vmtouch is a tool for learning about and controlling the file system cache of unix and unix-like systems. It is BSD licensed so you can basically do whatever you want with it.
$ gcc -Wall -O3 -o vmtouch vmtouch.c
$ su Password: # cp vmtouch /usr/local/bin/
How much of the /bin/ directory is currently in cache?
$ vmtouch /bin/ Files: 92 Directories: 1 Resident Pages: 348/1307 1M/5M 26.6% Elapsed: 0.003426 seconds
How much of big-dataset.txt is currently in memory?
$ vmtouch -v big-dataset.txt big-dataset.txt [ ] 0/42116 Files: 1 Directories: 0 Resident Pages: 0/42116 0/164M 0% Elapsed: 0.005182 secondsNone of it. Now let's bring part of it into memory with tail:
$ tail -n 10000 big-dataset.txt > /dev/nullNow how much?
$ vmtouch -v big-dataset.txt big-dataset.txt [ oOOOOOOO] 4950/42116 Files: 1 Directories: 0 Resident Pages: 4950/42116 19M/164M 11.8% Elapsed: 0.006706 secondsvmtouch tells us that 4950 pages at the end of the file are now resident in memory.
Let's bring the rest of big-dataset.txt into memory (pressing enter a few times to illustrate the animated progress bar you will see on your terminal):
$ vmtouch -vt big-dataset.txt big-dataset.txt [OOo oOOOOOOO] 6887/42116 [OOOOOOOOo oOOOOOOO] 10631/42116 [OOOOOOOOOOOOOOo oOOOOOOO] 15351/42116 [OOOOOOOOOOOOOOOOOOOOOo oOOOOOOO] 19719/42116 [OOOOOOOOOOOOOOOOOOOOOOOOOOOo oOOOOOOO] 24183/42116 [OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOo oOOOOOOO] 28615/42116 [OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOo oOOOOOOO] 31415/42116 [OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOo oOOOOOOO] 36775/42116 [OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOo oOOOOOOO] 39431/42116 [OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO] 42116/42116 Files: 1 Directories: 0 Touched Pages: 42116 (164M) Elapsed: 12.107 seconds
We have 3 big datasets, a.txt, b.txt, and c.txt but only 2 of them will fit in memory at once. If we have a.txt and b.txt in memory but would now like to work with b.txt and c.txt, we could just start loading up c.txt but then our system would evict pages from both a.txt (which we want) and b.txt (which we don't want).
So let's give the system a hint and evict a.txt from memory, making room for c.txt:
$ vmtouch -ve a.txt Evicting a.txt Files: 1 Directories: 0 Evicted Pages: 42116 (164M) Elapsed: 0.076824 seconds
Daemonise and lock all files in a directory into physical memory:
vmtouch -dl /var/www/htdocs/critical/