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Tim,
You are confirming my original understanding. However Kirti stated that
based on your figures if a sort was used then malloc would be called 4
times to get from 512000 to 2048000. I do not know the answer for certain
but Kirti's view seems logical because sort_area_size is the MAX size and it
is very likely that only the retained is allocated initially and then
increased in increments up to sort_area_retained_size. Any bigger and the
sort is done to the temporary tablespace.
John
-----Original Message-----
From: Tim Sawmiller [mailto:[EMAIL PROTECTED]]
Sent: 18 April 2001 17:26
To: Multiple recipients of list ORACLE-L
Subject: RE: Temporary Tablespace Design
Well, not quite. SORT_AREA_SIZE specifies the maximum
amount, in bytes, of memory to use for a sort. After the sort is complete
and all that remains to do is to return the rows, the memory is released
down to the size specified by SORT_AREA_RETAINED_SIZE. After the last row
is returned, all memory is freed. Temporary disk is used for the
intermediate sort runs only if the SORT_AREA_SIZE isn't large enough to
handle the sort in memory. The SORT_AREA_RETAINED_SIZE controls the size of
the read buffer which is used to maintain a portion of the sort in memory.
Myself, I use 2048000 for SORT_AREA_SIZE, and 512000 for
SORT_AREA_RETAINED_SIZE.
>>> [EMAIL PROTECTED] 04/18/01 11:36AM >>>
I think you are not correct. First - Oracle allocates memory
for sorting as
needed by 8K chunks up to sort_area_size. Second if your
sort_area_size is
large enough to do sort in memory and your
sort_area_retained_size <
sort_area_size oracle will dump sorted data into temporary
tablespace and
then read from this tablespace. So it is a tradeoff between
late memory
release and temporary tablespace I/O.
Alex Hillman
-----Original Message-----
Sent: Wednesday, April 18, 2001 10:40 AM
To: Multiple recipients of list ORACLE-L
Hi John,
As you mentioned, it depends on a number of factors. I
believe I replied to
a question about improving performance of index building
after an import.
Anyway, here is the reasoning behind why I said that....
When Oracle begins
sorting, it starts with the memory equal to
sort_area_retained_size value
and may eventually acquire memory equal to sort_area_size.
This involves at
OS level, malloc (and may be some other) memory mgmt
routine(s). After the
sort phase is complete, Oracle will trigger memory
de-allocation and the OS
will go at work again. If there are servile sessions
performing sorts, there
can be an increased OS level memory mgmt activity (while
attaining
sort_area_size and releasing it back to
sort_area_retained_size) . But if
the server has *enough* memory keeping
sort_area_retained_size equal to
sort_area_size may actually help. I also assume that these
days most
installations have ample memory. And that the advice of
using 50% (which I
had heard of) of sort area size for sort area retained size
should always be
viewed in light of one's requirements and resource
availability. But I can't
stop wondering... how come it was 50% and not 10% or 20%..
so it's all
relative. I have been using what I suggested, for a number
of years now
with no problems or complaints.
Cheers !
- Kirti Deshpande
Verizon Information Services
http://www.superpages.com
> -----Original Message-----
> From: Hallas, John [SMTP:[EMAIL PROTECTED]]
> Sent: Wednesday, April 18, 2001 6:21 AM
> To: Multiple recipients of list ORACLE-L
> Subject: RE: Temporary Tablespace Design
>
>
> Kirti Deshpande writes
>
> "Also, keep SORT_AREA_RETAINED_SIZE the same as
SORT_AREA_SIZE. As the
> former will be used while fetching data after sort phase
is completed."
>
> I thought best advice was to make the retained size
smaller (50%?) of
> sort_area_size to allow initial sort memory to be released
after the first
> part of the sort is managed and only the merge phase of a
disk sort is
> left
> to do.
>
> I appreciate that a lot depends on the amount of memory
available and the
> number of concurrent processes that may be performing
sorts but surely the
> aim is to free memory up as soon as possible down to the
> sort_area_retained_size
>
> Thanks
>
> John
>
> (PS I do apologise for the rubbish at the top of this mail
but it is
> inserted after I have sent the mail into our mail gateway)
>
>
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Author: Hallas, John
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