[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: "Raghavendra Gowdappa" <rgowdapp at redhat.com>
> Cc: "Jeff Darcy" <jdarcy at redhat.com>, "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: Tuesday, January 19, 2016 4:07:09 AM
> Subject: Re: [Gluster-devel] FreeBSD port of GlusterFS racks up a lot of CPU usage
> 
> Raghavendra Gowdappa wrote:
> > 
> > 
> > ----- 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
> > 
> Actually, the patch turned out to be a flop. Sometimes a fuse mount would end
> up with an empty file system with the patch. (I don't know why it was broken,
> but maybe the original author tan into issues as well?)

+static void
+event_pool_changed (struct event_pool *event_pool)
+{
+
+        /* Write a byte into the breaker pipe to wake up poll(). */
+        if (event_pool->breaker[1] >= 0)
+                write(event_pool->breaker[1], "X", 1);
+}

breaker is set to non-blocking on both read and write ends. So, probably write might be failing sometimes with EAGAIN/EBUSY and thereby preventing the socket from being registered. Probably that might be the reason?

        if (event_pool->breaker[1] >= 0) {
                 do {
                        ret = write(event_pool->breaker[1], "X", 1);
                 } while (ret != 1);
        }

Also similar logic might be required while flushing out junk from read end too.

> 
> Anyhow, I am now using the 3.7.6 event-poll.c code except that I have
> increased
> the timeout from 1msec->10msec. (Going from 1->5->10 didn't seem to cause a
> problem, but I got slower test runs when I increased to 20msec, so I've
> settled on
> 10mses. This does reduce the CPU usage when the GlusterFS file systems aren't
> active.)
> I will submit this one line change to your workflow if it continues to test
> ok.
> 
> Thanks for everyone's input, rick
> 
> > > 
> > > 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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> > > > 
> > > 
> > 
>