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Re: see higher CPU usage after increase SGA

From: Jonathan Lewis <>
Date: Wed, 9 Jun 2004 16:01:02 +0100
Message-ID: <022801c44e32$98a97430$7102a8c0@Primary>

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 The Co-operative Oracle Users' FAQ Optimising Oracle Seminar - schedule updated May 1st

: Hi,
: I once saw Jonathan said at metalink that huge SGA does not help in
: 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

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Received on Wed Jun 09 2004 - 09:58:33 CDT

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