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Re: Pg+Linux swap use

From: Shridhar Daithankar <shridhar_daithankar(at)persistent(dot)co(dot)in>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: Pg+Linux swap use
Date: 2003-10-31 15:54:43
Message-ID: 3FA285C3.8050904@persistent.co.in (view raw or flat)
Thread:
Lists: pgsql-performance
Jeff wrote:

> On Fri, 31 Oct 2003 09:31:19 -0600
> "Rob Sell" <lists(at)facnd(dot)com> wrote:
> 
> 
>>Not being one to hijack threads, but I haven't heard of this
>>performance hit when using HT, I have what should all rights be a
>>pretty fast server, dual 2.4 Xeons with HT 205gb raid 5 array, 1 gig
>>of memory. And it is only 50% as fast as my old server which was a
>>dual AMD MP 1400's with a 45gb raid 5 array and 1gb of ram. I have
>>read everything I could find on Pg performance tweaked all the
>>variables that were suggested and nothing. Which is why I subscribed
>>to this list, just been lurking so far but this caught my eye. 
>>
>>Rob
> There's benchmarks around that show in _some_ cases HT is not all it is
> cracked up to be, somtimes running slower.

To use HT effectively on needs.

1. A kernel that understands HT.
2. A task scheduler that understands HT
3. A CPU intensive load.

So if you are running a stock RedHat and production postgresql database, turn it 
off. It won't hurt certainly(Almost certainly)

> I'm guessing RH is running some useless stuff in the BG.. or maybe he's
> running a retarded kernel... or.. maybe.. just.. maybe.. little elves
> are doing it.

Too much..:-)

I guess Alexandre can tune bdflush to be little less agressive. Comparing 
bdflush values on two machines might turn up something.

His idea of compiling kernel is also good one. He can also try tuning some 
values in /proc/sys/vm but I don't find any documentation offhand.

I usually run slackware and a handcompiled 2.6-test4. None of them use any swap 
unless true memory starts falling low. This 
touch-swap-even-if-oodles-of-ram-is-free is something I have't experienced on my 
desktop for quite a while.

  Shridhar


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