Red Hat Enterprise Linux 5 Administration Unleashed

Optimizing Virtual Memory 429. vm.dirty_writeback_centisecsHow long a disk buffer can stay in RAM in a dirty state, specified in 100th of asecond. If this time is exceeded, the buffer is written back to disk when the pdflushdaemon runs next.21. vm.dirty_ratioLimit at which processes with dirty buffers start writing to disk (all at the sametime) instead of waiting on pdflush. The value is a percentage of total memory.. vm.dirty_background_ratioPercentage of memory. When this percentage is reached, dirty data is written out inthe background using the pdflush daemon.. vm.page-clusterSet to an integer value that controls the number of pages read at the same time,minimizing the number of disk reads. The number of pages read at once is equal to2^. For example, if this parameter is set to 3, 8 pages are read at once.. vm.overcommit_memoryThis parameter sets how the kernel handles memory allocation. If set to 0, thekernel will only assign memory to a program if there is enough free memory. If setto 1, the kernel will allocate memory even if all the memory has already been allocatedto other programs. If set to 2 and all the physical memory and swap spacehave already been allocated, the kernel will allow a certain amount of additionalmemory to be allocated based on the value of vm.overcommit_ratio. If set to 2, theswap space should be the amount of physical memory plus the amount of memorythe kernel is allowed to overcommit.CAUTIONUse caution when allowing the kernel to overallocate memory. If programs actually useall the memory they request, overallocating will slow down the performance of thesystem as memory has to be constantly swapped in and out as programs try tocompensate for the lack of available memory.. vm.overcommit_ratioIf vm.overcommit_memory is set to 2, for which the kernel allows memory allocationslarger than the total amount of available memory, this percentage value determineshow much additional memory can be allocated. It is expressed as a percentage of physicalRAM. For example, if a system has 8GB of RAM and 10 GB of swap space, a value of25 would allow the kernel to allocate an additional 2GB of memory (25% of 8GB).. vm.min_slab_ratioOnly for NUMA kernels. The percentage of total pages in each memory zone. Ifmore than this percentage of zone pages are reclaimable slab pages, slabs arereclaimed when a zone reclaim occurs.