Re: [PATCH RFC v2 3/3] qapi/iothread: introduce poll-weight parameter for aio-poll

[Markus Armbruster <armbru@redhat.com>]

JAEHOON KIM <jhkim@linux.ibm.com> writes:

> On 3/25/2026 9:04 AM, Markus Armbruster wrote:
>> 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>

[...]

>>> diff --git a/qapi/qom.json b/qapi/qom.json
>>> index c653248f85..feb80b6cfe 100644
>>> --- a/qapi/qom.json
>>> +++ b/qapi/qom.json
>>> @@ -606,6 +606,11 @@
>>>  #     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 current event interval contributes to
>>> +#     the next polling time calculation.  Valid values are 1 or
>>> +#     greater.  If set to 0, the default value of 3 is used.
>>
>> The commit message hints what the valid values mean, the doc comment
>> doesn't even that.  Do users need to know?
>>
>> Code [*] below uses it like time >> poll_weight, where @time is int64_t.
>> poll_weight > 63 is undefined behavior, which is a no-no.  Please reject
>> such values.  poll_weight == 64 results in zero.  Is that useful?
>>
>> Missing: (default: 0) (since 11.1)
>
> I agree. I will update the doc comment to give users a practical hint,
> for example like this:
>
> # @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)

Better, thanks!

> I will also a check in the code so that values exceeding the maximum
> allowed will revert to the system default.

Don't silently "correct" invalid input, reject it!

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