What size for the swap with a kernel 2.4x and 8GB RAM

Jean-David Beyer a écrit :

> In Red Hat Enterprise Linux 3, they _never_ raise the 2.4.21 number. They
> add suffixes to this to denote Red Hat changes. Red Hat has folded into the
> 2.4 kernel a lot of the 2.6 improvements, and all the security and bug-fix
> updates, so the 2.4.21 is a bit deceptive. The current kernel that I use is
> called 2.4.21-40.ELhugemem (the hugemem is what allows me to use more memory
> than 4 GBytes; of course the Xeon processors and E7501 chipset help too).
> >
> > If not - and I certainly recommend this for any hyperthreading-enabled
> > machine - I would advise you to install a far more recent distribution
> > with a 2.6 kernel.
> >
> Most of what is in 2.6 kernel has been backported into the RHEL 3 kernel.
>

My kernel is a 2.4.21-20.0.1.ELsmp, not a 2.4.21-20.0.1.ELhugemem.
Refering to
http://www.redhat.com/docs/manuals/e...ch-kernel.html,
does it mean that the kernel 2.4.21-20.0.1.ELsmp works with 8GB of RAM
but is not optimized at all? I understand that kernel-hugemem is
required for memory configurations higher than 16 GB, and
2.4.21-20.0.1.ELsmp is enough with 8 Gb.

I need the smp kernel because I have got 2 Intel Xeon.

Sylvain

Aragorn a écrit :

> On Monday 20 March 2006 20:53, Jean-David Beyer stood up and spoke the
> following words to the masses in /comp.os.linux.misc...:/
>
> The amount of usable swap space in the Linux kernel for IA32 systems is
> limited to around 2 GB, give and take a few bytes. In other words,
> unless your machine is an IA32-64 - e.g. AMD64 - you're wasting about 6
> GB of diskspace... ;-)
>

I have got a RedHat Linux IA32 but the cpus are 2 dual core Intel Xeon
3.4 Ghz. I agree that a real Linux 64 OS would better fit with my Xeons
but I have to deal with...
In my case, Linux 32 bits (RedHat AS 3.0 32 bits with a
2.4.21-20.0.1.ELsmp kernel _ this kernel from RedHat has some back
ports from kernel 2.6), am I wasting 6 Gb of diskspace on my swap
partition of 8 Gb? How can I confirm it? man on mkswap gives me "The
maximum useful size of a swap area now depends on the architecture. It
is roughly 2GiB on i386, PPC, m68k, ARM, 1GiB on sparc, 512MiB on
mips, 128GiB on alpha and 3TiB on sparc64."

Sylvain

On Friday 24 March 2006 18:33, s45_nospam@free.fr stood up and spoke the
following words to the masses in /comp.os.linux.misc...:/

>
> Aragorn a écrit :
>
>> On Monday 20 March 2006 20:53, Jean-David Beyer stood up and spoke
>> the following words to the masses in /comp.os.linux.misc...:/
>>
>> The amount of usable swap space in the Linux kernel for IA32 systems
>> is limited to around 2 GB, give and take a few bytes. In other
>> words, unless your machine is an IA32-64 - e.g. AMD64 - you're
>> wasting about 6 GB of diskspace... ;-)

I was corrected on the above, as it would seem that the size limit to
the swap pertains to one swap partition, and the Linux kernel can use
32 of them.

On /i386/ - i.e. 32-bit GNU/Linux - this would mean that the kernel can
access up to 64 GB worth of swap space, divided over 32 swap partitions
of 2 GB each.

> I have got a RedHat Linux IA32 but the cpus are 2 dual core Intel Xeon
> 3.4 Ghz. I agree that a real Linux 64 OS would better fit with my
> Xeons but I have to deal with...

Oh, so yours is an IA32-64 system running a 32-bit distribution. I've
got two hyperthreaded 32-bit Xeons here. ;-)

> In my case, Linux 32 bits (RedHat AS 3.0 32 bits with a
> 2.4.21-20.0.1.ELsmp kernel _ this kernel from RedHat has some back
> ports from kernel 2.6), am I wasting 6 Gb of diskspace on my swap
> partition of 8 Gb?

Yes. A swap partition can only be used for 2 GB by a 32-bit kernel on
an /x86/ machine. You'd have to remove your swap partition or resize
it, and then - should you so desire - create three more 2 GB swap
partitions.

> How can I confirm it? man on mkswap gives me "The maximum useful size
> of a swap area now depends on the architecture. It is roughly 2GiB
> on i386, PPC, m68k, ARM, 1GiB on sparc, 512MiB on mips, 128GiB on
> alpha and 3TiB on sparc64."

Well, that says it all, but those are the sizes _per_ swap partition.
If you really want 8 GB worth of swap - do you really need that? - then
I would advise creating four swap partitions of 2 GB each.

By the way, if you put two swap partitions on different disks, you can
add a striping effect to them by giving them an equal priority. ;-)

--
With kind regards,

*Aragorn*
(Registered GNU/Linux user #223157)

On Friday 24 March 2006 18:24, s45_nospam@free.fr stood up and spoke the
following words to the masses in /comp.os.linux.misc...:/

> My kernel is a 2.4.21-20.0.1.ELsmp, not a 2.4.21-20.0.1.ELhugemem.
> Refering to
>
http://www.redhat.com/docs/manuals/e...ch-kernel.html,
> does it mean that the kernel 2.4.21-20.0.1.ELsmp works with 8GB of
> RAM but is not optimized at all?

Negative... In order to make use of more than 4 GB worth of physical
memory on an IA32 machine (or an IA32-64 running a 32-bit operating
system), you need to have an /i686/ or better CPU - i.e. Pentium Pro
and above or equivalent - and you need a kernel that has been compiled
with support for up to 64 GB, which will be addressed in a paged mode
via three-level paging.

> I understand that kernel-hugemem is required for memory configurations
> higher than 16 GB, and 2.4.21-20.0.1.ELsmp is enough with 8 Gb.

Negative again. The /kernel-hugemem/ is required for memory
configurations above 4 GB.

There are only three memory capacity configurations in the Linux kernel:
- no /HIGHMEM/ support : supports up to 1 GB
- /HIGHMEM/ support : supports up to 4 GB
- /HIGHMEM_64/ support : supports up to 64 GB by running the CPU in PAE
(Physical Address Extension) mode.

The latter is why you need an /i686/ or better. /i586/ does not have a
PAE mode.

> I need the smp kernel because I have got 2 Intel Xeon.

Normally, the /kernel-hugemem/ will also support SMP. There's very
little point in having it configured as a uniprocessor kernel, as
32-bit machines with more than 4 GB of physical memory installed are
typically SMP machines. ;-)

--
With kind regards,

*Aragorn*
(Registered GNU/Linux user #223157)