[PATCH v2] migration/xbzrle: add encoding rate

[Wei Wang]

Users may need to check the xbzrle encoding rate to know if the guest
memory is xbzrle encoding-friendly, and dynamically turn off the
encoding if the encoding rate is low.

Signed-off-by: Yi Sun <address@hidden>
Signed-off-by: Wei Wang <address@hidden>
---
 migration/migration.c |  1 +
 migration/ram.c       | 38 ++++++++++++++++++++++++++++++++++++--
 monitor/hmp-cmds.c    |  2 ++
 qapi/migration.json   |  5 ++++-
 4 files changed, 43 insertions(+), 3 deletions(-)

ChangeLog:
- include the 3 bytes (ENCODING_FLAG_XBZRLE flag and encoded_len) when
  calculating the encoding rate. Similar to the compress rate
  calculation, the 8 byte RAM_SAVE_FLAG_CONTINUE flag isn't included in
  the calculation.

diff --git a/migration/migration.c b/migration/migration.c
index 187ac04..e404213 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -930,6 +930,7 @@ static void populate_ram_info(MigrationInfo *info, 
MigrationState *s)
         info->xbzrle_cache->pages = xbzrle_counters.pages;
         info->xbzrle_cache->cache_miss = xbzrle_counters.cache_miss;
         info->xbzrle_cache->cache_miss_rate = xbzrle_counters.cache_miss_rate;
+        info->xbzrle_cache->encoding_rate = xbzrle_counters.encoding_rate;
         info->xbzrle_cache->overflow = xbzrle_counters.overflow;
     }
 
diff --git a/migration/ram.c b/migration/ram.c
index 04f13fe..f46ab96 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -327,6 +327,10 @@ struct RAMState {
     uint64_t num_dirty_pages_period;
     /* xbzrle misses since the beginning of the period */
     uint64_t xbzrle_cache_miss_prev;
+    /* Amount of xbzrle pages since the beginning of the period */
+    uint64_t xbzrle_pages_prev;
+    /* Amount of xbzrle encoded bytes since the beginning of the period */
+    uint64_t xbzrle_bytes_prev;
 
     /* compression statistics since the beginning of the period */
     /* amount of count that no free thread to compress data */
@@ -696,6 +700,18 @@ static int save_xbzrle_page(RAMState *rs, uint8_t 
**current_data,
         return -1;
     }
 
+    /*
+     * Reaching here means the page has hit the xbzrle cache, no matter what
+     * encoding result it is (normal encoding, overflow or skipping the page),
+     * count the page as encoded. This is used to caculate the encoding rate.
+     *
+     * Example: 2 pages (8KB) being encoded, first page encoding generates 2KB,
+     * 2nd page turns out to be skipped (i.e. no new bytes written to the
+     * page), the overall encoding rate will be 8KB / 2KB = 4, which has the
+     * skipped page included. In this way, the encoding rate can tell if the
+     * guest page is good for xbzrle encoding.
+     */
+    xbzrle_counters.pages++;
     prev_cached_page = get_cached_data(XBZRLE.cache, current_addr);
 
     /* save current buffer into memory */
@@ -736,8 +752,12 @@ static int save_xbzrle_page(RAMState *rs, uint8_t 
**current_data,
     qemu_put_be16(rs->f, encoded_len);
     qemu_put_buffer(rs->f, XBZRLE.encoded_buf, encoded_len);
     bytes_xbzrle += encoded_len + 1 + 2;
-    xbzrle_counters.pages++;
-    xbzrle_counters.bytes += bytes_xbzrle;
+    /*
+     * Like compressed_size (please see update_compress_thread_counts),
+     * the xbzrle encoded bytes don't count the 8 byte header with
+     * RAM_SAVE_FLAG_CONTINUE.
+     */
+    xbzrle_counters.bytes += bytes_xbzrle - 8;
     ram_counters.transferred += bytes_xbzrle;
 
     return 1;
@@ -870,9 +890,23 @@ static void migration_update_rates(RAMState *rs, int64_t 
end_time)
     }
 
     if (migrate_use_xbzrle()) {
+        double encoded_size, unencoded_size;
+
         xbzrle_counters.cache_miss_rate = (double)(xbzrle_counters.cache_miss -
             rs->xbzrle_cache_miss_prev) / page_count;
         rs->xbzrle_cache_miss_prev = xbzrle_counters.cache_miss;
+        unencoded_size = (xbzrle_counters.pages - rs->xbzrle_pages_prev) *
+                         TARGET_PAGE_SIZE;
+        encoded_size = xbzrle_counters.bytes - rs->xbzrle_bytes_prev;
+        if (xbzrle_counters.pages == rs->xbzrle_pages_prev) {
+            xbzrle_counters.encoding_rate = 0;
+        } else if (!encoded_size) {
+            xbzrle_counters.encoding_rate = UINT64_MAX;
+        } else {
+            xbzrle_counters.encoding_rate = unencoded_size / encoded_size;
+        }
+        rs->xbzrle_pages_prev = xbzrle_counters.pages;
+        rs->xbzrle_bytes_prev = xbzrle_counters.bytes;
     }
 
     if (migrate_use_compression()) {
diff --git a/monitor/hmp-cmds.c b/monitor/hmp-cmds.c
index 9b94e67..c2a3a66 100644
--- a/monitor/hmp-cmds.c
+++ b/monitor/hmp-cmds.c
@@ -303,6 +303,8 @@ void hmp_info_migrate(Monitor *mon, const QDict *qdict)
                        info->xbzrle_cache->cache_miss);
         monitor_printf(mon, "xbzrle cache miss rate: %0.2f\n",
                        info->xbzrle_cache->cache_miss_rate);
+        monitor_printf(mon, "xbzrle encoding rate: %0.2f\n",
+                       info->xbzrle_cache->encoding_rate);
         monitor_printf(mon, "xbzrle overflow: %" PRIu64 "\n",
                        info->xbzrle_cache->overflow);
     }
diff --git a/qapi/migration.json b/qapi/migration.json
index eca2981..358e402 100644
--- a/qapi/migration.json
+++ b/qapi/migration.json
@@ -70,6 +70,8 @@
 #
 # @cache-miss-rate: rate of cache miss (since 2.1)
 #
+# @encoding-rate: rate of encoded bytes (since 5.1)
+#
 # @overflow: number of overflows
 #
 # Since: 1.2
@@ -77,7 +79,7 @@
 { 'struct': 'XBZRLECacheStats',
   'data': {'cache-size': 'int', 'bytes': 'int', 'pages': 'int',
            'cache-miss': 'int', 'cache-miss-rate': 'number',
-           'overflow': 'int' } }
+           'encoding-rate': 'number', 'overflow': 'int' } }
 
 ##
 # @CompressionStats:
@@ -337,6 +339,7 @@
 #             "pages":2444343,
 #             "cache-miss":2244,
 #             "cache-miss-rate":0.123,
+#             "encoding-rate":80.1,
 #             "overflow":34434
 #          }
 #       }
-- 
1.8.3.1