Re: [PATCH v2] migration/xbzrle: add encoding rate

[Dr. David Alan Gilbert]

* Wei Wang (address@hidden) wrote:
> 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++;

Can you explain how that works with overflow?  Doesn't overflow return
-1 here, not increment the bytes, so it looks like you've xbzrle'd a
page, but the encoding rate hasn't incremented.

Dave

>      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
> 
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK