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

[Roman Kagan <rkagan@virtuozzo.com>]

On Tue, Nov 28, 2017 at 08:50:54PM +0100, Paolo Bonzini wrote:
> 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?

I wonder why dedicated pCPU would matter at all?  The behavior change is
in the guest scheduler.

> 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?

We have sibling vCPUs in terms of cores, not threads.  I.e. the
configuration in the test was sockets=1,cores=8,threads=1.  Are you
suggesting that it shouldn't be used without pCPU binding?

Roman.