linux-kernel - Re: [PATCH] eventfd: support delayed wakeup for non-semaphore eventfd to reduce cpu utilization

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Message-ID: <7dded5a8-32c1-e994-52a0-ce32011d5e6b@kernel.dk>
Date:   Tue, 18 Apr 2023 20:15:03 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Wen Yang <wenyang.linux@...mail.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>
Cc:     Christoph Hellwig <hch@....de>, Dylan Yudaken <dylany@...com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Paolo Bonzini <pbonzini@...hat.com>, Fu Wei <wefu@...hat.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] eventfd: support delayed wakeup for non-semaphore eventfd
 to reduce cpu utilization

On 4/17/23 10:32?AM, Wen Yang wrote:
> 
> ? 2023/4/17 22:38, Jens Axboe ??:
>> On 4/16/23 5:31?AM, wenyang.linux@...mail.com wrote:
>>> From: Wen Yang <wenyang.linux@...mail.com>
>>>
>>> For the NON SEMAPHORE eventfd, if it's counter has a nonzero value,
>>> then a read(2) returns 8 bytes containing that value, and the counter's
>>> value is reset to zero. Therefore, in the NON SEMAPHORE scenario,
>>> N event_writes vs ONE event_read is possible.
>>>
>>> However, the current implementation wakes up the read thread immediately
>>> in eventfd_write so that the cpu utilization increases unnecessarily.
>>>
>>> By adding a configurable delay after eventfd_write, these unnecessary
>>> wakeup operations are avoided, thereby reducing cpu utilization.
>> What's the real world use case of this, and what would the expected
>> delay be there? With using a delayed work item for this, there's
>> certainly a pretty wide grey zone in terms of delay where this would
>> perform considerably worse than not doing any delayed wakeups at all.
> 
> 
> Thanks for your comments.
> 
> We have found that the CPU usage of the message middleware is high in
> our environment, because sensor messages from MCU are very frequent
> and constantly reported, possibly several hundred thousand times per
> second. As a result, the message receiving thread is frequently
> awakened to process short messages.
> 
> The following is the simplified test code:
> https://github.com/w-simon/tests/blob/master/src/test.c
> 
> And the test code in this patch is further simplified.
> 
> Finally, only a configuration item has been added here, allowing users
> to make more choices.

I think you'd have a higher chance of getting this in if the delay
setting was per eventfd context, rather than a global thing.

-- 
Jens Axboe