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Re: High context switches occurring

From: Sven Geisler <sgeisler(at)aeccom(dot)com>
To: Anjan Dave <adave(at)vantage(dot)com>
Cc: Scott Marlowe <smarlowe(at)g2switchworks(dot)com>,Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Vivek Khera <vivek(at)khera(dot)org>,Postgresql Performance <pgsql-performance(at)postgresql(dot)org>
Subject: Re: High context switches occurring
Date: 2005-11-24 10:38:47
Message-ID: 43859837.8040101@aeccom.com (view raw or flat)
Thread:
Lists: pgsql-performance
Hi Anjan,

I can support Scott. You should turn on HT if you see high values for CS.

I do have a few customers running a web-based 3-tier application with 
PostgreSQL. We had to turn off HT to have better overall performance.
The issue is the behavior under high load. I notice that HT on does 
collapse faster.

Just a question. Which version of XEON do you have? What is does the 
server have as memory architecture.

I think, Dual-Core XEON's are no issue. One of our customers does use a 
4-way Dual-Core Opteron 875 since a few months. We have Pg 8.0.3 and it 
runs perfect. I have to say that we use a special patch from Tom which 
fix an issue with the looking of shared buffers and the Opteron.
I notice that this patch is also useful for XEON's with EMT64.

Best regards
Sven.

Anjan Dave schrieb:
> Yes, it's turned on, unfortunately it got overlooked during the setup,
> and until now...!
> 
> It's mostly a 'read' application, I increased the vm.max-readahead to
> 2048 from the default 256, after which I've not seen the CS storm,
> though it could be incidental.
> 
> Thanks,
> Anjan
> 
> -----Original Message-----
> From: Scott Marlowe [mailto:smarlowe(at)g2switchworks(dot)com] 
> Sent: Tuesday, November 22, 2005 3:38 PM
> To: Anjan Dave
> Cc: Tom Lane; Vivek Khera; Postgresql Performance
> Subject: Re: [PERFORM] High context switches occurring
> 
> On Tue, 2005-11-22 at 14:33, Anjan Dave wrote:
> 
>>Is there any way to get a temporary relief from this Context Switching
>>storm? Does restarting postmaster help?
>>
>>It seems that I can recreate the heavy CS with just one SELECT
>>statement...and then when multiple such SELECT queries are coming in,
>>things just get hosed up until we cancel a bunch of queries...
> 
> 
> Is your machine a hyperthreaded one?  Some folks have found that turning
> off hyper threading helps.  I knew it made my servers better behaved in
> the past.
> 
> 
> ---------------------------(end of broadcast)---------------------------
> TIP 2: Don't 'kill -9' the postmaster

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Sven Geisler <sgeisler(at)aeccom(dot)com> Tel +49.30.5362.1627 Fax .1638
Senior Developer,    AEC/communications GmbH    Berlin,   Germany

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