Re: [PATCH] rfc: threaded epoll_wait thundering herd

[Davi Arnaut <davi@haxent.com.br>]

Davide Libenzi wrote:
> On Fri, 4 May 2007, Davi Arnaut wrote:
> 
>> Hi,
>>
>> If multiple threads are parked on epoll_wait (on a single epoll fd) and
>> events become available, epoll performs a wake up of all threads of the
>> poll wait list, causing a thundering herd of processes trying to grab
>> the eventpoll lock.
>>
>> This patch addresses this by using exclusive waiters (wake one). Once
>> the exclusive thread finishes transferring it's events, a new thread
>> is woken if there are more events available.
>>
>> Makes sense?
> 
> Theorically, make sense. I said theorically because all the use 
> epoll_wait MT use cases I've heard of, use a single thread that does the 
> epoll_wait, and then dispatch to worker threads. So thundering herd is not 
> in the picture. OTOH, it does not hurt either.

A google search turns up a few users. It also addresses some complaints
from Drepper.

> But, that code is completely changed with the new single-pass epoll delivery 
> code that is in -mm. So, I'd either wait for that code to go in, or I 
> (or you, if you like) can make a patch against -mm.

Fell free to update the patch for -mm and to include my signed-of-by.
Thanks.

--
Davi Arnaut

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