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. > > Some background on my workload: I'm using IDE emulation, the guest is an > old RTOS that doesn't support virtio, using 'aio=native' isn't possible > either (unaligned IO accesses). I thought QEMU's block layer creates bounce buffers for unaligned accesses, handling both memory buffer alignment and LBA alignment necessary for aio=native,cache=none?
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