postgresql and smp linux kernel

[zhicheng wang]

Dear all

i have a dedicated server mainly running postgresql,
currently rh-posgtresql-7.3.6.7 (and samba, ypbind
etc). The server is a dual processor machine and the
OS is RHEL AS 3.

I am currently running kernel-smp-2.4.21-15.0.4.EL.

My question is: do i need to run smp kernel for
postgresql? What is benefit of it? If I run UP kernel,
what do I lose from the point view of PG?

Thanks
=====
Best wishes
Z C Wang

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[Oliver Elphick]

On Mon, 2004-08-09 at 11:10, zhicheng wang wrote:

Dear all

i have a dedicated server mainly running postgresql,
currently rh-posgtresql-7.3.6.7 (and samba, ypbind
etc). The server is a dual processor machine and the
OS is RHEL AS 3.

I am currently running kernel-smp-2.4.21-15.0.4.EL.

My question is: do i need to run smp kernel for
postgresql? What is benefit of it? If I run UP kernel,
what do I lose from the point view of PG?

You do not NEED it for Pg, but it will improve performance. Each
separate session gets a different process and therefore has a change to
run on the second processor concurrently with another session on the
first processor. The same goes for any other processes that are
running, of course.

If you do not use an SMP kernel, one of your processors will sit there
for ever doing nothing, which is money down the drain.

--
Oliver Elphick ol**@lfix.co.uk
Isle of Wight http://www.lfix.co.uk/oliver
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[Scott Marlowe]

On Mon, 2004-08-09 at 04:10, zhicheng wang wrote:

Dear all

i have a dedicated server mainly running postgresql,
currently rh-posgtresql-7.3.6.7 (and samba, ypbind
etc). The server is a dual processor machine and the
OS is RHEL AS 3.

I am currently running kernel-smp-2.4.21-15.0.4.EL.

My question is: do i need to run smp kernel for
postgresql? What is benefit of it? If I run UP kernel,
what do I lose from the point view of PG?

Let's be clear here, PostgreSQL isn't responsible for exploiting your
second CPU, the OS is. The face that PostgreSQL spawns a new backend
process for each connection means that it can use about as many
processors as it has connections, plus maybe one for the OS.

However, the OS will not schedule anything to run at all on the second
CPU if you run a UP kernel.

Generally, the linux and bsd kernels both tend to get a scaling factor
of near unity for the second CPU with enough things to do in parallel.
There's almost no loss for single processes hogging one CPU, and even
these tasks are often sped up by the OS having a CPU to run on while
your one big process is hogging the other one.

Considering you're running samba, which is also process based, it would
be wise to enable the second CPU in the kernel, otherwise pull it, put
in the terminator, and save it in case of future CPU failure. It won't
be doing you any good.
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