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Re: Is it better to set the freelist to a prime number ?

From: Jonathan Lewis <>
Date: Wed, 24 Jul 2002 17:15:47 +0100
Message-ID: <>

Now there's an interesting point.

Unless I missed something, I was thinking that the PID referenced was v$ (the Oracle process id) whereas you seem to be suggesting that it is v$process.spid (the operating system process id). Do you want to do the test, or shall I ? I have a free slot some time next March ;)

(Note too, that your argument would only require an ODD number of freelists, not
a prime number).

I agree with your comment about pattern
avoidance, though.

Jonathan Lewis

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Andrew Mobbs wrote in message ...

>Jonathan Lewis <> wrote:
>>Thanks for the reference, which I've followed up.
>>Suddenly it's all clear.
>>It makes no difference whether the value is prime
>>or not. The significance appears I think when you
>>use multiple free lists AND multiple freelist groups
>>in a single instance.
>>In this case, the number of freelists and the number
>>of freelist groups must be relatively prime (co-prime ?)
>>or you don't make use of all the freelists in all the
>>freelist blocks.
>Not quite, it still can make a difference in some cases. For example,
>if there's some characteristic of your environment that makes the PID
>Say I start up a batch job that runs as 64 parallel processes on the same
>system as the database server is running. Each batch job has an Oracle
>shadow process. It's quite likely (on some OSes) to have the application
>and shadow processes interleaved, i.e. all the application processes to
>end up with an odd PID, and all the Oracle shadow processes to end up
>with an even PID (or vice-versa). There's an example of something very
>similar on my other terminal about two feet away :-)
>If you had a even number of freelists, you'd only end up using half
>the freelists.
>PIDs have no guarantees about their numerical properties, so may well
>fall into a pattern (possibly one more complex than just all being
>even). Primes have the greatest resistance to matching a pattern.
>Andrew Mobbs -
Received on Wed Jul 24 2002 - 11:15:47 CDT

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