Red Hat Enterprise Linux 5 Administration Unleashed

428CHAPTER 21Monitoring and Tuning the Kernel. vm.percpu_pagelist_fractionMaximum pages allocated in each zone for any per_cpu_pagelist, as a fraction. Forexample, if this value is 50, a maximum of 1/50 of the pages in each zone can beallocated for each per_cpu_pagelist. Minimum value it can be set to is 8.. vm.min_free_kbytesMinimum number of kilobytes to keep free so the kernel can reserve pages for eachlowmem zone.. vm.drop_cachesUsed to manually free page cache, dentries, and inodes. Does not free dirty objects,so the sync command should be run first to force dirty objects to be written to disk.Use the echo command as root to set it to one of the following values (such as, echo1 > /proc/sys/vm/drop_caches):1: Free page cache2: Free dentries and inodes3: Free page cache, dentries, and inodes. vm.lowmem_reserve_ratioSets ratio of total pages to free pages for each memory zone.. vm.hugetlb_shm_groupGroup ID of the non-root group allowed to allocate huge pages for SHM_HUGETLBshm segments. Useful when using the Oracle® Database.. vm.nr_hugepagesCurrent number of reserved hugetlb pages. Change this value to allocate or deallocatehugepages.. vm.swappinessHow much the kernel should swap memory into swap space. The value is a percentageof physical memory.. vm.nr_pdflush_threadsNumber of pdflush threads running. Cannot be changed because it is a count ofrunning threads. Will be between the minimum and maximum pdflush threadsallowed. Useful in monitoring disk activity.. vm.dirty_expire_centisecsWhen dirty data can be written to disk by pdflush, in 100th of a second. After dirtydata has been in memory for this amount of time, it is written to disk the next timethe pdflush daemon writes dirty data to disk.