linux-kernel - Re: 2 epoll questions

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Date:	Tue, 30 Nov 2010 08:03:51 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Mike Blumenkrantz <mike@...tific.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: 2 epoll questions

Le lundi 29 novembre 2010 à 19:17 -0500, Mike Blumenkrantz a écrit :
> Hi,
> 
> I am not subscribed to the list yet, and I would appreciate it if all replies
> could be CCed to me :).
> 
> I have two questions regarding epoll.  The epoll code that I am using can all
> be found in the file here for those interested:
> https://svn.enlightenment.org/svn/e/trunk/ecore/src/lib/ecore/ecore_main.c
> 
> 1) Is there any known reason why adding fds to an epoll fd would begin to slow
> down dramatically after ~6000 fds are added?  It is entirely possible that the
> problem is not epoll at all, but I wanted to ask and see if there was some O(n)
> code somewhere that was known.
> 

No particular slowdown, I once used more than 1.5 million fds in epoll
fd.

Of course, if you only add/remove fds as fast as possible, its going to
be slower, since epoll fds are stored in a tree, and the time of
insert/delete of one item depends on the tree depth. Maybe this is your
case and your working set is larger than your cpu caches after ~6000
fds ?


> 2) If a select on an epoll fd (not one inside the epoll, the actual epoll fd
> itself) returns EBADF, is there a way to determine which fd is causing the
> error and then remove it?  I am occasionally running into this issue, and it
> seems to be caused by code which I do not necessarily control closing a fd that
> is part of my epoll fd without my knowledge (meaning that the closed fd is
> still in epoll).

Strange, I never got this EBADF error. On epoll_ctl() or epoll_wait() ?
(I dont understand your "select on epoll fd"). epoll is meant to replace
select()...

When you close a file descriptor, its automatically removed from epoll
as well.



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