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 <nsaen...@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 -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|