Re: [Qemu-devel] [PATCH v4] XBZRLE delta for live migration of large memory apps

[Avi Kivity <avi@redhat.com>]

On 08/08/2011 11:42 AM, Shribman, Aidan wrote:
> Subject: [PATCH v4] XBZRLE delta for live migration of large memory apps
> From: Aidan Shribman<aidan.shribman@sap.com>
>
> By using XBZRLE (Xor Binary 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.
>
> 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 64 MB). The
> receiving side uses the existing page content and XBZRLE to decode the new page
> content.
>
> 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 -x -d tcp:destination.host:4444
>      {qemu} info migrate
>      ...
>      transferred ram-duplicate: A kbytes
>      transferred ram-duplicate: B pages
>      transferred ram-normal: C kbytes
>      transferred ram-normal: D pages
>      transferred ram-xbrle: E kbytes
>      transferred ram-xbrle: F pages
>      overflow ram-xbrle: G pages
>      cache-hit ram-xbrle: H pages
>      cache-lookup ram-xbrle: J 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(".");
> ..        }
> ..    }
>
>


Please provide documentation in docs/ of the compression format.

IMO it should be disabled by default (with an option to disable it, via, 
sat, migrate-set-options, so we can migrate to older hosts).

The protocol should allow XBZRLE to turn itself off if it detects that 
it isn't effective.

-- 
error compiling committee.c: too many arguments to function