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

[Raghavendra Gowdappa]

----- Original Message -----
> From: "Rick Macklem" <rmacklem at uoguelph.ca>
> To: "Jeff Darcy" <jdarcy at redhat.com>
> Cc: "Raghavendra G" <raghavendra at gluster.com>, "freebsd-fs" <freebsd-fs at freebsd.org>, "Hubbard Jordan"
> <jkh at ixsystems.com>, "Xavier Hernandez" <xhernandez at datalab.es>, "Gluster Devel" <gluster-devel at gluster.org>
> Sent: Saturday, January 9, 2016 7:29:59 AM
> Subject: Re: [Gluster-devel] FreeBSD port of GlusterFS racks up a lot of CPU usage
> 
> 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.

I did a check on history, but couldn't find any information on why it was removed. Can you send this patch to http://review.gluster.org ? We can review and merge the patch over there. If you are not aware, development work flow can be found at:

http://www.gluster.org/community/documentation/index.php/Developers

> 
> 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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>