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

[Stefan Hajnoczi]

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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