linux-kernel - Re: PROBLEM: Epoll frequently fails to notify connects at connect bursts

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Date:	Tue, 25 Aug 2009 18:14:55 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Will Brown <dr.hfuhruhurr@...il.com>
cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: PROBLEM: Epoll frequently fails to notify connects at connect
 bursts

On Wed, 26 Aug 2009, Will Brown wrote:

> PLEASE CC: ME ON ANY REPLIES, thank you.
> 
> *** 1. Epoll frequently fails to notify connects at connect bursts
> 
> *** 2. Sometimes, when several connects come in quickly, epoll
> establishes tcp connections (according to 'netstat') but then fails to
> notify the application by returning from epoll_wait(). In those cases
> a subsequent connection or an EPOLLIN or EPOLLOUT event can cause a
> return from epoll_wait(), which then causes the previously ignored
> connection to be reported. But then the more recent event is not
> notified, the whole sequence lags and everything gets out of whack.
> The problems can easily be reproduced on my Ubuntu machine which runs
> a kernel version 2.6.28 using supplied test program. I also have a
> Ubuntu hosted in VMWare running a 2.6.27 version kernel, which appears
> to NOT have the same problem.

Multiple quasi-simultaneous events for the same fd are merged. This is why 
on POLLIN on accepting sockets you want to do something like:

	while ((afd = accept(sfd, ...)) != -1)
		add_client(afd);



- Davide


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