Re: [RFC] thread-pool: Add option to fix the pool size

[Daniel P . Berrangé]

On Thu, Feb 03, 2022 at 10:53:07AM +0000, Stefan Hajnoczi wrote:
> On Wed, Feb 02, 2022 at 06:52:34PM +0100, Nicolas Saenz Julienne wrote:
> > The thread pool regulates itself: when idle, it kills threads until
> > empty, when in demand, it creates new threads until full. This behaviour
> > doesn't play well with latency sensitive workloads where the price of
> > creating a new thread is too high. For example, when paired with qemu's
> > '-mlock', or using safety features like SafeStack, creating a new thread
> > has been measured take multiple milliseconds.
> > 
> > In order to mitigate this let's introduce a new option to set a fixed
> > pool size. The threads will be created during the pool's initialization,
> > remain available during its lifetime regardless of demand, and destroyed
> > upon freeing it. A properly characterized workload will then be able to
> > configure the pool to avoid any latency spike.
> > 
> > Signed-off-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>
> > 
> > ---
> > 
> > The fix I propose here works for my specific use-case, but I'm pretty
> > sure it'll need to be a bit more versatile to accommodate other
> > use-cases.
> > 
> > Some questions:
> > 
> > - Is unanimously setting these parameters for any pool instance too
> >   limiting? It'd make sense to move the options into the AioContext the
> >   pool belongs to. IIUC, for the general block use-case, this would be
> >   'qemu_aio_context' as initialized in qemu_init_main_loop().
> 
> Yes, qemu_aio_context is the main loop's AioContext. It's used unless
> IOThreads are configured.
> 
> It's nice to have global settings that affect all AioContexts, so I
> think this patch is fine for now.
> 
> In the future IOThread-specific parameters could be added if individual
> IOThread AioContexts need tuning (similar to how poll-max-ns works
> today).
> 
> > - Currently I'm setting two pool properties through a single qemu
> >   option. The pool's size and dynamic behaviour, or lack thereof. I
> >   think it'd be better to split them into separate options. I thought of
> >   different ways of expressing this (min/max-size where static happens
> >   when min-size=max-size, size and static/dynamic, etc..), but you might
> >   have ideas on what could be useful to other use-cases.
> 
> Yes, "min" and "max" is more flexible than fixed-size=n. fixed-size=n is
> equivalent to min=n,max=n. The current default policy is min=0,max=64.
> If you want more threads you could do min=0,max=128. If you want to
> reserve 1 thread all the time use min=1,max=64.
> 
> I would go with min and max.

This commit also exposes this as a new top level command line
argument. Given our aim to eliminate QemuOpts and use QAPI/QOM
properties for everything I think we need a different approach.

I'm not sure which exisiting QAPI/QOM option it most appropriate
to graft these tunables onto ?  -machine ?  -accel ?  Or is there
no good fit yet ?

Regards,
Daniel
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