|Date: ||Tue, 9 Sep 2003 13:35:05 -0400|
|Reply-To: ||Michael Raithel <RAITHEM@WESTAT.COM>|
|Sender: ||"SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>|
|From: ||Michael Raithel <RAITHEM@WESTAT.COM>|
|Subject: ||Re: Index performance question|
The amazingly prolific David Cassell posted the following, in reply to my
own musings on this topic:
> I was wondering the same thing myself, Mikeeeeee. It seems
> to me that there is another possibility. The OP may be using
> caching controllers for his hard drives, and the information
> may still be cached in the RAM of the controller so that the
> OS is handed the info without having to go hunt on the drive again.
> But (if this wild, unsubstantiated speculation is true) this
> means that an early DATA _NULL_ to get the info into the
> controller cache won't help when more work is done later,
> obliterating the controller cache with new data from
> different drive sectors. This leads to a logical test. The
> OP can do the same test as before, but adding a fast read of,
> say, the first 2 Megs of the file in between the early
> indexing and the second indexing. We might see the time
> required to do the second index suddenly jump back up to what
> we would normally expect.
David, well they don't call you "Senior Computing Specialist" for nothing;
that's for sure! Just before reading your own post, I received the
following from a "birdie" that is a long-time technical guru of note:
"If the operating system is Windows or Unix, this is most likely due to file
caching by the operating system. SAS doesn't do any file caching between
steps unless you use the SASFILE in-memory option."
So, it looks like you hit that nail directly on the head with your "wild,
unsubstantiated speculation." Not too shabby. So, could you please send me
a "wild, unsubstantiated speculation" on the next set of winning Powerball
numbers in a private reply?
I hope that this suggestion proves helpful now, and in the future!
Of course, all of these opinions and insights are my own, and do not reflect
those of my organization or my associates. All SAS code and/or methodologies
specified in this posting are for illustrative purposes only and no warranty
is stated or implied as to their accuracy or applicability. People deciding
to use information in this posting do so at their own risk.
Michael A. Raithel
"The man who wrote the book on performance"
Author: Tuning SAS Applications in the MVS Environment
Author: Tuning SAS Applications in the OS/390 and z/OS Environments, Second
All programmers are playwrights and all computers are
lousy actors. - Unknown