btrfs-cleaner leaves CPU pinned to 100%

btrfs-cleaner leaves CPU pinned to 100%

[Adam G]

Hello,

After a recent kernel upgrade I noticed the fans blowing at full
throttle on my laptop.

There is currently very low load on the machine and nothing out of the
ordinary has occurred to lead me to believe this is normal behavior.

Here is a link to the perf output I managed to capture

http://pastebin.com/a5cAy7Dw

Please let me know if I may be of any further assistance.

Regards,

Adam Gradzki

Re: btrfs-cleaner leaves CPU pinned to 100%

[Adam Gradzki]

Adam G <adam.gradzki <at> gmail.com> writes:

> 
> Hello,
> 
> After a recent kernel upgrade I noticed the fans blowing at full
> throttle on my laptop.
> 
> There is currently very low load on the machine and nothing out of the
> ordinary has occurred to lead me to believe this is normal behavior.
> 
> Here is a link to the perf output I managed to capture
> 
> http://pastebin.com/a5cAy7Dw
> 
> Please let me know if I may be of any further assistance.
> 
> Regards,
> 
> Adam Gradzki
> --
> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
> the body of a message to majordomo <at> vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> 
> 

I would like to add an updated perf report I took shortly after the first 
one yesterday (600 seconds this time).

http://www.pastebin.ca/2492119

Removing autodefrag from /etc/fstab has ameliorated my troubles for the time 
being.

Re: btrfs-cleaner leaves CPU pinned to 100%

[Josef Bacik]

Adam Gradzki <adam.gradzki@gmail.com> wrote on Wed [2013-Dec-04 15:27:58 +0000]:
> Adam G <adam.gradzki <at> gmail.com> writes:
> 
> > 
> > Hello,
> > 
> > After a recent kernel upgrade I noticed the fans blowing at full
> > throttle on my laptop.
> > 
> > There is currently very low load on the machine and nothing out of the
> > ordinary has occurred to lead me to believe this is normal behavior.
> > 
> > Here is a link to the perf output I managed to capture
> > 
> > http://pastebin.com/a5cAy7Dw
> > 
> > Please let me know if I may be of any further assistance.
> > 
> > Regards,
> > 
> > Adam Gradzki
> > --
> > To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
> > the body of a message to majordomo <at> vger.kernel.org
> > More majordomo info at  http://vger.kernel.org/majordomo-info.html
> > 
> > 
> 
> I would like to add an updated perf report I took shortly after the first 
> one yesterday (600 seconds this time).
> 
> http://www.pastebin.ca/2492119
> 
> Removing autodefrag from /etc/fstab has ameliorated my troubles for the time 
> being.
> 

Do you have a lot of snapshots?  I'm working on this currently so hopefully it
will be fixed soon.  If you have issues again can you use perf record -g so it
pulls the stacktraces and then you can drill down before you post so I can
figure out where we're spending all of our time.  Thanks,

Josef

Re: btrfs-cleaner leaves CPU pinned to 100%

[Adam G]

Good news:

I recreated the problem by booting with autodefrag enabled and loading
a large VirtualBox image of Windows 8.1. The CPU is pinned to 100%
even after closing the VirtualBox software.

Here is the perf with your suggested configuration:

http://pastebin.ca/2492270

~ Adam Gradzki

On Wed, Dec 4, 2013 at 6:58 PM, Adam G <adam.gradzki@gmail.com> wrote:
> Hi Josef,
>
> I do not have any snapshots at the moment. This is a new filesystem
> created with the 3.12 kernel last week. The filesystem is on a laptop
> that sees very little load on the file system and there is plenty of
> free space available on my two btrfs partitions.
>
> I will reboot with autodefrag enabled in fstab right now to see if I
> can capture the stacktraces for you.
>
> Regards,
> Adam Gradzki

Re: btrfs-cleaner leaves CPU pinned to 100%

[Duncan]

Adam G posted on Wed, 04 Dec 2013 19:19:15 -0500 as excerpted:

> I recreated the problem by booting with autodefrag enabled and loading a
> large VirtualBox image of Windows 8.1. The CPU is pinned to 100%
> even after closing the VirtualBox software.

Virtual images!

That's your problem!  Any large and frequently internally written files 
(as opposed to appended such as logs or write-once files such as most 
media files), including both large databases and virtual machine images, 
are going to be an issue on any COW (copy-on-write) filesystem such as 
btrfs, since they'll very quickly become very heavily fragmented due to 
the constant internal writes creating fragments.

And virtual machine images are a classic example, often being several 
gigs (if not tens of gigs) in size, with frequent writes to data in the 
center of the image.

As a consequence, it's strongly recommended that VM images are set NOCOW, 
so writes will happen in-place, instead of fragmenting the image.  
Without that... well, the poor defragmenter was working overtime to try 
and fix the fragments, and all those fragments were certainly dragging 
down your VM performance as well.

For curiosity sake, you might try running filefrag on that image and see 
just how fragmented it was.

As for setting NOCOW, note that the attribute must be set before the file 
has any data in it.  There are two common ways to do that.  The easy way 
is to create a directory for your VM images and set the attribute on it, 
then copy the vm images into it.  An alternative is to touch the 
individual file to create it at zero size, set the attribute, then copy 
the data into it from the existing file.

After doing that with all your VM images and deleting the old/fragmented 
copies, the pegged cpu defrag thread issue should disappear or at least 
dramatically reduce, and you may well notice a performance boost in your 
VMs as well. =:^)

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman