linux-kernel - epoll and multiple processes - eliminate unneeded process wake-ups

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Date:	Mon, 13 Jul 2015 15:34:12 +0300
From:	Madars Vitolins <m@...odev.com>
To:	<linux-kernel@...r.kernel.org>
Subject: epoll and multiple processes - eliminate unneeded process wake-ups

Hi Folks,

I am developing kind of open systems application, which uses multiple 
processes/executables where each of them monitors some set of resources 
(in this case POSIX Queues) via epoll interface. For example when 10 
processes on same queue are in state of epoll_wait() and one message 
arrives, all 10 processes gets woken up and all of them tries to read 
the message from Q. One succeeds, the others gets EAGAIN error. The 
problem is with those others, which generates extra context switches - 
useless CPU usage. With more processes inefficiency gets higher.

I tried to use EPOLLONESHOT, but no help. Seems this is suitable for 
multi-threaded application and not for multi-process application.

Ideal mechanism for this would be:
1. If multiple epoll sets in kernel matches same event and one or more 
processes are in state of epoll_wait() - then send event only to one 
waiter.
2. If none of processes are in wait state, then send the event to all 
epoll sets (as it is currently). Then the first free process will grab 
the event.

How do you think, would it be real to implement this? How about 
concurrency?
Can you please give me some hints from which points in code to start to 
implement these changes?

Thanks a lot in advance,
Madars
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