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

[Martin Sustrik]

Hi Eric,

On 08/02/13 23:21, Eric Wong wrote:
> Martin Sustrik<sustrik@xxxxxxxxxx>  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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