linux-kernel - Re: epoll behaviour after running out of descriptors

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Date:	Sun, 02 Nov 2008 20:10:37 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Olaf van der Spek <olafvdspek@...il.com>
CC:	Davide Libenzi <davidel@...ilserver.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: epoll behaviour after running out of descriptors

Olaf van der Spek a écrit :
> On Sun, Nov 2, 2008 at 7:48 PM, Davide Libenzi <davidel@...ilserver.org> wrote:
>> Why don't you grep for TIME_WAIT?
> 
> Because I don't have access to the test environment at the moment.

Hello Olaf

If your application calls accept() and accept() returns EMFILE, its a nullop.

On listen queue, socket is still ready for an accept().


Since you use edge trigered epoll, you'll only reveive new notification.

You probably had in you app a : listen(sock, 10), so after 10 notifications,
your listen queue is full and TCP stack refuses to handle new connections.

In order to cope with this kind of thing the trick I personnally use is to always keep
around a *free* fd, that is :

At start of program, reserve an "emergency fd"
free_fd = open("/dev/null", O_RDONLY)

Then later :

newfd = accept(...)
if (newfd == -1 && errno == EMFILE) {
   /* emergency action : clean listen queue */
   close(free_fd);
   newfd = accept(...);
   close(newfd); /* forget this incoming connection, we dont have enough fd */
   free_fd = open("/dev/null"; O_RDONLY);
}

Of course, if your application is multi-threaded, you might adapt (and eventually reserve
one emergency fd per thread)


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