IBM REDP-4285-00 User Manual
Chapter 1. Understanding the Linux operating system
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Draft Document for Review May 4, 2007 11:35 am
4285ch01.fm
1.2 Linux memory architecture
To execute a process, the Linux kernel allocates a portion of the memory area to the
requesting process. The process uses the memory area as workspace and performs the
required work. It is similar to you having your own desk allocated and then using the desktop
to scatter papers, documents and memos to perform your work. The difference is that the
kernel has to allocate space in more dynamic manner. The number of running processes
sometimes comes to tens of thousands and amount of memory is usually limited. Therefore,
Linux kernel must handle the memory efficiently. In this section, we describe the Linux
memory architecture, address layout, and how the Linux manages memory space efficiently.
requesting process. The process uses the memory area as workspace and performs the
required work. It is similar to you having your own desk allocated and then using the desktop
to scatter papers, documents and memos to perform your work. The difference is that the
kernel has to allocate space in more dynamic manner. The number of running processes
sometimes comes to tens of thousands and amount of memory is usually limited. Therefore,
Linux kernel must handle the memory efficiently. In this section, we describe the Linux
memory architecture, address layout, and how the Linux manages memory space efficiently.
1.2.1 Physical and virtual memory
Today we are faced with the choice of 32-bit systems and 64-bit systems. One of the most
important differences for enterprise-class clients is the possibility of virtual memory
addressing above 4 GB. From a performance point of view, it is therefore interesting to
understand how the Linux kernel maps physical memory into virtual memory on both 32-bit
and 64-bit systems.
important differences for enterprise-class clients is the possibility of virtual memory
addressing above 4 GB. From a performance point of view, it is therefore interesting to
understand how the Linux kernel maps physical memory into virtual memory on both 32-bit
and 64-bit systems.
As you can see in Figure 1-10 on page 12, there are obvious differences in the way the Linux
kernel has to address memory in 32-bit and 64-bit systems. Exploring the physical-to-virtual
mapping in detail is beyond the scope of this paper, so we highlight some specifics in the
Linux memory architecture.
kernel has to address memory in 32-bit and 64-bit systems. Exploring the physical-to-virtual
mapping in detail is beyond the scope of this paper, so we highlight some specifics in the
Linux memory architecture.
On 32-bit architectures such as the IA-32, the Linux kernel can directly address only the first
gigabyte of physical memory (896 MB when considering the reserved range). Memory above
the so-called ZONE_NORMAL must be mapped into the lower 1 GB. This mapping is
completely transparent to applications, but allocating a memory page in ZONE_HIGHMEM
causes a small performance degradation.
gigabyte of physical memory (896 MB when considering the reserved range). Memory above
the so-called ZONE_NORMAL must be mapped into the lower 1 GB. This mapping is
completely transparent to applications, but allocating a memory page in ZONE_HIGHMEM
causes a small performance degradation.
On the other hand, with 64-bit architectures such as x86-64 (also x64), ZONE_NORMAL
extends all the way to 64GB or to 128 GB in the case of IA-64 systems. As you can see, the
overhead of mapping memory pages from ZONE_HIGHMEM into ZONE_NORMAL can be
eliminated by using a 64-bit architecture.
extends all the way to 64GB or to 128 GB in the case of IA-64 systems. As you can see, the
overhead of mapping memory pages from ZONE_HIGHMEM into ZONE_NORMAL can be
eliminated by using a 64-bit architecture.