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

From: Tejun Heo
Date: Thu Mar 31 2016 - 13:14:42 EST

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.