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Home -> Community -> Mailing Lists -> Oracle-L -> Response times with different multiblock read count
Hi All,
I am running a test query with different db_file_multiblock_read_count to test the overall throughput.
environment : oracle 8.1.6 on hp-ux v11
db_block_size : 16k
system is using LVM and file system is using buffered I/O and disk is mirrored but not
stripped.
I was expecting that having larger multiblock count would result in better performance. But, when I was testing the query with smaller multiblock count,the response time is better.
Here is the testing result.
Multiblock time in Secs ---------- ------------ 1 12.5 better 2 12.5 better 4 12.6 better 8 27 bad 16 27 bad 32 18.6 okay 64 19.0 okay
I don't understand why 8,16 are taking longer time. I did make sure that oracle is issuing proper multiblock read count with the help of multiblock_read_test.sql which is available in Steve Adams's site. Thank you steve for providing valuable information & scripts on your web site.
select statement that I am using is,
select /*+ full(t) noparallel(t) nocache(t) */ count(*) from &Table t;
I am clueless why it is behaving like this. Please pass your suggestions. If you need more info like oracle/OS settings etc, please let me know.
When I was doing some reading on steve adam's site, he suggests the following.
Date: 29-Dec-2000 20:55
Nevertheless, it remains best to allow Oracle to use large multiblock reads, as long
as the optimizer is not doing
inappropriate full scans. The explanation relates to the system call and context
switch overheads associated with I/O.
It is cheaper in CPU usage terms to request a 256K I/O and allow the operating system
to service it in 64K chunks,
than to request 4 I/O's of 64K each. As is mentioned in the tip on Why Large
Multiblock Reads, the increased
latency is not an issue because multiblock reads should not be used in cases where
"first rows" response time is
critical.
Thanks in advance.
Best Regards,
Prasad
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Author:
INET: Prasada.Gunda1_at_hartfordlife.com
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