Re: [PATCH v4 3/3] util/event-loop-base: Introduce options to set the thread pool size

[Markus Armbruster <armbru@redhat.com>]

Nicolas Saenz Julienne <nsaenzju@redhat.com> writes:

> 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 'EventLoopBase'
> property to set the thread pool size. The threads will be created during
> the pool's initialization or upon updating the property's value, 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 spikes.
>
> Signed-off-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>
> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

[...]

> diff --git a/qapi/qom.json b/qapi/qom.json
> index e5f31c4469..06b8c3d10b 100644
> --- a/qapi/qom.json
> +++ b/qapi/qom.json
> @@ -506,9 +506,17 @@
>  #
>  # @aio-max-batch: maximum number of requests in a batch for the AIO engine,
>  #                 0 means that the engine will use its default.
> +#
> +# @thread-pool-min: minimum number of threads readily available in the thread
> +#                   pool (default:0, since 7.1)

What do you mean by "readily available in the thread pool"?

> +#
> +# @thread-pool-max: maximum number of threads the thread pool can contain
> +#                   (default:64, since 7.1)
>  ##

If you add "Since: 7.1" in the previous patch, then the "since 7.1" here
need to go.

>  { 'struct': 'EventLoopBaseProperties',
> -  'data': { '*aio-max-batch': 'int' } }
> +  'data': { '*aio-max-batch': 'int',
> +            '*thread-pool-min': 'int',
> +            '*thread-pool-max': 'int' } }
>  
>  ##
>  # @IothreadProperties:

[...]