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Re: [PATCH v2] Saving a few cycles, part 3/3

From: Stefan Sperling <stsp_at_elego.de>
Date: Tue, 11 May 2010 13:56:55 +0200

On Tue, May 11, 2010 at 07:43:33AM -0400, Mark Phippard wrote:
> On Tue, May 11, 2010 at 7:27 AM, Stefan Sperling <stsp_at_elego.de> wrote:
> > On Tue, May 11, 2010 at 01:36:26AM +0200, Johan Corveleyn wrote:
> >> As I understand your set of patches, you're mainly focusing on saving
> >> cpu cycles, and not on avoiding I/O where possible (unless I'm missing
> >> something). Maybe some of the low- or high-level algorithms in the
> >> back-end can be reworked a bit to reduce the amount of I/O? Or maybe
> >> some clever caching can avoid some file accesses?
> >
> > In general, I think trying to work around I/O slowness by loading
> > stuff into RAM (caching) is a bad idea. You're just taking away memory
> > from the OS buffer cache if you do this. A good buffer cache in the OS
> > should make open/close/seek fast. (So don't run a windows server if
> > you can avoid it.)
> >
> > The only point where it's worth thinking about optimizing I/O
> > access is when you get to clustered, distributed storage, because
> > at that point every I/O request translated into a network packet.
>
> You had me until that last part. I think we should ALWAYS be thinking
> about optimizing I/O. I have little doubt that is where the biggest
> performance bottlenecks live (other than network of course). I agree
> that making a big cache is probably not the best way to go, but I
> think we should always be looking for optimizations where we avoid
> repeated open/closes that are not necessary.

That's true. Avoiding repeated open/close of the same file
is a good optimisation. Even with a good buffer cache it will
make a difference.

So s/The only point/One point/ :)

> I think it is extremely common that our customers have their
> repositories on NFS-mounted or SAN storage. While these often have
> fast disk subsystems there is still a noticeable penalty for file
> opens. Have you looked at Blair's wiki before?
>
> http://www.orcaware.com/svn/wiki/Server_performance_tuning_for_Linux_and_Unix

Thanks, that was an interesting read.

Of course, network filesystems like NFS have the same network
overhead penalty (except that caching on the local client is
probably a bit easier than with truly distributed storage,
but that's a minor detail).

Stefan
Received on 2010-05-11 13:57:30 CEST

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