For those interested I've had a reply off Dell and Redhat for this issue:
We have done some research & this is what our software tech & RH have stated --> i.e.: you REALLY needs to use a 64bit OS. This is a typical behavior of 32 bit systems with a lot of memory. In Linux, the system memory is divided into 3 zones, namely: zone DMA (first 16 MB), zone Normal (from 16 MB - ~1 GB) and zoen HighMem (remainder 1 GB - 64 GB here). Unfortunately, in the 32 bits intel platforms, the kernel is restricted to do all its memory allocaltions in the zone Normal. With 64 GB of memory, more than half of the zone Normal is consumed by data structures used to describe and manage the rest of the memory. Since those are kernel data structures, they cannot be moved to Highmem. This leaves very little room for the kernel to run its operations. This is why the OOM killer is triggering despite the fact that the system still has 48 GB of memory free (MemFree: 48190756 kB). However, the free memory is all in the Highmem zone which cannot be used by kernel alloctions. Kernel allocations are restricted to the Normal zone (low memory) where you have only 50 MB free (LowFree: 51084) and I guess there was even less when the OOM killer triggered. The RHEL 5 PAE kernel is support with up to 16 GB of RAM, and should work correctly with up to 32 GB of RAM, but beyond that, it is strongly recommended that you switch to a 64 bit kernel. We do not ship a kernel-hugemem in RHEL 5 because of several reasons: 1. the RHEL 5 PAE kernel handles memory in a better way and we do not require the 4GB/4GB address space split used by -hugemem 2. it became extremely difficult to maintain a reliable -hugemem kernel in addition to the Xen patches in RHEL 5 3. almost all new servers are 64 bit anyway and we'd like to discourage running large memory systems in 32 bit mode because it causes too many problems 4. now, it is very safe to run 32 bit applications on 64 bits kernels Kind Regards Nick Lunt Managed Services and O/S Analyst Patech Solutions Limited Tel: 01543 444 707 Fax: 01543 444 709 Tame House, Fradley Park, Lichfield, Staffordshire, WS13 8RZ www.patech-solutions.com <http://www.patech-solutions.com/home.htm> From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Nick Lunt Sent: 24 September 2008 11:46 To: Red Hat Enterprise Linux 5 (Tikanga) discussion mailing-list Subject: [rhelv5-list] PAE Kernel 32 bit RHEL 5.2 64GB RAM Hi folks, We've got a RHEL 5.2 32 bit server with 64GB of RAM. We had to use 32 bit Redhat due to the application. Were using 2.6.18-92.1.10.el5PAE kernel and the OS can see all 64GB, we can also stress the server and it will use all 64GB. The problem is oom-killer is going ballistic, killing processes all over the place. I've logged a call with Dell about this and they say we cannot use > 16GB of RAM on a 32 bit kernel. RHEL 5 doesn't have a Hugemem kernel hence the PAE kernel. I got the following info from here http://en.wikipedia.org/wiki/Physical_Address_Extension "The x86 processor hardware is augmented with additional address lines used to select the additional memory, so physical address size is increased from 32 bits to 36 bits. This increases maximum physical memory size from 4 GB to 64 GB." So can anyone confirm that we 'cannot' use 64GB RAM on a 32 bit system with the PAE kernel, or offer any other advice please ? Kind Regards Nick Lunt Managed Services and O/S Analyst Patech Solutions Limited Tel: 01543 444 707 Fax: 01543 444 709 Tame House, Fradley Park, Lichfield, Staffordshire, WS13 8RZ www.patech-solutions.com <http://www.patech-solutions.com/home.htm>
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