Re: [Qemu-devel] slow xbzrle

[Orit Wasserman]

On 10/25/2012 02:18 PM, Stefan Priebe - Profihost AG wrote:
> Am 25.10.2012 13:39, schrieb Orit Wasserman:
>> On 10/25/2012 12:35 PM, Stefan Priebe - Profihost AG wrote:
>>> Am 25.10.2012 11:44, schrieb Orit Wasserman:
>>>>> Is this known or is something wrong?
>>>> My guess this workload migrates fine without XBZRLE so it is not the speed 
>>>> or downtime :).
>>>> it could be that the cache size is too small resulting with a lot of cache 
>>>> misses which means
>>>> XBZRLE makes things worse and just uses CPU to maintain the cache.
>>>> Other option is that this workload changes the pages too much which can 
>>>> result with a lot of over flows
>>>> which result in XBZRLE to waste CPU scanning the pages.
>>>> As this is idle mysql my guess it the first scenario.
>>>>
>>>> Can you send the cache size, cache miss and overflow (from 'info migrate')?
>>>
>>> Example got from a log file (it's the info from info migrate just in 
>>> another format):
>>> migration xbzrle
>>> cachesize: 1073741824
>>> transferred 0
>>> pages 0
>>> cachemiss 1315374
>> Looks like a lot of cache miss, you can try increasing the cache size 
>> (migrate_set_cache_size).
>> But you should remember that for an idle guest XBZRLE is wasteful,
>> it is useful for workload that changes the same memory pages frequently.
> 
> sure here are some values from a loaded VM (20% cpu load with MySQL) - with 
> an increased cache size does not look much better:
> 
> Oct 25 14:16:39 migration xbzrle cachesize: 536870912 transferred 0 pages 0 
> cachemiss 159634 overflow 0
> Oct 25 14:16:41 migration xbzrle cachesize: 536870912 transferred 0 pages 0 
> cachemiss 173631 overflow 0
> Oct 25 14:16:43 migration xbzrle cachesize: 536870912 transferred 0 pages 0 
> cachemiss 187627 overflow 0
> Oct 25 14:16:45 migration xbzrle cachesize: 536870912 transferred 0 pages 0 
> cachemiss 201624 overflow 0
It looks like the cache got smaller 1073741824 > 536870912 ...
try 2G cache size 
Orit
> 
> Greets,
> Stefan