Re: Large difference between elapsed time and run time

Hi,

I have noted similar issues in the past - and seemed then that most of the overhead bottleneck was due to establishing a new connection in the front end. As soon as I started using connection pooling, with connections made when the app initialises, and then recycled for each request (i.e. the connections never close) then the execution time was far quicker.
I have also noticed that sparc processor speed, num processors, disk space and memory seems to makes little difference with postgres (for us anyway!) performance - e.g. performance no better with dual sparc 450mhz, 2 scsi disks, 1Gb mem - than on a single processor 400 mhz Netra, 256Mb ram with a single IDE disk!

Nikk

-----Original Message-----
From: Scott Buchan [mailto:sbuchan(at)technicacorp(dot)com]
Sent: 10 March 2003 18:58
To: pgsql-performance(at)postgresql(dot)org
Subject: [PERFORM] Large difference between elapsed time and run time for queries

Hello,

While running benchmarks for my database, I am seeing a large difference in the elapsed time (from stats collected in the logs) and run time (running explain analyze on the query using ./psql <database>) for each of my queries. The database is being ran on a sunfire 880 with 4 750mhz processors with 8 G RAM running solaris 8

I am simulating 200 user connections each running 6 select queries on 1 indexed table with 50,000 records. The elapsed time for the queries average around 2.5 seconds while if I run the query using explain analyze while the test is running, the run time is around 300 ms although it takes much longer (few seconds) to display the results. If I reduce the number of concurrent connections to 100 then the run time and elapsed time for the queries are the same.

I have tried numerous configurations in the postgresql.conf file. I have set the shared_buffers with values ranging from 75 MB to 4000MB with no luck. I have also tried increasing the sort_mem with no luck.

When the test is running, the cpu is well over 50% idle and iostat shows that the processes are not waiting for i/o and disk usage percentage is low.

Any help would be appreciated.

Thanks.