Re: see higher CPU usage after increase SGA
- From: "Jonathan Lewis" <jonathan@xxxxxxxxxxxxxxxxxx>
- To: <oracle-l@xxxxxxxxxxxxx>
- Date: Wed, 9 Jun 2004 16:01:02 +0100
A couple of thoughts:
Increasing the db_cache_size means you may have
sufficient space to keep more clones of CR blocks,
which means that the number of clones checked for
the correct SCN could have increased - this would
use more CPU directly (checking the next clone) and
indirectly (holding the latch longer for the check causes
others to spin for longer).
Increasing the db_cache_size will also increase the
size of a 'small table', so more tablescans would go into
the middle of the LRU rather than the end. This could
have some effect on CPU usage if you previous has a
number of tablescans which just fell short of 'small'.
I can't see where the 1.5% drop in logical I/O would
come from - but it's small enough to be noise - I think.
These are ideas are just conjecture, of course, and may
have nothing to do with the real reason for your increased
CPU.
Regards
Jonathan Lewis
http://www.jlcomp.demon.co.uk
http://www.jlcomp.demon.co.uk/faq/ind_faq.html
The Co-operative Oracle Users' FAQ
http://www.jlcomp.demon.co.uk/seminar.html
Optimising Oracle Seminar - schedule updated May 1st
----- Original Message -----
From: "zhu chao" <chao_ping@xxxxxxxxxxx>
To: <oracle-l@xxxxxxxxxxxxx>
Sent: Wednesday, June 09, 2004 2:31 PM
Subject: see higher CPU usage after increase SGA
: Hi,
: I once saw Jonathan said at metalink that huge SGA does not help in
many
: case, But no further discuss at that topic later. Last night we added 1Gb
: to oracle sga and we see fewer disk read but higher CPU usage.
:
: Fewer disk read of course cut CPU usage, but larger buffer cache
: management in unix and oracle, seems caused higher CPU usage. Has someone
: also have similar experience? How to explain the higher CPU usage?
:
: We have a 16GB memory sun 880 with 10G data cache. As disk read get
: higher and higher , and not much SQL to tune we deciede to increase data
: buffer from 10G to 11GB, as there is still 1.5G free memory on the host.
:
: We expect to see some CPU usage drop, as disk read drop by 30%. But
: after 1 day's run, we saw higher CPU usage then before we increase the
SGA.
:
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