Red Hat Enterprise Linux 5 Administration Unleashed

430CHAPTER 21Monitoring and Tuning the Kernel. vm.min_unmapped_ratioOnly for NUMA kernels. The percentage of total pages in each memory zone. If, andonly if, more than this percentage of zone pages are file backed and mapped, zonereclaim occurs. This setting ensures local pages are still available for file input andoutput even if a node is overallocated.. vm.panic_on_oomIf set to 0, the Out of Memory (OOM) killer is enabled. The OOM killer is enabled bydefault in the Red Hat Enterprise Linux kernel. When there is no memory left on thesystem and more memory is requested by an application, the kernel uses a complexalgorithm to determine which process to kill based on the usage of the process andhow much memory it has been allocated. This usually allows the system to stay upand running. If set to 1, the kernel panics when out-of-memory occurs.. vm.zone_reclaim_modeSets how memory is reclaimed when a zone runs out of memory. If set to 0, noreclaim occurs, and memory is allocated from other zones or nodes on the system.The value of this setting should be the ORed value from the following:. 1: Zone reclaim enabled. Easily reusable pages are reclaimed before allocatingpages in a different node.. 2: Zone reclaim writes dirty pages out. Dirty pages are written out if a zone fillsup and slows down the system.. 4: Zone reclaim swaps pages. Swapping pages limits allocations on local nodes.The default value is sometimes 1 instead of 0 if the kernel determines at bootup thatpages from remote zones will cause a significant decrease in performance.Managing Memory with NUMANon-Uniform Memory Access (NUMA) is a memory management technology available forsome multi-processor systems. NUMA works by having dedicated memory for eachprocessor to decrease access time and limit delays by preventing more than one processorfrom trying to access the same memory at the same time. When the same data inmemory is needed by more than one processor, the data must be moved betweenmemory banks, which slows down the process. The time saved by using NUMA dependson many factors including how many processors would otherwise be sharing the samememory bus, how often memory is accessed, whether the applications running werewritten for a NUMA system, and how much data in memory must be shared betweenprocessors.NUMA is enabled by default in Red Hat Enterprise Linux 5. In some older versions of RedHat Enterprise Linux, NUMA is disabled by default. Be sure to read the Release Notes todetermine whether NUMA is enabled or disabled by default.