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5.5.3.1 Support for NUMA servers
NUMA is an architecture wherein the memory access time for different regions of memory from a given
processor varies according to the nodal distance of the memory region from the processor. Each region of
memory, to which access times are the same from any CPU, is called a node. The NUMA architecture
surpasses the scalability limits of SMP (Symmetric Multi-Processing) architecture.
With SMP, all memory accesses are posted to the same shared memory bus. This works fine for a relatively
small number of CPUs, but there is a problem with hundreds of CPUs competing for access to this shared
memory bus. NUMA alleviates these bottlenecks by limiting the number of CPUs on any one memory bus
and connecting the various nodes by means of a high-speed interconnect. NUMA allows SLES Server to
scale more efficiently for systems with dozens or hundreds of CPUs, because CPUs can access a dedicated
memory bus for local memory. It also supports multiple interconnected memory nodes, each supporting a
smaller number of CPUs.
Figure 5-64 shows a sample NUMA design. Each node in the system is simply a 4 processor SMP system.
Each CPU in the node contains a L1 and L2 cache. The node contains an L3 cache, which is shared by all
processors in the node. Nodes are connected together via an Interface/Node, which contains the SCI
interface.
The SLES kernel includes various data structures and macros for determining the layout of the memory and
processors on the system. These enable the VM subsystem to make decisions on the optimal placement of
memory for processes. For more information about NUMA, see http://lse.sourceforge.net/numa/.
5.5.3.2 Reverse map Virtual Memory
Reverse map Virtual Memory (Rmap VM) improves the operation of the kernel memory management
subsystem, resulting in improved performance for memory constrained systems, NUMA systems, and
systems with large aggregate virtual address spaces. Previously, the Linux memory management system
could efficiently perform virtual-to-physical address translation using page tables. However, translating a
physical address to its corresponding virtual addresses was inefficient and time consuming, because it
required the memory management software to traverse every page table in the system. In today's large
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Figure 5-64: NUMA Design