Re: Performance Issues

[Josef Bacik <jbacik@fb.com>]

On 09/19/2014 11:51 AM, Rob Spanton wrote:
> Hi,
>
> Thanks for the response everyone.
>
> I wrote:
>> I have a particularly uncomplicated setup (a desktop PC with a hard
>> disk) and I'm seeing particularly slow performance from btrfs.  A `git
>> status` in the linux source tree takes about 46 seconds after dropping
>> caches, whereas on other machines using ext4 this takes about 13s.  My
>> mail client (evolution) also seems to perform particularly poorly on
>> this setup, and my hunch is that it's spending a lot of time waiting on
>> the filesystem.
>
> The evolution problem has been improved: the sqlite db that it was using
> had over 18000 fragments, so I got evolution to recreate that file with
> nocow set.  It now takes "only" 30s to load my mail rather than 80s,
> which is better...
>
> On Fri, 2014-09-19 at 11:05 -0400, Josef Bacik wrote:
>> Weird, I get the exact opposite performance.  Anyway it's probably
>> because of your file layouts, try defragging your git dir and see if
>> that helps.  Thanks,
>
> Defragging has improved matters a bit: it now takes 26s (was 46s) to run
> git status.  Still not amazing, but at the moment I have no evidence to
> suggest that it's not something to do with the machine's hardware.  If I
> get time over the weekend I'll dig out an external hard disk and try a
> couple of benchmarks with that.
>
> For reference, these are the mount flags:
>          /dev/sda4 on / type btrfs (rw,noatime,space_cache)
>          /dev/sda4 on /home type btrfs (rw,noatime,space_cache)
>

You have an awful lot of metadata, do you have a lot of snapshots?  Also 
I'd be interested in making sure most of this is just from shitty 
metadata layout, could you make sure you have a recent version of 
trace-cmd and then drop caches and do

trace-cmd record -e sched:sched_switch git status

and send me the trace.dat so I can see where all the time is spent?  Thanks,

Josef