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Re: [Qemu-devel] [PATCH] i386: turn off l3-cache property by default
From: |
Roman Kagan |
Subject: |
Re: [Qemu-devel] [PATCH] i386: turn off l3-cache property by default |
Date: |
Wed, 29 Nov 2017 07:56:22 +0300 |
User-agent: |
Mutt/1.9.1 (2017-09-22) |
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.
- [Qemu-devel] [PATCH] i386: turn off l3-cache property by default, Denis Plotnikov, 2017/11/24
- Re: [Qemu-devel] [PATCH] i386: turn off l3-cache property by default, Michael S. Tsirkin, 2017/11/28
- Re: [Qemu-devel] [PATCH] i386: turn off l3-cache property by default, Eduardo Habkost, 2017/11/28
- Re: [Qemu-devel] [PATCH] i386: turn off l3-cache property by default, Longpeng (Mike), 2017/11/29
- Re: [Qemu-devel] [PATCH] i386: turn off l3-cache property by default, Roman Kagan, 2017/11/29
- Re: [Qemu-devel] [PATCH] i386: turn off l3-cache property by default, Longpeng (Mike), 2017/11/29
- Re: [Qemu-devel] [PATCH] i386: turn off l3-cache property by default, Eduardo Habkost, 2017/11/29