Subject: Re: [PATCH 1/1] eventfd: implementation of EFD_MASK flag

From: Martin Sustrik <[email-masked]>
Date: Sat, 09 Feb 2013 03:40:32 +0100

Hi Eric,

On 08/02/13 23:21, Eric Wong wrote:
> Martin Sustrik<[email-masked]>  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