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

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Message-ID: <tencent_9D8583482619D25B9953FCA89E69AA92A909@qq.com>
Date:   Tue, 18 Apr 2023 00:32:00 +0800
From:   Wen Yang <wenyang.linux@...mail.com>
To:     Jens Axboe <axboe@...nel.dk>,
        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


在 2023/4/17 22:38, Jens Axboe 写道:
> 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.


--

Best wishes,

Wen