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

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Message-ID: <alpine.DEB.2.00.0908251823470.18518@makko.or.mcafeemobile.com>
Date:	Tue, 25 Aug 2009 18:24:47 -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 Tue, 25 Aug 2009, Davide Libenzi wrote:

> 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);

Of course, with  sfd  in non-blocking mode. Alternatively, you drop EPOLLET 
for the  sfd  socket.


- Davide


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