HIGH MEM suport for up to 64GB

Steffen Persvold sp at scali.com
Wed Apr 17 14:44:10 EDT 2002


On 17 Apr 2002, Leandro Tavares Carneiro wrote:

> Hi,
>
> 	Anyone have tesed or used an machine with more than 4GB of RAM or paging
> in virtual memory on intel machines?
> 	He have an linux beowulf cluster and one of ours developers have asked
> us for how much memory an process can allocate to use. In the tests we
> have made, we cannot allocate much more than 3GB, using an dual PIII
> with 1GB of ram and 12Gb of swap area for testing.
> 	We can use 2 process alocating more or less 3Gb, but one process alone
> canot pass this test.
> 	We have using redhat linux 7.2, kernel 2.4.9-21, recompiled with High
> Mem suport.
> 	I have tested the same test aplication on an Itanium machine, with 1GB
> of ram and 16Gb of swap area, and they passed. The aplication can
> alocate more than 5GB of memory, using swap. In this machine, we are
> using turbolinux 7, with kernel version 2.4.4-010508-18smp.
> 	If this works, we can improve our applications.
>

There is simply no way you can make a 32 bit machine address more than 4GB
of memory in a single application simply because memory pointers are
only 32 bit (2^32 = 4GB). The reason why you can only address 3GB on Linux
is that normally 1GB of the virtual memory area is reserved for the kernel
(can be trimmed down to 512MB, which gives you 3.5GB accessible from
userspace). Sure, you can have several applications each using 3GB if you
have the memory for it (either swap or real) providing that you use the
64GB option in Linux. Huge memory requirement from applications is one of
the reasons people choose a 64bit platform (ppc, sparc, s390, alpha and
ia64).

Regards,
-- 
  Steffen Persvold   | Scalable Linux Systems |   Try out the world's best
 mailto:sp at scali.com |  http://www.scali.com  | performing MPI implementation:
Tel: (+47) 2262 8950 |   Olaf Helsets vei 6   |      - ScaMPI 1.13.8 -
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