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Re: Hardware suggestions for Linux/PGSQL server

From: Jeff Bohmer <bohmer(at)visionlink(dot)org>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: Hardware suggestions for Linux/PGSQL server
Date: 2003-12-11 22:02:11
Message-ID: p04330116bbfe9076106b@[] (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
>Properly tuned, PG on Linux runs really nice. A few people have 
>mentioned the VM swapping algorithm on Linux is semi-dumb. I get 
>around that problem by having a ton of memory and almost no swap.

I think we want your approach: enough RAM to avoid swapping altogether.

>With 4GB of RAM, you're already running into bigmem. By default, 
>Linux gives 2GB of address space to programs and 2GB to kernel.

It seems I don't fully understand the bigmem situation.  I've 
searched the archives, googled, checked RedHat's docs, etc.  But I'm 
getting conflicting, incomplete and/or out of date information.  Does 
anyone have pointers to bigmem info or configuration for the 2.4 

If Linux is setup with 2GB for kernel and 2GB for user, would that be 
OK with a DB size of 2-2.5 GB?  I'm figuring the kernel will cache 
most/all of the DB in it's 2GB and there's 2GB left for PG processes. 
Where does PG's SHM buffers live, kernel or user?  (I don't plan on 
going crazy with buffers, but will guess we'd need about 128MB, 256MB 
at most.)

>I usually see people quote 5%-15% penalty in general for using PAE 
>versus a flat address space. I've seen simple MySQL benchmarks where 
>64-bit versions run 35%+ faster versus 32-bit+PAE but how that 
>translates to PG, I dunno yet.
>>We'd like to always have enough RAM to cache the entire database. 
>>While 64bit is in our long-term future, we're willing to stick with 
>>32bit Linux until 64bit Linux on Itanium/Opteron and 64bit 
>>PostgreSQL "settle in" to proven production-quality.
>Well if this is the case, you probably should get an Opteron server 
>*now* and just run 32-bit Linux on it until you're sure about the 
>software. No point in buying a Xeon and then throwing the machine 
>away in a year when you decide you need 64-bit for more speed.

That's a good point.  I had forgotten about the option to run 32bit 
on an Operton.  If we had 3GB or 4GB initially on an Opteron, we'd 
need bigmem for 32bit Linux, right?

This might work nicely since we'd factor in the penalty from PAE for 
now and have the performance boost from moving to 64bit available on 
demand.  Not having to build another DB server in a year would also 
be nice.

FYI, we need stability first and performance second.

Thank you,
- Jeff


Jeff Bohmer
VisionLink, Inc.
People. Tools. Change. Community.

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Subject: Re: Hardware suggestions for Linux/PGSQL server
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