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Re: [RFC] thread-pool: Add option to fix the pool size
From: |
Stefan Hajnoczi |
Subject: |
Re: [RFC] thread-pool: Add option to fix the pool size |
Date: |
Thu, 3 Feb 2022 10:53:07 +0000 |
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.
>
> 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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- [RFC] thread-pool: Add option to fix the pool size, Nicolas Saenz Julienne, 2022/02/02
- Re: [RFC] thread-pool: Add option to fix the pool size,
Stefan Hajnoczi <=
- Re: [RFC] thread-pool: Add option to fix the pool size, Daniel P . Berrangé, 2022/02/03
- Re: [RFC] thread-pool: Add option to fix the pool size, Stefan Hajnoczi, 2022/02/03
- Re: [RFC] thread-pool: Add option to fix the pool size, Nicolas Saenz Julienne, 2022/02/11
- Re: [RFC] thread-pool: Add option to fix the pool size, Kevin Wolf, 2022/02/11
- Re: [RFC] thread-pool: Add option to fix the pool size, Stefan Hajnoczi, 2022/02/14
- Re: [RFC] thread-pool: Add option to fix the pool size, Kevin Wolf, 2022/02/14
- Re: [RFC] thread-pool: Add option to fix the pool size, Stefan Hajnoczi, 2022/02/14
- Re: [RFC] thread-pool: Add option to fix the pool size, Kevin Wolf, 2022/02/14
Re: [RFC] thread-pool: Add option to fix the pool size, Nicolas Saenz Julienne, 2022/02/07