qemu-devel
[Top][All Lists]
Advanced

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

[Qemu-devel] [PATCH v4 5/8] XBZRLE: optimize XBZRLE to decrease the cach


From: arei.gonglei
Subject: [Qemu-devel] [PATCH v4 5/8] XBZRLE: optimize XBZRLE to decrease the cache misses
Date: Thu, 27 Mar 2014 11:18:20 +0800

From: ChenLiang <address@hidden>

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
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
The value of cache-miss-rate decrease to 48.59%.

Signed-off-by: ChenLiang <address@hidden>
Signed-off-by: Gonglei <address@hidden>
Reviewed-by: Eric Blake <address@hidden>
---
 arch_init.c                    |  8 +++++---
 docs/xbzrle.txt                |  8 ++++++++
 include/migration/page_cache.h | 10 +++++++---
 page_cache.c                   | 23 +++++++++++++++++++----
 4 files changed, 39 insertions(+), 10 deletions(-)

diff --git a/arch_init.c b/arch_init.c
index 15ca4c0..84a4bd3 100644
--- a/arch_init.c
+++ b/arch_init.c
@@ -343,7 +343,8 @@ 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
@@ -355,10 +356,11 @@ 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
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.
diff --git a/include/migration/page_cache.h b/include/migration/page_cache.h
index 2d5ce2d..10ed532 100644
--- a/include/migration/page_cache.h
+++ b/include/migration/page_cache.h
@@ -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
diff --git a/page_cache.c b/page_cache.c
index b033681..c78157b 100644
--- a/page_cache.c
+++ b/page_cache.c
@@ -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 {
@@ -121,7 +124,8 @@ 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;
 
@@ -130,7 +134,12 @@ 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)
@@ -150,7 +159,8 @@ 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;
@@ -161,6 +171,11 @@ 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_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);
@@ -173,7 +188,7 @@ 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;
-- 
1.7.12.4





reply via email to

[Prev in Thread] Current Thread [Next in Thread]