From: Ayson, Alison {Info~Palo Alto} (ALISON.AYSON@ROCHE.COM)
Date: Sat Sep 07 2002 - 13:23:07 EDT
Thanks for the many many replies. I now have the answers that I needed and
I know exactly what's wrong (but what to do about it is still in question).
I won't include my whole post as it was pretty long. Basically our DBA's
were running into problems with "out of memory" errors while trying to
install a third instance. (HP-UX 11.0, Oracle 8.1.7/32 bit). I noticed
that dynamic buffer caching was set very high (50% of RAM) and I wonder if
that was causing my problems. Here is a list of my questions and the
answers:
1) Why is the dynamic caching set so high? (dbc_min_pct=5,
dbc_max_pct=50).
Answer: These are the default values. There was 100% agreement that these
values are way too high. The most common recommendation was to change these
to dbc_min_pct=2 and dbc_max_pct=15.
2) Is dynamic buffer cache able to dynamically deallocate it's memory
should there be a demand for it from another process?
Answer: Wellll....yeah, supposedly. But I got many replies which claimed
that this doesn't always work that well.
3) Could the large dynamic buffer cache reponsible for the memory problems
with Oracle?
No. The problem is with Oracle 32 bit. From Bill Hassell:
"The problem is that all instances of 32bit versions of Oracle must share
one single space for SGA and the default is 960 megs, with some relinking
options, 1750 megs MAX. You can put 20Gb of RAM in the machine and those
programs must share that same 1750megs of RAM."
I checked the databases sizes and they are configured to use roughly 592
megs of Shared Memory. If you multiply 592 by 3 (the number of instances we
are trying to run) you get 1776 meg which is too large to fit in the 1.75 GB
of Share memory available.
So what to do? Well I have three choices:
1) Get the DBA's to review their SGA settings. Can they be lowered? (<--
this is more of a "quick fix")
2) Upgrade to Oracle 64 bit which does not have this memory limitation.
3) Set up memory windows. Bill Hassell mentioned this so casually but I'd
never heard of "memory windows". I found several whitepapers describing how
to set up memory windows at HP's website "www.docs.hp.com". Bill's
explanation of how this could help my particular situation:
"Setup memory windows. Each instance will now run in it's own 1750 meg
prvate space, thus SGA can be 900 megs or 1500 megs and there will be no
problem in allocating sort areas, room for in-memory insertions, etc.
Now the DBAs will go crazy because they can eliminate a huge amount of disk
I/O by making SGA very large, but if each instance uses 1500 megs of shared
memory (which will work fine due to virtual memory handling), there will be
some swapping which degrades the performance gains in SGA."
Thanks again for all the helpful replies. I've learned a lot from the
emails I received on this particular problem.
-- Alison Ayson
Roche Bioscience
Palo Alto, CA
(650) 855-5425
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