Verify Cache Memory with "cp" Command

Linux Tutorials - Herong's Tutorial Examples

∟Verify Cache Memory with "cp" Command

This section provides a tutorial example to verify the cache memory usage in the RAM free space using a single 'cp' command.

To verify my understanding of the cache memory in the RAM free space, I tried the following test:

1. Create a 100000 KiB file using the "dd" command on a large file.

herong$ dd count=200000 if=large.bin of=100000KiB.bin  

herong$ ls -l 100000KiB.bin
-rw-r--r--. 1 herong herong 102400000 Aug 14 09:16  100000KiB.bin

2. Free cache memory as much as possible using the "drop_caches" system parameter.

herong$ sudo sysctl -w vm.drop_caches=3

vm.drop_caches = 3

3. Check the cache memory with the "free" command.

herong$ free -w

          total     used     free  shared  buffers   cache  available
Mem:   15731448  9702632  5472488  149684     6284  550044    5570860
Swap:   8019964  3478492  4541472

4. Copy the 100000 KiB file to /dev/null. It took about 0.4 seconds.

herong$ time -p command cp 100000KiB.bin /dev/null

real 0.40
...

5. Check the cache memory again. I see the cache memory size increased about 100000 KiB (649892 - 550044 = 99848 KiB), matching well with 100000KiB.bin file size.

herong$ free -w

          total     used     free  shared  buffers   cache  available
Mem:   15731448  9703108  5372164  149684     6284  649892    5536464
Swap:   8019964  3478492  4541472

6. Copy the 100000 KiB file to /dev/null again. It took only 0.01 seconds this time, because the entire file is in the cache memory.

herong$ time -p command cp 100000KiB.bin /dev/null

real 0.01
...

7. Check the cache memory again. It had a small change.

herong$ free -w

          total     used     free  shared  buffers   cache  available
Mem:   15731448  9702812  5371988  149684     6284  650364    5536800
Swap:   8019964  3478492  4541472