Re: [Qemu-devel] [PATCH] i386: turn off l3-cache property by default

[Paolo Bonzini]

On 28/11/2017 19:54, Michael S. Tsirkin wrote:
>> Now there's a downside: with L3 cache the Linux scheduler is more eager
>> to wake up tasks on sibling CPUs, resulting in unnecessary cross-vCPU
>> interactions and therefore exessive halts and IPIs.  E.g. "perf bench
>> sched pipe -i 100000" gives
>>
>> l3-cache     #res IPI /s     #HLT /s         #time /100000 loops
>> off          200 (no K)      230             0.2 sec
>> on           400K            330K            0.5 sec
>>
>> In a more realistic test, we observe 15% degradation in VM density
>> (measured as the number of VMs, each running Drupal CMS serving 2 http
>> requests per second to its main page, with 95%-percentile response
>> latency under 100 ms) with l3-cache=on.
>>
>> We think that mostly-idle scenario is more common in cloud and personal
>> usage, and should be optimized for by default; users of highly loaded
>> VMs should be able to tune them up themselves.

Hi Denis,

thanks for the report.  I think there are two cases:

1) The dedicated pCPU case: do you still get the performance degradation
with dedicated pCPUs?

2) The non-dedicated pCPU case: do you still get the performance
degradation with threads=1?  If not, why do you have sibling vCPUs at
all, if you don't have a dedicated physical CPU for each vCPU?

Thanks,

Paolo