[Qemu-devel] [PATCH v8 00/10] XBZRLE delta for live migration of large memory app

[Orit Wasserman]

Changes from v7:
        Copy current page before encoding it, this will prevents page content
        change during the encoding.
        Allow changing the cache size during an active migration.
        Fix comments by Avi.

Changes from v6:
 1) add assert checks to ULEB encoding/decoding
 2) no need to send last zero run
        
Changes from v5:
1) Add migration capabilities
2) Use ULEB to encode run length
3) Do not send unmodified (dirty) page
3) Fix other patch comments

Using GCache or GHashTable requires allocating new buffer on every content 
change, 
so I decided to keep the simple cache implementation.

Changes from v4:
1) Rebase
2) divide patch into 9 patches
3) move memory allocation into cache_insert

Future work :
     Use SSE for encoding.
     Page ranking acording to their dirty rate and automatic 
activation/deactivation of the feature - will be sent in a separate patch 
series.  

By using XBZRLE (Xor Based Zero Run Length Encoding) we can reduce VM downtime
and total live-migration time of VMs running memory write intensive workloads
typical of large enterprise applications such as SAP ERP Systems, and generally
speaking for any application with a sparse memory update pattern.

The compression format uses the fact that we will have many zero (zero 
represents
an unchanged value). 
We repesent the page data delta by zero and non zero runs.
We represent a zero run with it's length (in bytes). 
We represent a non zero run with it's length (in bytes) and the data.
The run length is encoded using ULEB128 (http://en.wikipedia.org/wiki/LEB128)

page = zrun nzrun
       | zrun nzrun page

zrun = length

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 an LRU cache (default size of 512 MB). The
receiving side uses the existing page content and XBZRLE to decode the new page
content.

This is a more compact way to store the delta than the previous version.

This work was originally based on research results published VEE 2011: 
Evaluation of
Delta Compression Techniques for Efficient Live Migration of Large Virtual
Machines by Benoit, Svard, Tordsson and Elmroth. Additionally the delta encoder
XBRLE was improved further using XBZRLE 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.

A typical usage scenario:
    {qemu} migrate_set_cachesize 256m
    {qemu} migrate -d -x tcp:destination.host:4444
    {qemu} info migrate
    ...
    transferred ram-duplicate: A kbytes
    transferred ram-normal: B kbytes
    transferred ram-xbrle: C kbytes
    overflow ram-xbrle: D pages
    cache-miss ram-xbrle: E pages

Testing: live migration with XBZRLE completed in 110 seconds, without live
migration was not 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(".");
..        }
..    }

Signed-off-by: Benoit Hudzia <address@hidden>
Signed-off-by: Petter Svard <address@hidden>
Signed-off-by: Aidan Shribman <address@hidden>

Orit Wasserman (10):
  Add cache handling functions
  Add uleb encoding/decoding functions
  Add save_block_hdr function
  Add host_from_stream_offset_versioned function
  Add MigrationParams structure
  Add XBZRLE to ram_save_block and ram_save_live
  Add XBZRLE option to migrate command
  Add migration capabilites
  Add set_cachesize command
  Add XBZRLE statstics information

 arch_init.c       |  477 +++++++++++++++++++++++++++++++++++++++++++++++++---
 block-migration.c |    8 +-
 hmp-commands.hx   |   35 +++-
 hmp.c             |   46 +++++-
 hmp.h             |    2 +
 migration.c       |   91 ++++++++++-
 migration.h       |   32 ++++-
 monitor.c         |    7 +
 qapi-schema.json  |   60 +++++++-
 qemu-common.h     |    1 + qmp-commands.hx   |   50 ++++++-
 savevm.c          |  133 ++++++++++++++-
 sysemu.h          |    3 +-
 vmstate.h         |    2 +-
 14 files changed, 889 insertions(+), 58 deletions(-)

-- 
1.7.7.6