Re: [Qemu-devel] slow xbzrle

[Orit Wasserman <owasserm@redhat.com>]

On 10/25/2012 10:12 AM, Stefan Priebe - Profihost AG wrote:
> Hello list,
> 
> i'm using 1.2 stable and wanted to use xbzrle but xbzrle is extremely slow.
> 
> While trying to transfer a simple VM with 4GB memory through a 10GBe nic while running a MySQL (with NO LOAD) it takes up to 10 - 15 minutes.
> 
> Remaining is often jumping or just lowering pretty slow.
This happens if the guest is dirtying his memory faster than the speed qemu is sending it.
It can happen with regular migration if the speed is too low or the downtime is too low.
> 
> 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')?

Thanks,
Orit
> 
> Greets,
> Stefan
>