Re: [PATCH v3 3/3] qapi/iothread: introduce poll-weight parameter for aio-poll

[Markus Armbruster <armbru@redhat.com>]

Jaehoon Kim <jhkim@linux.ibm.com> writes:

> Introduce a configurable poll-weight parameter for adaptive polling
> in IOThread. This parameter replaces the hardcoded POLL_WEIGHT_SHIFT
> constant, allowing runtime control over how much the most recent
> event interval affects the next polling duration calculation.
>
> The poll-weight parameter uses a shift value where larger values
> decrease the weight of the current interval, enabling more gradual
> adjustments. When set to 0, a default value of 3 is used (meaning
> the current interval contributes approximately 1/8 to the weighted
> average).
>
> This patch also removes the hardcoded default values for poll-grow
> and poll-shrink parameters from the grow_polling_time() and
> shrink_polling_time() functions, as these defaults are now properly
> initialized in iothread.c during IOThread creation.
>
> Signed-off-by: Jaehoon Kim <jhkim@linux.ibm.com>
> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

[...]

> diff --git a/qapi/misc.json b/qapi/misc.json
> index 28c641fe2f..554b0c5717 100644
> --- a/qapi/misc.json
> +++ b/qapi/misc.json
> @@ -85,6 +85,12 @@

Note: this is IOThreadInfo, used only as return value of
query-iothreads.

>  # @poll-shrink: how many ns will be removed from polling time, 0 means
>  #     that it's not configured (since 2.9)
>  #
> +# @poll-weight: the weight factor for adaptive polling.
> +#     Determines how much the current event interval contributes to
> +#     the next polling time calculation.  Valid values are 1 or
> +#     greater.  0 selects the system default value which is currently
> +#     3 (since 11.1)

As far as I can tell, query-iothreads never returns "poll-weight": 0.
So, scratch the last sentence.

> +#
>  # @aio-max-batch: maximum number of requests in a batch for the AIO
>  #     engine, 0 means that the engine will use its default (since 6.1)
>  #
> @@ -96,6 +102,7 @@
>             'poll-max-ns': 'int',
>             'poll-grow': 'int',
>             'poll-shrink': 'int',
> +           'poll-weight': 'int',
>             'aio-max-batch': 'int' } }
>  
>  ##
> diff --git a/qapi/qom.json b/qapi/qom.json
> index c653248f85..dd45ac1087 100644
> --- a/qapi/qom.json
> +++ b/qapi/qom.json
> @@ -606,6 +606,13 @@
>  #     algorithm detects it is spending too long polling without
>  #     encountering events.  0 selects a default behaviour (default: 0)
>  #
> +# @poll-weight: the weight factor for adaptive polling.  Determines
> +#     how much the most recent event interval affects the next
> +#     polling duration calculation.  If set to 0, the system default
> +#     value of 3 is used.  Typical values: 1 (high weight on recent
> +#     interval), 2-4 (moderate weight on recent interval).
> +#     (default: 0) (since 11.1)
> +#
>  # The @aio-max-batch option is available since 6.1.
>  #
>  # Since: 2.0
> @@ -614,7 +621,8 @@
>    'base': 'EventLoopBaseProperties',
>    'data': { '*poll-max-ns': 'int',
>              '*poll-grow': 'int',
> -            '*poll-shrink': 'int' } }
> +            '*poll-shrink': 'int',
> +            '*poll-weight': 'int' } }
>  
>  ##
>  # @MainLoopProperties:
> diff --git a/qemu-options.hx b/qemu-options.hx
> index 69e5a874c1..8ddf6c8d36 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -6413,7 +6413,7 @@ SRST
>  
>              CN=laptop.example.com,O=Example Home,L=London,ST=London,C=GB
>  
> -    ``-object iothread,id=id,poll-max-ns=poll-max-ns,poll-grow=poll-grow,poll-shrink=poll-shrink,aio-max-batch=aio-max-batch``
> +    ``-object iothread,id=id,poll-max-ns=poll-max-ns,poll-grow=poll-grow,poll-shrink=poll-shrink,poll-weight=poll-weight,aio-max-batch=aio-max-batch``
>          Creates a dedicated event loop thread that devices can be
>          assigned to. This is known as an IOThread. By default device
>          emulation happens in vCPU threads or the main event loop thread.
> @@ -6449,6 +6449,11 @@ SRST
>          the polling time when the algorithm detects it is spending too
>          long polling without encountering events.
>  
> +        The ``poll-weight`` parameter is the weight factor used in the
> +        adaptive polling algorithm. It determines how much the most
> +        recent event interval affects the calculation of the next
> +        polling duration.

This is now less detailed than the version in qom.json.  Care to add the
details here, too?

> +
>          The ``aio-max-batch`` parameter is the maximum number of requests
>          in a batch for the AIO engine, 0 means that the engine will use
>          its default.

[...]