Hi Konstantin,

---------- Original Message -----------
From: Konstantin Olchanski <[log in to unmask]>
To: Tim Edwards <[log in to unmask]>
Cc: "[log in to unmask]" <[log in to unmask]>
Sent: Thu, 28 Jan 2010 13:38:59 -0800
Subject: Re: Memory limits for Scientific Linux kernels

> On Thu, Jan 28, 2010 at 02:56:31PM +0100, Tim Edwards wrote:
> > We're trying to work out memory limits for 32-bit versions of SL4 and 5.
> 
> AFAIC, the effective hard limits for 32-bit Linux are 3 GBytes per user
> process (they may have squeezed it up to 3.5 GBytes) and 4 GBytes
> of physical RAM for the machine.
> 
> If your machine has more than 4 Gbytes of physical RAM, you should
> be running a 64-bit OS. In this case, 32-bit processes are still
> limited to 3-3.5 Gbytes of memory, there is no way around that.
> 
> The "by the specs" "paper" limits may say that 32-bit Linux kernels
> can use more memory, and it might even work in practice, but be aware
> that it requires that the Linux kernel use some page table magic
> and relies on funny Intel CPU extenstions (PAE & co). As I 
> understand, this leads to noticable drop in performance.

Just asking for some advice based on your comments above. 

I have several HP ProLiant DL360 G3 servers, their CPU's are 32 bit only (G4
and above are 64bit), as such I run 32 bit Linux on them. However, many of
them have more than 4Gb of RAM (2x2Gb modules plus 2x512Mb is typical) and I
use the kernel-PAE to access that extra RAM.

The servers typically don't use more than 4Gb of RAM, actually, I hardly ever
see them do, they typically access 2-3Gb for normal daily operations.
Performance isn't an issue either however, are you saying in my situation I
would be better off, performance wise, to just keep the 2x 2Gb DIMMs in them,
yank the 2x 512Mb and revert back to the standard "kernel" RPM's without PAE
to get a noticeable increase in performance?

I wonder if there's some weblinks anywhere which could show performance graphs
of with and without PAE's?

Thanks.

Michael.

> I am now curious why are you interested in running 32-bit Linux
> when 64-bit Linux was "invented" specifically to "fix" the
> memory limits.
> 
> K.O.
> 
> > Redhat's page (http://www.redhat.com/rhel/compare/) says that the
> > maximums are 64GB or RHEL4 and 16GB for RHEL5 (I guess because they
> > dropped the HUGEMEM kernel RPM in RHEL5).
> > 
> > This page (http://www.scientificlinux.org/documentation/misc/limits)
> > says that it's 64GB in SL4 but gives no information for SL5. So two
> > questions:
> > 
> > Does SL4 i386 have a 'HUGEMEM' kernel build or do you just build those
> > features into the normal -smp kernel build in order to support 64GB RAM?
> > 
> > What is the memory limit on SL5 i386?
> > 
> > Thanks
> > 
> > Tim Edwards
> 
> -- 
> Konstantin Olchanski
> Data Acquisition Systems: The Bytes Must Flow!
> Email: olchansk-at-triumf-dot-ca
> Snail mail: 4004 Wesbrook Mall, TRIUMF, Vancouver, B.C., V6T 2A3, Canada
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