From: Thomas Huth <[email protected]> xbzrle is a feature of migration and thus this file should go into the docs/devel/migration/ folder. While we're at it, turn it into proper .rst format, too.
Signed-off-by: Thomas Huth <[email protected]> --- docs/devel/migration/features.rst | 1 + .../migration/xbzrle.rst} | 106 +++++++++++------- 2 files changed, 65 insertions(+), 42 deletions(-) rename docs/{xbzrle.txt => devel/migration/xbzrle.rst} (74%) diff --git a/docs/devel/migration/features.rst b/docs/devel/migration/features.rst index 8f431d52f9d..9aef79e7fa4 100644 --- a/docs/devel/migration/features.rst +++ b/docs/devel/migration/features.rst @@ -15,3 +15,4 @@ Migration has plenty of features to support different use cases. qpl-compression uadk-compression qatzip-compression + xbzrle diff --git a/docs/xbzrle.txt b/docs/devel/migration/xbzrle.rst similarity index 74% rename from docs/xbzrle.txt rename to docs/devel/migration/xbzrle.rst index bcb3f0c901a..d4f40072611 100644 --- a/docs/xbzrle.txt +++ b/docs/devel/migration/xbzrle.rst @@ -20,7 +20,7 @@ A small cache size will result in high cache miss rate. Cache size can be changed before and during migration. Format -======= +------ The compression format performs a XOR between the previous and current content of the page, where zero represents an unchanged value. @@ -29,17 +29,19 @@ A zero run is represented by its length (in bytes). A non zero run is represented by its length (in bytes) and the new data. The run length is encoded using ULEB128 (http://en.wikipedia.org/wiki/LEB128) -There can be more than one valid encoding, the sender may send a longer encoding -for the benefit of reducing computation cost. +There can be more than one valid encoding, the sender may send a longer +encoding for the benefit of reducing computation cost. -page = zrun nzrun - | zrun nzrun page +:: -zrun = length + page = zrun nzrun + | zrun nzrun page -nzrun = length byte... + zrun = length -length = uleb128 encoded integer + nzrun = length byte... + + length = uleb128 encoded integer On the sender side XBZRLE is used as a compact delta encoding of page updates, retrieving the old page content from the cache (default size of 64MB). The @@ -55,24 +57,34 @@ instead. XBZRLE has a sustained bandwidth of 2-2.5 GB/s for typical workloads making it ideal for in-line, real-time encoding such as is needed for live-migration. -Example +Example: + old buffer: -1001 zeros -05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 68 00 00 6b 00 6d -3074 zeros + +.. code:: batch + + 1001 zeros + 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 68 00 00 6b 00 6d + 3074 zeros new buffer: -1001 zeros -01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 68 00 00 67 00 69 -3074 zeros + +.. code:: batch + + 1001 zeros + 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 68 00 00 67 00 69 + 3074 zeros 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 +.. code:: batch + + 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 decreasing 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 @@ -80,21 +92,27 @@ 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. - {qemu} info migrate_capabilities - {qemu} xbzrle: off , ... +----- -2. Activate xbzrle on both source and destination: - {qemu} migrate_set_capability xbzrle on +1. Verify the destination QEMU version is able to decode the new format:: + + (qemu) info migrate_capabilities + xbzrle: off + ... + +2. Activate xbzrle on both source and destination:: + + (qemu) migrate_set_capability xbzrle on 3. Set the XBZRLE cache size - the cache size is in MBytes and should be a -power of 2. The cache default value is 64MBytes. (on source only) - {qemu} migrate_set_parameter xbzrle-cache-size 256m + power of 2. The cache default value is 64 MBytes (on source only):: + + (qemu) migrate_set_parameter xbzrle-cache-size 256m -4. Start outgoing migration - {qemu} migrate -d tcp:destination.host:4444 - {qemu} info migrate +4. Start outgoing migration:: + + (qemu) migrate -d tcp:destination.host:4444 + (qemu) info migrate capabilities: xbzrle: on Migration status: active transferred ram: A kbytes @@ -114,6 +132,7 @@ power of 2. The cache default value is 64MBytes. (on source only) xbzrle cache miss: the number of cache misses to date - high cache-miss rate indicates that the cache size is set too low. + xbzrle overflow: the number of overflows in the decoding which where the delta could not be compressed. This can happen if the changes in the pages are too large or there are many short changes; for example, changing every second byte @@ -123,16 +142,19 @@ Testing: Testing indicated that live migration with XBZRLE was completed in 110 seconds, whereas without it would not be able to complete. A simple synthetic memory r/w load generator: -.. 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("."); -.. } -.. } + +.. code-block:: c + + #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("."); + } + } -- 2.53.0
