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SubjectRe: [PATCH] rfc: threaded epoll_wait thundering herd
On Sat, 5 May 2007, 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.
> 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.
>

*raises hand*

I'm working on event handling code for multiple projects right now, and my
method of calling epoll_wait() is to do so from several threads. I've
glanced at the epoll code but obviously haven't noticed the wake-all
behavior... good to know. I suppose I'm going to have to hack around this
problem by wrapping epoll_wait() calls in a mutex. That sucks - it means
other threads won't be able to 'get ahead' by preparing their wait before
it is their turn to dequeue events.

In any case, I think having multiple threads blocking on epoll_wait() is a
much saner idea than one thread which then passes out events, so I must
voice my support for fixing this case. Why this is the exception instead
of the norm is a little baffling, but I've seen so many perverse things in
multi-threaded code...

>
> - Davide
>

Thanks,
Chase
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