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Linux Cached Memory

October 29, 2008 11 comments

In Linux, reading from a disk is very slow compared to accessing real memory. In addition, it is common to read the same part of a disk several times during relatively short periods of time. For example, one might first read an e-mail message, then read the letter into an editor when replying to it, then make the mail program read it again when copying it to a folder. Or, consider how often the command ls might be run on a system with many users. By reading the information from disk only once and then keeping it in memory until no longer needed, one can speed up all but the first read. This is called disk buffering, and the memory used for the purpose is called the buffer cache.

Unlike Windows other operating systems, Linux administers memory the smartest way it can.

Since unused memory is next to worthless, the filesystem takes whatever memory is left and caches it in order to speed up disk access. When the cache fills up, the data that has been unused for the longest time is discarded and the memory thus freed is used for the new data.

Whenever an application needs memory, the kernel makes the cache smaller; You do not need to do anything to make use of the cache, it happens completely automatically.

Freeing memory buffer does not make your programs faster… Actually it makes disk access slower.

BUT if for some reason (kernel debugging for example) you want to force the buffer to be freed, you need to set the drop_caches value to 1:

$ echo 1 > /proc/sys/vm/drop_caches

Issuing this command will release all the cached memory and also will stop collecting I/O buffer blocks. Let’s see an example of the effect:

Under normal situations, most of the memory is already cached by the system. But if we force the system to free the memory, you can see in the graph how the memory is suddenly dropped.

The technical details of how this works are explained on the Linux API

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Categories: Memory Tags: ,