RE: SORT_AREA_SIZE question

Thanks Dennis. My question was not about the memory release, but more of the way memory is allocated to the PGA. Given that Oracle allocates memory in small chunks to the PGA until it approaches the sort area size, if I run a huge sort, should I expect the PGA memory to increase, until it reaches sort_area_size?

1. Initial dedicated connection established. The PGA used should be very minimal.
2. Say, I invoke a small sort on DBA_USERS. The PGA (or UGA) should increase by a very small value.
3. I invoke a sort on a huge table. I expect the PGA to keep increasing, and flatten out once it reaches the SORT_AREA_SIZE.

I tried to monitor the memory utilization by the session using pmap, and v$sysstat, during these 3 steps, and did not see the expected results. It was a rudimentary test on a high volume database, so I guess I could have been wrong. I remember I tried this coz I read a post from Jonathan Lewis, suggesting that the initial memory for sorting is allocated in the UGA upto sort_area_retained_size, after which it is allocated from the PGA upto sort_area_size.

Thanks
Raj

                                                                                       
                             
                    DENNIS WILLIAMS                                                    
                             
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                    06/25/2003                                                         
                             
                    04:27 PM                                                           
                             
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Raj

   From our perspective as users of the operating system we expect when we release memory that the O.S. will immediately release it to other users.
>From the O.S. perspective, that is treated more like a suggestion. The O.S.
has many tasks to accomplish and reusing released memory is pretty low on the list of priorities.

Dennis Williams
DBA, 80%OCP, 100% DBA
Lifetouch, Inc.
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Sent: Wednesday, June 25, 2003 1:31 PM
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Since we are talking of Sort area sizes here, I tried to collect some statistics on the amount of memory used by each dedicated Oracle connection. As I understand it, the PGA is allocated in chunks upto the SORT_AREA_SIZE. Initially, I just started a session, and noted down the memory used using pmap. Then, I did a small sort, and then, a huge sort, noting the memory utilization each time. I was even monitoring the PGA statistics in v$sysstat. I was expecting the sizes to increase, and then decrease when the sorting was done with. (retained_size was 0). I dont recall my observations rite now, but it was definitely not what I expected.

Has anyone done something similar? Are my expectations rite?

Thanks
Raj

                    DENNIS WILLIAMS

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                    06/25/2003

                    12:30 PM

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Bart -

   No it hasn't been recently asked.

1. SORT_AREA_SIZE is per-process. So be a little wary. Keep in mind that in-memory sorts are much faster than disk sorts. But you knew that.
2. Ask your system administrator to monitor whether your system is getting paging/swapping.
3. Find out how many disk sorts are occurring. I prefer STATSPACK. Try to get a handle on how may disk vs. memory sorts are occurring. Try to increase SORT_AREA_SIZE until you have few disk sorts, but not so large you cause paging/swapping. Also look at your temp space settings to make sure when disk is used, it is used most efficiently. With a data mart, you may have some really large sorts that will always be too large for memory sorts.

Dennis Williams
DBA, 80%OCP, 100% DBA
Lifetouch, Inc.
[EMAIL PROTECTED] -----Original Message-----
Sent: Wednesday, June 25, 2003 9:39 AM
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Hello,

I joined this list last week, so I apologize in advance if I'm asking a question that has previously been answered.

I am responsible for a reporting database/data mart that is approximately 175 GB. Our main fact table ranges from 1-14 GB depending upon how far along we are into our financial year. I have large reports that run full table scans on this table daily. In an effort to keep as much of the sorting in memory as possible I have specified SORT_AREA_SIZE to be 100MB. Some of the tuning books I am reading now are making me second-guess myself and I am wondering if this is overkill.

Can anyone provide some advice on how large they are setting their SORT_AREA_SIZE values for their DSS systems?

Thanks in advance,

Bart

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