From: Eric Wong <normalperson@...>
Date: 2013-08-17T19:37:43+00:00
Subject: [ruby-core:56700] Re: [ruby-trunk - Feature #8788][Open] use eventfd on newer Linux instead of pipe for timer thread

SASADA Koichi <ko1@atdot.net> wrote:
> (2013/08/16 10:47), normalperson (Eric Wong) wrote:
> > eventfd is a cheaper alternative to pipe for self-notification (signals) on Linux
> > 
> > I will submit patches in the next few days/weeks unless there are objections
> > (or somebody else wants to do it sooner).  I'd also like to cleanup some of the existing #ifdefs in that area while I'm at it.
> 
> Can we see the performance comparison?
> If we can see the clear difference, it can be acceptable.

It's not for speed (signal handling performance should not be a
bottleneck), but halve FD use in userspace and reduce memory use inside
the kernel.

AFAIK, writing to a empty pipe still allocates a 4K page, eventfd avoids
that allocation/deallocation.  Since Ruby is CoW/fork-friendly, this
should allow running more Ruby processes on a system.

I also thought my own code had an FD leak when timer_thread_pipe_low was
introduced.  Maybe this will reduce confusion for users who lsof Ruby
processes, since there are more pipe users than eventfd users.

> (If we can't see any difference, it only increase the source code
> complexity).

I've tried to minimize the impact of my patch and keep the eventfd/pipe
difference minimal.