Re: Poor HVM performance with 8 vcpus

[Juergen Gross <juergen.gross@ts.fujitsu.com>]

Tim Deegan wrote:
> At 09:16 +0100 on 14 Oct (1255511785), Juergen Gross wrote:
>> as the performance of BS2000 seems to be hit by OOS optimization, I'm
>> thinking of making a patch to disable this feature by a domain parameter.
>>
>> Is there a way to do this without having to change all places where the
>> #if statements are placed?
>> I think there should be some central routines where adding an "if" could
>> be enough (setting oos_active to 0 seems not to be enough, I fear).
>>
>> Do you have any hint?
> 
> The simplest way is to cause sh_unsync() to immediately return 0.  That
> won't be quite as fast as #defining it all away but will avoid the
> expensive paths that cause lock contention.  You can add your flag to
> the big if statement that's already there to avoid unsafe cases.

Thanks, I'll try.

> 
> Incidentally, although your benchmark does poorly on 8 VCPUs it might be
> worth trying a less aggressively targeted benchmark -- we found on
> Windows VMs that more realistic tests (e.g. Sysmark) still showed a
> slight improvement from the OOS optimization at 8 vcpus.

The benchmark is designed to be a realistic simulation of a typical customer
batch load. Many BS2000 customers would see similar performance effects.


Juergen

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Juergen Gross                 Principal Developer Operating Systems
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