Re: udevd is killing file write performance.

[David Chinner <dgc@sgi.com>]

On Thu, Feb 23, 2006 at 06:56:39AM -0600, Robin Holt wrote:
> On Wed, Feb 22, 2006 at 12:05:47PM -0800, Andrew Morton wrote:
> > Robin Holt <holt@sgi.com> wrote:
> > >
> > > Let me reiterate, I know _VERY_ little about filesystems.  Can the
> > >  dentry->d_lock be changed to a read/write lock?
> > 
> > Well, it could, but I suspect that won't help - the hold times in there
> > will be very short so the problem is more likely acquisition frequency.
> > 
> > However it's a bit strange that this function is the bottleneck.  If their
> > workload is doing large numbers of reads or writes from large numbers of
> > processes against the same file then they should be hitting heavy
> > contention on other locks, such as i_sem and/or tree_lock and/or lru_lock
> > and others.
> > 
> > Can you tell us more about the kernel-visible behaviour of this app?
> 
> I looked at a little more of the output.  All I have to go on is a few
> back traces generated by kdb of the entire system a few seconds apart.
> 
> In all of the traces, the first chunk of cpus are the only ones doing
> writes.  I have not counted exactly, but I think it is around 32.  There
> may be more or less, but that is the feeling (sometimes they are doing
> reads as well).

Robin, is the app doing direct or buffered I/O?

Cheers,

Dave.
-- 
Dave Chinner
R&D Software Enginner
SGI Australian Software Group