Re: [Qemu-devel] slow xbzrle

[Stefan Priebe - Profihost AG <s.priebe@profihost.ag>]

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

Greets,
Stefan