[Gluster-devel] FreeBSD port of GlusterFS racks up a lot of CPU usage

[Rick Macklem]

Jeff Darcy wrote:
> > > I don't know anything about gluster's poll implementation so I may
> > > be totally wrong, but would it be possible to use an eventfd (or a
> > > pipe if eventfd is not supported) to signal the need to add more
> > > file descriptors to the poll call ?
> > >
> > >
> > > The poll call should listen on this new fd. When we need to change
> > > the fd list, we should simply write to the eventfd or pipe from
> > > another thread.  This will cause the poll call to return and we will
> > > be able to change the fd list without having a short timeout nor
> > > having to decide on any trade-off.
> > 
> >
> > Thats a nice idea. Based on my understanding of why timeouts are being
> > used, this approach can work.
> 
> The own-thread code which preceded the current poll implementation did
> something similar, using a pipe fd to be woken up for new *outgoing*
> messages.  That code still exists, and might provide some insight into
> how to do this for the current poll code.
I took a look at event-poll.c and found something interesting...
- A pipe called "breaker" is already set up by event_pool_new_poll() and
  closed by event_pool_destroy_poll(), however it never gets used for anything.

So, I added a few lines of code that writes a byte to it whenever the list of
file descriptors is changed and read when poll() returns, if its revents is set.
I also changed the timeout to -1 (infinity) and it seems to work for a trivial
test.
--> Btw, I also noticed the "changed" variable gets set to 1 on a change, but
    never reset to 0. I didn't change this, since it looks "racey". (ie. I
    think you could easily get a race between a thread that clears it and one
    that adds a new fd.)

A slightly safer version of the patch would set a long (100msec ??) timeout instead
of -1.

Anyhow, I've attached the patch in case anyone would like to try it and will
create a bug report for this after I've had more time to test it.
(I only use a couple of laptops, so my testing will be minimal.)

Thanks for all the help, rick

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