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Optimizing Postgresql - Page 2

by: PHP Builder Staff
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August 21, 2001

Optimizing

One of the first things I noticed after turning on the CGI program, was that although queries were returned almost as fast as from the previous MySQL based system, the load on the server was much higher -- in fact almost 90-percent! Then I started to go down into the nitty-gritty of things. I had optimized MySQL before by greatly increasing cache and buffer sizes and by throwing more ram towards the problem.
The single biggest thing that you have to do before running Postgresql, is to provide enough shared buffer space. Let me repeat: provide enough buffer space! Let's say you have about 512MB of ram on a dedicated database server, then you need to turn over about 75-percent of it to this shared buffer. Postgresql does best when it can load most or -- even better -- all of a table into its shared memory space. In our case, since our database was fairly small, I decided to allocate 128MB of RAM towards the shared buffer space.
The file /var/lib/pgsql/data/postgresql.conf contains settings for the database server. Postgresql uses system shared memory as a buffer. On a Linux system, you can see how much shared memory was allocated by your system by running the command:
cat /proc/sys/kernel/shmmax
And to view shared memory use on the system:
ipcs
The result will be in bytes. By default RedHat 7.1 allocates 32MB of shared memory, hardly enough for postgresql. I increased this limit to 128MB by doing the command:
echo 128000000 > /proc/sys/kernel/shmmax
Be aware that once you reboot the server, this setting will disappear. You need to place this line in your postgresql startup file, or by editing the /etc/sysctl.conf file for a more permanent setting.
Then in our postgresql.conf I set shared_buffers to 15200. Because Postgresql uses 8K segments, I made a calculation of 128000/8192 plus a 512K overhead. I also set our sort_mem to 32168 (32Megs for a sort memory area). Since connection pooling was in effect, I set max_connections to 64. And fsync was also set to false.
shared_buffers = 15200
sort_mem = 32168
max_connections=64
fsync=false
You can read the manual to tweak other settings, but I never had the need to do so. Note that if you set shared_buffers to more than what your shared memory limit is, postgresql will refuse to start. This confused us for a while, since no logging was taking place. You can tweak the startup file in /etc/init.d for the postmaster to write its output to a log file. Change the fragment from:
/postmaster start > /dev/null 2>
to
/postmaster start > /var/lib/pgsql.log 2>
(or wherever you want to store the log.)

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