linux-kernel - Re: [RFC PATCH 0/4] cgroup aware workqueues

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Message-Id: <201603310617.u2V6HIkt008006@d06av12.portsmouth.uk.ibm.com>
Date:	Thu, 31 Mar 2016 08:17:13 +0200
From:	"Michael Rapoport" <RAPOPORT@...ibm.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Bandan Das <bsd@...hat.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, mst@...hat.com, jiangshanlai@...il.com
Subject: Re: [RFC PATCH 0/4] cgroup aware workqueues

Hello,

> Tejun Heo <htejun@...il.com> wrote on 03/30/2016 08:04:19 PM:
> 
> Hello,
> 
> On Mon, Mar 21, 2016 at 09:58:39AM +0200, Michael Rapoport wrote:
> > I did some performance evaluation of different threading models in 
vhost, 
> > and in most tests replacing vhost kthread's with workqueues degrades 
the 
> > performance. Moreover, having thread management inside the vhost 
provides 
> 
> There really shouldn't be any difference when using unbound
> workqueues.  workqueue becomes a convenience thing which manages
> worker pools and there shouldn't be any difference between workqueue
> workers and kthreads in terms of behavior.

I agree that there really shouldn't be any performance difference, but the 
tests I've run show otherwise. I have no idea why and I hadn't time yet to 
investigate it.
 
> > opportunity for optimization, at least for some workloads...
> 
> What sort of optimizations are we talking about?

Well, if we take Evlis (1) as for the theoretical base, there could be 
benefit of doing I/O scheduling inside the vhost.

[1] https://www.usenix.org/system/files/conference/atc13/atc13-harel.pdf

> Thanks.
> 
> -- 
> tejun

--
Sincerely yours,
Mike.