====== Ubuntu - Memory - Tune virtual memory ======

Tune the virtual memory subsystem under Linux for better performance.

<WRAP warning>
**WARNING:**  Tuning the memory subsystem is a challenging task.  Wrong settings can affect the overall performance of your system.  It is recommended to modify one setting at a time and monitor your system for sometime.  If performance increases keep the settings else revert back.
</WRAP>

----

===== /proc/sys/vm =====

The files in the **/proc/sys/vm** directory can be used to tune the operation of the virtual memory (VM) subsystem of the Linux kernel:

<code bash>
ls -al /proc/sys/vm
</code>

Sample outputs:

<code bash>
total 0
-rw-r--r-- 1 root root 0 Oct 16 04:21 block_dump
-rw-r--r-- 1 root root 0 Oct 16 04:21 dirty_background_ratio
-rw-r--r-- 1 root root 0 Oct 16 04:21 dirty_expire_centisecs
-rw-r--r-- 1 root root 0 Oct 16 04:21 dirty_ratio
-rw-r--r-- 1 root root 0 Oct 16 04:21 dirty_writeback_centisecs
-rw-r--r-- 1 root root 0 Oct 16 04:21 drop_caches
-rw-r--r-- 1 root root 0 Oct 16 04:21 flush_mmap_pages
-rw-r--r-- 1 root root 0 Oct 16 04:21 hugetlb_shm_group
-rw-r--r-- 1 root root 0 Oct 16 04:21 laptop_mode
-rw-r--r-- 1 root root 0 Oct 16 04:21 legacy_va_layout
-rw-r--r-- 1 root root 0 Oct 16 04:21 lowmem_reserve_ratio
-rw-r--r-- 1 root root 0 Oct 16 04:21 max_map_count
-rw-r--r-- 1 root root 0 Oct 16 04:21 max_writeback_pages
-rw-r--r-- 1 root root 0 Oct 16 04:21 min_free_kbytes
-rw-r--r-- 1 root root 0 Oct 16 04:21 min_slab_ratio
-rw-r--r-- 1 root root 0 Oct 16 04:21 min_unmapped_ratio
-rw-r--r-- 1 root root 0 Oct 16 04:21 mmap_min_addr
-rw-r--r-- 1 root root 0 Oct 16 04:21 nr_hugepages
-r--r--r-- 1 root root 0 Oct 16 04:21 nr_pdflush_threads
-rw-r--r-- 1 root root 0 Oct 16 04:21 overcommit_memory
-rw-r--r-- 1 root root 0 Oct 16 04:21 overcommit_ratio
-rw-r--r-- 1 root root 0 Oct 16 04:21 pagecache
-rw-r--r-- 1 root root 0 Oct 16 04:21 page-cluster
-rw-r--r-- 1 root root 0 Oct 16 04:21 panic_on_oom
-rw-r--r-- 1 root root 0 Oct 16 04:21 percpu_pagelist_fraction
-rw-r--r-- 1 root root 0 Oct 16 04:21 swappiness
-rw-r--r-- 1 root root 0 Oct 16 04:21 swap_token_timeout
-rw-r--r-- 1 root root 0 Oct 16 04:21 vfs_cache_pressure
-rw-r--r-- 1 root root 0 Oct 16 04:21 zone_reclaim_mode
</code>

----

===== pdflush =====

Type the following command to see current wake up time of pdflush:

<code bash>
sysctl vm.dirty_background_ratio
</code>

Sample outputs:

<code bash>
sysctl vm.dirty_background_ratio = 10
</code>

**vm.dirty_background_ratio** contains 10, which is a percentage of total system memory, the number of pages at which the pdflush background writeback daemon will start writing out dirty data.  However, for fast RAID based disk system this may cause large flushes of dirty memory pages.  If you increase this value from 10 to 20 (a large value) will result into less frequent flushes:

<code bash>
sysctl -w vm.dirty_background_ratio=20
</code>

----

===== swappiness =====

Type the following command to see current default value:

<code bash>
sysctl vm.swappiness
</code>

Sample outputs:

<code bash>
vm.swappiness = 60
</code>

The value 60 defines how aggressively memory pages are swapped to disk.  If you do not want swapping, than lower this value.  However, if your system process sleeps for a long time you may benefit with an aggressive swapping behavior by increasing this value.  For example, you can change swappiness behavior by increasing or decreasing the value:

<code bash>
# sysctl -w vm.swappiness=100
</code>

----

===== dirty_ratio =====

Type the following command:

<code bash>
sysctl vm.dirty_ratio
</code>

Sample outputs:

<code bash>
vm.dirty_ratio = 40
</code>

The value 40 is a percentage of total system memory, the number of pages at which a process which is generating disk writes will itself start writing out dirty data.  This is nothing but the ratio at which dirty pages created by application disk writes will be flushed out to disk.  A value of 40 mean that data will be written into system memory until the file system cache has a size of 40% of the server’s RAM.  So if you have 12GB ram, data will be written into system memory until the file system cache has a size of 4.8G.  You change the dirty ratio as follows:

<code bash>
sysctl -w vm.dirty_ratio=25
</code>

----

===== Making Changes To VM Permanently =====

You need to add the settings to **/etc/sysctl.conf**.  


===== References =====

  * The /proc filesystem
  * man page sysctl