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SubjectRe: XFS lstat() _very_ slow on SMP
On Wed, May 18, 2005 at 07:45:30PM +0200, Jan Kasprzak wrote:
> Christoph Hellwig wrote:
> : On Mon, May 16, 2005 at 06:25:06PM +0200, Jan Kasprzak wrote:
> : > Hi all,
> : >
> : > I have a big XFS volume on my fileserver, and I have noticed that
> : > making an incremental backup of this volume is _very_ slow. The incremental
> : > backup essentially checks mtime of all files on this volume, and it
> : > takes ~4ms of _system_ time (i.e. no iowait or what) to do a lstat().
> :
> : Thanks a lot for the report, I'll investigate what's going on once I get
> : a little time. (Early next week I hope)
>
> Hmm, I feel like I am hunting ghosts - after a fresh reboot
> of the 4-CPU server I did four runs of 128*128*128 files with various
> sizes of the underlying filesystem (in order to eliminate the volume
> size as a problematic factor). I've got the following numbers:
>
> Volume size create time find -mtime +1000 cost of lseek()
> 5GB 55m77 real 52m51 sys 1m1 real 0m53 sys 19 usecs
> 25GB 58m15 real 55m27 sys 83m47 real 82m15 sys 2171 usecs (!!!!!!)
> 125GB 67m0 real 61m35 sys 0m55 real 0m48 sys 18 usecs
> 625GB 68m30 real 62m38 sys 0m57 real 0m49 sys 18 usecs
>
> So the results are probably not dependent on the volume size,
> but on something totally random (such as which cpu the command
> ends up running on or something like that), or on the system uptime
> (and implied fragmentation of memory or what).
>
> I've tried to re-run the same test the next day (i.e. on
> server with longer uptime), but the server crashed - my test script
> ended locked up somewhere in kernel (probably holding some locks),
> and then other processes started to lock up after accessing the file
> system (my top(1) was running OK, but when I tried to "touch newfile"
> in another shell, it locked up as well). So I had to reset this server
> again.
>
> I am not really sure where exactly the problem is. I think
> it is related to XFS, big memory of this server (26 GB), four CPUs,
> and maybe even the x86_64 architecture. I was not able to reproduce
> the problem on the same HW using ext3fs, and the problem is also
> a magnitude smaller on 2-way system with 4GB of RAM. Maybe I should
> try to reproduce this on our Altix box to eliminate the x86_64 as the
> possible source of problems.
>
> I use the attached "bigtree.pl" to create the directory structure
> ("time ./bigtree.pl /new-volume 3 128" for 128*128*128 files), and then
> "strace -c find /new-volume -type f -mtime +1000 -print" (the numbers
> without strace are almost the same, so strace is not a problem here).

I couldn't reproduce the odd case here. Could you try to get some profiling
data with oprofile for the odd and one of the normal cases?
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