Kwan Lowe wrote:
> Michael D. Berger
> > I have a C++ program that opens, gets a little data from, and closes
> > about 5000 files. Now if I run the program when I first boot up,
> > the running time is from about 10 seconds to a minute. Subsequently,
> > the program runs in well inter one second. Buffering -- right?
> >
> > So I changed a parameter, so the program gets a different (albeit
> > similar) 5000 files from a different directory. Running time on
> > the first try: well under one second.
> >
> > How about that?
> >
> > [~]$ uname -a
> > Linux xxxxx 2.6.18-194.32.1.el5 #1 SMP Wed Jan 5 17:53:09 EST 2011 i686
> > i686 i386 GNU/Linux
>
> Buffering, certainly.. but maybe on several layers. The physical
> disk may have a read ahead cache. The OS may also buffer.
>
> You may also be pulling in libraries on the initial run which stay
> resident for awhile..
You might explore what vmstat says about disk activity during an
initial vs follow-on run. Also, I wonder what "top" shows in
the WCHAN and nFLT columns.
--
Charles Polisher
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