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SubjectRe: [RFC PATCH 0/4] cgroup aware workqueues
Date
"Michael Rapoport" <RAPOPORT@il.ibm.com> writes:

> Hi Bandan,
>
>> From: Bandan Das <bsd@redhat.com>
>>
>> At Linuxcon last year, based on our presentation "vhost: sharing is
> better" [1],
>> we had briefly discussed the idea of cgroup aware workqueues with Tejun.
> The
>> following patches are a result of the discussion. They are in no way
> complete in
>> that the changes are for unbounded workqueues only, but I just wanted to
> present my
>> unfinished work as RFC and get some feedback.
>>
>> 1/4 and 3/4 are simple cgroup changes and add a helper function.
>> 2/4 is the main implementation.
>> 4/4 changes vhost to use workqueues with support for cgroups.
>>
>> Example:
>> vhost creates a worker thread when invoked for a kvm guest. Since,
>> the guest is a normal process, the kernel thread servicing it should be
>> attached to the vm process' cgroups.
>
> I did some performance evaluation of different threading models in vhost,
> and in most tests replacing vhost kthread's with workqueues degrades the

Workqueues us kthread_create internally and if calling one over the
other impacts performace, I think we should investigate that. Which
patches did you use ? Note that an earlier version of workqueue patches
that I posted used per-cpu workqueues.

> performance. Moreover, having thread management inside the vhost provides

What exactly is the advantage doing our own thread management ? Do you have
any examples ? (Besides for doing our own scheduling like in the original Elvis
paper which I don't think is gonna happen). Also, note here that there is
a possibility to affect how our work gets executed by using optional switches to
alloc_workqueue() so all is not lost.

> opportunity for optimization, at least for some workloads...
> That said, I believe that switching vhost to use workqueues is not that
> good idea after all.
>
>> Netperf:
>> Two guests running netperf in parallel.
>> Without patches With
> patches
>>
>> TCP_STREAM (10^6 bits/second) 975.45 978.88
>> TCP_RR (Trans/second) 20121 18820.82
>> UDP_STREAM (10^6 bits/second) 1287.82 1184.5
>> UDP_RR (Trans/second) 20766.72 19667.08
>> Time a 4G iso download 2m 33 seconds 3m 02 seconds
>
> --
> Sincerely yours,
> Mike.
>
> --
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