Date: Thu, 31 Mar 2016 13:14:35 -0400
From: Tejun Heo <tj@kernel.org>
To: Michael Rapoport <RAPOPORT@il.ibm.com>
Cc: Bandan Das <bsd@redhat.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org, mst@redhat.com, jiangshanlai@gmail.com
Subject: Re: [RFC PATCH 0/4] cgroup aware workqueues
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Hello, Michael.

On Thu, Mar 31, 2016 at 08:17:13AM +0200, Michael Rapoport wrote:
> > 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.

I'd be happy to help digging into what's going on.  If kvm wants full
control over the worker thread, kvm can use workqueue as a pure
threadpool.  Schedule a work item to grab a worker thread with the
matching attributes and keep using it as it'd a kthread.  While that
wouldn't be able to take advantage of work item flushing and so on,
it'd still be a simpler way to manage worker threads and the extra
stuff like cgroup membership handling doesn't have to be duplicated.

> > > 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.

Yeah, if that actually is beneficial, take full control of the
kworker thread.

Thanks.

-- 
tejun