Re: Weird performance decrease

[Shailendra Tripathi <stripathi@xxxxxxxxx>]

Hi Sascha,
                 Did you notice the iostat -x on the device ? Please 
verify the turnaround time of the device when you are getting the slowdown.
For example, if %b is closer towards 100, perhaps you are maxing out on 
the disk I/O ops per sec.
Since you have only one disk, once I/O becomes random, the disk wouldn't 
be able to do more than 200-250 disk ops per sec.
# iostat -x sda 1
                            extended device statistics
device mgr/s mgw/s    r/s    w/s    kr/s    kw/s   size queue   wait 
svc_t  %b
sda        3     7    3.5   18.5   157.0   112.9   12.3   0.2    9.7   
1.7   4

Regards,
Shailendra

Sascha Nitsch wrote:
> Hi,
>
> I'm observing a rather strange behaviour of the filesystem cache algorithm.
>
> I have a server running the following app scenario:
>
> A filesystem tree with a depth of 7 directories and 4 character directory 
> names.
> In the deepest directories are files.
> filesize from 100 bytes to 5kb.
> Filesystem is XFS.
>
> The app creates dirs in the tree and reads/writes files into the deepest dirs 
> in the tree.
> CPU: Dual Xeon 3.0 Ghz w/HT 512KB cache each, 2GB RAM, SCSI-HDD 15k RPM
>
> The first while, all is fine and extremely fast. After a while the buffer size 
> is about 3.5 MB
> and cache size about 618 MB.
> Until that moment ~445000 directories and ~106000 files have been created
>
> Thats where the weird behaviour starts.
>
> The buffer size drops to ~200 kb and cache size starts decreasing fast.
> This results in a drastic performace drop in my app.
> (avg. read/write times increase from 0.3ms to 4ms)
> not a constant increase, a jumping increase. During the next while it 
> constantly gets slower (19ms and more).
> After running a while (with still reducing cache size) the buffer size stays 
> at
> ~700kb and cache about 400 MB. Performane is terrible. Way slower than 
> starting up with no cache.
> restarting the app makes no change, neither remounting the partition.
>
> cmd to create the fs:
> mkfs.xfs -b size=512 -i maxpct=0 -l version=2 -n size=16k /dev/sdc
> mounting with
> mount /dev/sdc /data
>
> I'm open for suggestion on mkfs calls, mount options and kernel tuning via 
> procfs.
> I have a testcase to reproduce the problem. It happens after ~45 minutes.
>
> xfs_info /data/
> meta-data=/data                  isize=256    agcount=16, agsize=8960921 blks
>          =                       sectsz=512
> data     =                       bsize=512    blocks=143374736, imaxpct=0
>          =                       sunit=0      swidth=0 blks, unwritten=1
> naming   =version 2              bsize=16384
> log      =internal               bsize=512    blocks=65536, version=2
>          =                       sectsz=512   sunit=0 blks
> realtime =none                   extsz=65536  blocks=0, rtextents=0
>
> kernel:
> a 2.6.9-34.0.2.ELsmp #1 SMP Mon Jul 17 21:41:41 CDT 2006 i686 i686 i386 
> GNU/Linux
> filesystem usage is < 1%
>
>
>