Re: [libvirt] Overhead for a default cpu cg placement scheme
On Thu, Jun 11, 2015 at 02:16:50PM +0300, Andrey Korolyov wrote:
On Thu, Jun 11, 2015 at 2:09 PM, Daniel P. Berrange
<berrange(a)redhat.com> wrote:
> On Thu, Jun 11, 2015 at 01:50:24PM +0300, Andrey Korolyov wrote:
>> Hi Daniel,
>>
>> would it possible to adopt an optional tunable for a virCgroup
>> mechanism which targets to a disablement of a nested (per-thread)
>> cgroup creation? Those are bringing visible overhead for many-threaded
>> guest workloads, almost 5% in non-congested host CPU state, primarily
>> because the host scheduler should make a much more decisions with
>> those cgroups than without them. We also experienced a lot of host
>> lockups with currently exploited cgroup placement and disabled nested
>> behavior a couple of years ago. Though the current patch is simply
>> carves out the mentioned behavior, leaving only top-level per-machine
>> cgroups, it can serve for an upstream after some adaptation, that`s
>> why I`m asking about a chance of its acceptance. This message is a
>> kind of 'request of a feature', it either can be accepted/dropped from
>> our side or someone may give a hand and redo it from scratch. The
>> detailed benchmarks are related to a host 3.10.y, if anyone is
>> interested in the numbers for latest stable, I can update those.
>
> When you say nested cgroup creation, as you referring to the modern
> libvirt hierarchy, or the legacy hierarchy - as described here:
>
> http://libvirt.org/cgroups.html
>
> The current libvirt setup used for a year or so now is much shallower
> than previously, to the extent that we'd consider performance problems
> with it to be the job of the kernel to fix.
Thanks, I`m referring to a 'new nested' hiearchy for an overhead
mentioned above. The host crashes I mentioned happened with old
hierarchy back ago, forgot to mention this. Despite the flattening of
the topo for the current scheme it should be possible to disable fine
group creation for the VM threads for some users who don`t need
per-vcpu cpu pinning/accounting (though overhead caused by a placement
for cpu cgroup, not by accounting/pinning ones, I`m assuming equal
distribution with such disablement for all nested-aware cgroup types),
that`s the point for now.
Ok, so the per-vCPU cgroups are used for a couple of things
- Setting scheduler tunables - period/quota/shares/etc
- Setting CPU pinning
- Setting NUMA memory pinning
In addition to the per-VCPU cgroup, we have one cgroup fr each
I/O thread, and also one more for general QEMU emulator threads.
In the case of CPU pinning we already have automatic fallback to
sched_setaffinity if the CPUSET controller isn't available.
We could in theory start off without the per-vCPU/emulator/I/O
cgroups and only create them as & when the feature is actually
used. The concern I would have though is that changing the cgroups
layout on the fly may cause unexpected sideeffects in behaviour of
the VM. More critically, there would be alot of places in the code
where we would need to deal with this which could hurt maintainability.
How confident are you that the performance problems you see are inherant
to the actual use of the cgroups, and not instead as a result of some
particular bad choice of default parameters we might have left in the
cgroups ? In general I'd have a desire to try to work to eliminate the
perf impact before we consider the complexity of disabling this feature
Regards,
Daniel
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