netdev - Re: [PATCH 1/1] eventfd: implementation of EFD

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Date:	Sat, 09 Feb 2013 03:40:32 +0100
From:	Martin Sustrik <sustrik@...bpm.com>
To:	Eric Wong <normalperson@...t.net>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Sha Zhengju <handai.szj@...bao.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org
Subject: Re: [PATCH 1/1] eventfd: implementation of EFD_MASK flag

Hi Eric,

On 08/02/13 23:21, Eric Wong wrote:
> Martin Sustrik<sustrik@...bpm.com>  wrote:
>> On 07/02/13 23:44, Andrew Morton wrote:
>>> That's a nice changelog but it omitted a critical thing: why do you
>>> think the kernel needs this feature?  What's the value and use case for
>>> being able to poll these descriptors?
>>
>> To address the question, I've written down detailed description of
>> the challenges of the network protocol development in user space and
>> how the proposed feature addresses the problems.
>>
>> It's too long to fit into ChangeLog, but it may be worth reading
>> when trying to judge the merit of the patch.
>>
>> It can be found here: http://www.250bpm.com/blog:16
>
> Using one eventfd per userspace socket still seems a bit wasteful.

Wasteful in what sense? Occupying a slot in file descriptor table? 
That's the price for having the socket uniquely identified by the fd.

> Couldn't you use a single pipe for all sockets and write the efd_mask to
> the pipe for each socket?
>
> A read from the pipe would behave like epoll_wait.
>
> You might need to use one-shot semantics; but that's probably
> the easiest thing in multithreaded apps anyways.

Having multiple sockets represented by a single eventfd. how would you 
distinguish where did individual events came from?

   struct pollfd pfd;
   ...
   poll (pfd, 1, -1);
   if (pfd.revents & POLLIN) /* Incoming data on which socket? */
     ...

Martin
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