On Thu, Feb 03, 2022 at 10:56:49AM +0000, Daniel P. Berrangé wrote: > 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 ?
Yep, I didn't comment on this because I don't have a good suggestion. In terms of semantics I think we should have: 1. A global default value that all new AioContext take. The QEMU main loop's qemu_aio_context will use this and all IOThread AioContext will use it (unless they have been overridden). I would define it on --machine because that's the "global" object for a guest, but that's not very satisfying. 2. (Future patch) --object iothread,thread-pool-min=N,thread-pool-max=M just like poll-max-ns and friends. This allows the values to be set on a per-IOThread basis. Stefan
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