The memory represented by "buffers/cache" in
free is your filesystem cache, which Linux caches to speed up reading data from your disk, as hitting the disk is generally a fairly slow way to access data repeatedly. As such, they are cached in memory, and transparently served from there if available.
You can see which blocks are currently in your cache by using
fincore. Here is an example from the project page:
# fincore --pages=false --summarize --only-cached *
stats for CLUSTER_LOG_2010_05_21.MYI: file size=93840384 , total pages=22910 , cached pages=1 , cached size=4096, cached perc=0.004365
stats for CLUSTER_LOG_2010_05_22.MYI: file size=417792 , total pages=102 , cached pages=1 , cached size=4096, cached perc=0.980392
stats for CLUSTER_LOG_2010_05_23.MYI: file size=826368 , total pages=201 , cached pages=1 , cached size=4096, cached perc=0.497512
stats for CLUSTER_LOG_2010_05_24.MYI: file size=192512 , total pages=47 , cached pages=1 , cached size=4096, cached perc=2.127660
stats for CLUSTER_LOG_2010_06_03.MYI: file size=345088 , total pages=84 , cached pages=43 , cached size=176128, cached perc=51.190476
As for how to clear them, from
man 5 proc:
/proc/sys/vm/drop_caches (since Linux 2.6.16)
Writing to this file causes the kernel to drop clean caches, dentries, and inodes from memory, causing that memory to become free. This can be useful for memory management testing and performing reproducible filesystem benchmarks. Because writing to this file causes the benefits of caching to be lost, it can degrade overall system performance.
- To free pagecache, use:
echo 1 > /proc/sys/vm/drop_caches
- To free dentries and inodes, use:
echo 2 > /proc/sys/vm/drop_caches
- To free pagecache, dentries and inodes, use:
echo 3 > /proc/sys/vm/drop_caches
Because writing to this file is a nondestructive operation and dirty objects are not freeable, the user should run sync(8) first.
You generally don't want to flush the cache, as its entire purpose is to improve performance, but for debugging purposes you can do so by using
drop_caches like so (note: you must be root to use drop_caches, but sync can be done as any user):
# sync && echo 3 > /proc/sys/vm/drop_caches