache->max_item_age;
it->it_age = current_age;
it->it_addr = addr;
return 0;