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Posted by ascultradio on September 3, 2009

Introduction :

This article discusses Red Hat Enterprise Linux optimizations for x86 (32 bit) and x86-64 (64 bit) platforms running Oracle 9i R2 (32bit/64bit) and Oracle 10g R1/R2 (32bit/64bit) standalone and RAC databases. This guide covers Red Hat Enterprise Linux Advanced Server 2.1, 3, and 4. Various workarounds covered in this article are due to the 32-bit address limitations of the x86 platform. However, many steps described in this document also apply to x86-64 platforms. Sections that do not specifically say that its only applicable to 32-bit or 64-bit apply to both platforms. If you think that a section is not very clear on that, let me know. For supported system configurations and limits for Red Hat Enterprise Linux releases, see http://www.redhat.com/rhel/details/limits/.

Note this document comes without warranty of any kind. But every effort has been made to provide the information as accurate as possible. I welcome emails from any readers with comments, suggestions, and corrections at webmaster_at_puschitz.com.

Hardware Architectures and Linux Kernels


When it comes to large databases the hybrid x86-64 architecture platform is strongly recommended over the 32-bit x86 platform. 64-bit platforms can access more than 4GB of memory without workarounds. With 32-bit platforms there are several issues that require workaround solutions for databases that use lots of memory, for example refer to Using Very Large Memory (VLM). If you are not sure whether you are on a 32-bit or 64-bit hardware, run dmidecode or cat /proc/cpuinfo. Running uname -a can be misleading since 32-bit Linux kernels can run on x86-64 platforms. But if uname -a displays x86_64, then you are running a 64-bit Linux kernel on a x86-64 platform.

32-bit Architecture and the hugemem Kernel

The RHEL 3/4 smp kernel can be used on systems with up to 16 GB of RAM. The hugemem kernel is required in order to use all the memory on systems that have more than 16GB of RAM up to 64GB. However, I recommend the hugemem kernel even on systems that have 8GB of RAM or more due to the potential issue of “low memory” starvation (see next section) that can happen on database systems with 8 GB of RAM. The stability you get with the hugemem kernel on larger systems outperforms the performance overhead of address space switching.

With x86 architecture the first 16MB-896MB of physical memory is known as “low memory” (ZONE_NORMAL) which is permanently mapped into kernel space. Many kernel resources must live in the low memory zone. In fact, many kernel operations can only take place in this zone. This means that the low memory area is the most performance critical zone. For example, if you run many resources intensive applications/programs and/or use large physical memory, then “low memory” can become low since more kernel structures must be allocated in this area. Low memory starvation happens when LowFree in /proc/meminfo becomes very low accompanied by a sudden spike in paging activity. To free up memory in the low memory zone, the kernel bounces buffers aggressively between low memory and high memory which becomes noticeable as paging (don’t confuse it with paging to the swap partition). If the kernel is unable to free up enough memory in the low memory zone, then the kernel can hang the system.

Paging activity can be monitored using the vmstat command or using the sar command (option ‘-B’) which comes with the sysstat RPM. Since Linux tries to utilize the whole low memory zone, a low LowFree in /proc/meminfo does not necessarily mean that the system is out of low memory. However, when the system shows increased paging activity when LowFree gets below 50MB, then the hugemem kernel should be installed. The stability you gain from using the hugemem kernel makes up for any performance impact resulting from the 4GB-4GB kernel/user memory split in this kernel (a classic 32-bit x86 system splits the available 4 GB address space into 3 GB virtual memory space for user processes and a 1 GB space for the kernel). To see some allocations in the low memory zone, refer to /proc/meminfo and slabtop(1) for more information. Note that Huge Pages would free up memory in the low memory zone since the system has less bookkeeping to do for that part of virtual memory, see Large Memory Optimization (Big Pages, Huge Pages).

If you install the RHEL 3/4 hugemem kernel ensure that any proprietary drivers you are using (e.g. proprietary multipath drivers) are certified with the hugemem kernel.

In RHEL 2.1, the smp kernel is capable of handling up to 4GB of RAM. The kernel-enterprise kernel should be used for systems with more than 4GB of RAM up to 16GB.

64-bit Architecture

This is the architecture that should be used whenever possible. If you can go with a x86-64 platform ensure that all drivers you need are supported on x86-64 (e.g. proprietary multipath drivers etc.) Furthermore, ensure that all the required applications are supported on x86-64 as well.

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