Re: [Qemu-devel] [PATCH 1/1] util/thread-pool: add parameter to limit maximum threads in thread pool

["Daniel P. Berrangé" <berrange@redhat.com>]

On Fri, Jul 27, 2018 at 11:18:26AM +0100, Stefan Hajnoczi wrote:
> On Fri, Jul 13, 2018 at 03:48:27PM +0800, FelixYao wrote:
> > Hi all:
> > 
> > Max threads in thread pool is fixed at 64 before which is not
> > propriate in some situations. For public cloud environment,
> > there are lots of VMs in one host machine. We should limit the worker thread
> > numbers for each machine alone based on its ability to prevent impacting
> > each other. Although we can use iotune to limit bandwidth or IOPS of block
> > device. That is not enough. Lots of worker threads still impact other VMs.
> > I think the ability to limit maximum threads in thread pool is necessary.
> > 
> > What's your opinion about this patch? please review it.
> 
> I haven't seen any discussion so here are some thoughts:
> 
> It sounds like you are trying to control I/O resources so that guests do
> not affect each other.  The threads are an implementation detail and not
> the root of the problem.
> 
> For example, if you use -drive aio=native then reads and writes will not
> use the thread pool and you will setting the thread pool size won't
> solve the problem.
> 
> Have you tried QEMU's I/O throttling feature?  You can set bandwidth
> (bps) and IOPS limits, including bursts if you wish to allow temporary
> spikes.  The basic parameters are -drive bps=BYTES_PER_SECOND,iops=IOPS.
> See the QEMU or libvirt documentation for details.

Correct me if I'm wrong, but would using the iothreads feature already
enable us to put a cap on the number of threads used for I/o operations,
but assigning dedicated threads to each device ?

Regards,
Daniel
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