Table of Contents
Advertisement
Chapter 2.
Hardware Architectures and Linux
Kernels
2.1. General
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
Chapter 17, Using Very Large Memory
databases that use lots of memory, for example refer to
(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.
2.2. 32 bit Architecture and the hugemem Kernel
In Red Hat Enterprise Linux 3, 4 or 5 the 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, it is recommend to use 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
resource-intensive applications or have a lot of memory installed and are simultaneously running
resource-intensive applications, memory addresses below 896MB can become constrained. This
happens because more kernel structures must be allocated to these low memory addresses. 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 (do
not 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
Chapter 17, Using Very Large Memory
(VLM).
memory, see
5
Advertisement
Table of Contents
