[plug] maximum RAM linux can cope with

[Ian Kent]

On Mon, 2007-05-07 at 23:20 +0800, Patrick Coleman wrote:
> On 5/7/07, David Buddrige <dbuddrige at bigpond.com> wrote:
> >
> >  Hi all,
> >
> >  I am currently helping a friend spec out a computer which is to be used for
> > creating video productions.  The plan is to run a Linux based system
> > utilising the various Free/OSS tools available for this task.  Given the
> > task at hand, we are spec-ing it out with as much RAM, the best CPU,
> > video-card and sound-card we can afford.  A question came up as to how much
> > RAM is Linux capable of using [also whether it is capable of fully utilising
> > quad-core CPU's.  We could for example stick 6 GB of RAM into this machine
> > [the motherboard we're looking at has slots to allow for this] but I am not
> > able to say at the moment [for certain] whether Linux is capable of using
> > this much RAM - particularly using a bog-standard distro.  If we were to
> > load it up with the latest fedora, would [fedora] be capable of making use
> > of 6GB RAM?

No sure either but FC6 ships with a 2.6.20 kernel and the HIGNMEM
options are enabled. Older (read really old) RedHat distributions used
to ship with a seperate "highmem" kernel variant that enabled use of the
PAE extensions (ie. up to 64G).

You need to remember that on 32 bit archs a process address space is
restricted to 4G, which is usually mapped as 3G user and 1G kernel. I'm
not sure if this has division has changed much in recent kernels but
there were patches around to change the allocation proportions.

So if your app needs to handle large amounts of data in program memory
you need to go for a 64-bit machine.

> 
> Hi David,
> 
> I'm not familar with Fedora, but linux < 2.6.12 has a CONFIG_NOHIGHMEM
> option (where you choose between having <1GB, 1-4GB or 4-64GB of
> memory). I imagine fedora won't ship with this set to 'enormous', so
> you may have to install a separate kernel package (probably named
> along the lines of kernel-x.x.xx-highmem). Apparently the worst that
> will happen if you get it wrong is that you will only see, say, 4GB of
> the 6GB of memory.
> 
> linux >= 2.6.12 apparently (I don't have sources handy to check) deals
> with this automatically; you don't have to set the CONFIG_HIGHMEM
> option in the kernel. If Fedora therefore ships with a linux kernel >
> 2.6.12 (which it should do) there should be no issues with the memory.
> 
> > Also, if we put in a CORE 2 Quad Q 6600 [as per
> > http://www.ple.com.au/?p=extremeint ] would it be able to
> > use all 4 cores
> 
> Yep, should just work. If it doesn't automatically you may have to
> install the fedora SMP (ie. multiprocessor, named along the lines of
> kernel-x.x.xx-smp) kernel.
> 
> > [I'm not up-to-speed with whether a quad core system
> > actually comprises 4 cpu's or a single cpu with 4 internal processing units
> > or something].
> 
> A quad core CPU contains four actual CPUs wacked onto one chip. I'm
> not completely sure if they share the same cache or not - I don't
> think so (wasn't that why 'hyperthreading' sucked?), but I could be
> wrong.
> 
> >  I've done a bit of a google on this but while it seems there are at least
> > configurations out there that can utilise as much as 64GB of RAM, I don't
> > know if you have to do some special configuration to allow linux to utilise
> > this kind of hardware, or whether a bog-standard distro will recognise and
> > use it?
> 
> It should do by itself if > 2.6.12 and is an SMP kernel; at worst if
> it doesn't and there isn't a preconfigured fedora package you will
> have to recompile a custom kernel.

The Fedora kernels have been SMP enabled for some time now.

> 
> Cheers,
> 
> Patrick
> 
>