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Re: Reason for PG being seemingly I/O bound?

From: Harshad RJ <harshad(dot)rj(at)gmail(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: pgsql-novice(at)postgresql(dot)org
Subject: Re: Reason for PG being seemingly I/O bound?
Date: 2009-09-14 04:34:22
Message-ID: 5f2b35a60909132134q2380717dna6043ce6f81d39a2@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-novice
On Mon, Sep 14, 2009 at 12:45 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Harshad <harshad(dot)rj(at)gmail(dot)com> writes:
> Did you watch "vmstat 1" or something
> similar to confirm that a lot of I/O is really happening?
>

No, I hadn't confirmed; I was just guessing, because I thought the CPU
utilisation was low. It turns out I was wrong in measuring the CPU
utilisation (forgot about system utilisation); and vmstat confirms that
there isn't any block i/o happening.

(PS. That's an incredibly useful command. Thanks! I was using 'grep' on
/proc/vmstat and it was cumbersome)


>
> However I still don't see how it would be I/O bound; the kernel
> certainly ought to have everything needed in disk cache after a couple
> of cycles.  On my machine a similar test immediately pins the CPU
> with about half user, half system time.
>
>

I hadn't noted down the system utilisation (sorry). My net CPU utilisation
(user + system) is about  50%, but it's a dual core system, so it is not all
that bad.

The CPU utilisation on my production server is lower, but I will
test/research further before asking questions here.

thanks & sorry for the false alarm,
-- 
Harshad RJ

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