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Re: SGA Free memory

From: Jonathan Lewis <>
Date: Thu, 9 Feb 2006 19:31:37 -0000
Message-ID: <01eb01c62daf$71e94ec0$0200a8c0@Primary>


This is not entirely normal - the scale of the flush is a little extreme, and ties in with your earlier question about "session param values".

The reply you got about increasing the reserved shared pool was correct, and is probably relevant in this case as well.

The sequence of events for allocating memory from the shared pool is:

    Request chunk in shared pool
    if no available free chunks large enough

        if chunksize > 'reserved pool min alloc' (ca. 4400 bytes)
            search reserved pool for large enough free chunk
            if space available, 
                allocate in reserved pool

    end if


        detach some items from end of library cache LRU
        distribute items to shared pool free list - coalescing where possible
        check for large enough chunk on free list
        return if allocation succeeds

    end loop

    There may be a limit on the number of times round the loop -     but I've seen highly concurrent OLTP systems flush several     hundred megabytes from the library cache - hammering the     library cache and shared pool latches to death for seconds     at a time doing this.

You can get problems on well-designed systems with large numbers of users because (a) the free memory eventually degrades into lots of very small pieces, (b) the session param values is about 22 KB and
(c) the default of 5% for the reserved pool may not be enough.

As a rough guide line, if you are seeing catastrophic memory thrashing, check that your shared_pool_reserved is about 25KB * sessions plus a few MB. (A query against x$ksmspr will tell you how much of the memory is used for things other than session parameters), and that's at least the size you need for the "few MB extra").

    select sum(KSMCHSIZ) from x$ksmspr
    where ksmchcom not in ('free memory','session param valu')

(Don't try this against x$ksmsp, the shared pool, it could
cripple your system for anything up to a couple of minutes if it's a system with a large shared pool and lots of users).
(the session parameter component has a couple of changes
in its name across Oracle versions).

You could also check to see how many of your session's have got their parameters into the reserved pool - this is quite a nice query, and not too aggressive as a more general check:

    select ksmchcom,count(*),sum(ksmchsiz)     from x$ksmspr
    group by ksmchcom;

(again - DON'T run it against x$ksmsp).


Jonathan Lewis The Co-operative Oracle Users' FAQ Cost Based Oracle: Fundamentals Public Appearances - schedule updated 2nd Feb 2006

This is normal behavior as chunks are flushed from and added to the shared pool, the memory fluctuates. As long as you are not seeing ORA-04031 error or contention on the shared pool latch is not too high, you are fine. What RDBMS version are you on?

-----Original Message-----
[] On Behalf Of
Sent: Wednesday, February 08, 2006 12:28 PM To:
Subject: Re: SGA Free memory

Hi all:

I am watching the usage of the SGA on my database via the following command:

select to_char(sysdate,'mm-dd-yyyy hh24:mi:ss'), name, bytes/(1024*1024) from v$sgastat, dual
where pool like '%shared%' and name = 'free memory' order by bytes asc ;

I am running it every 10 seconds. Recently I saw the free memory going from
25M to 145M over the course
of 10 sec. I'm wondering what could have caused it. Would appreciate any suggestions

thank you

Gene Gurevich
Oracle Engineering



Received on Thu Feb 09 2006 - 13:31:37 CST

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