vmtouch - the Virtual Memory Toucher

Portable file system cache diagnostics and control

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.

Quick install guide:

$ git clone https://github.com/hoytech/vmtouch.git $ cd vmtouch $ make $ sudo make install

What is it good for?

Discovering which files your OS is caching

Telling the OS to cache or evict certain files or regions of files

Locking files into memory so the OS won't evict them

Preserving virtual memory profile when failing over servers

Keeping a "hot-standby" file-server

Plotting filesystem cache usage over time

Maintaining "soft quotas" of cache usage

Speeding up batch/cron jobs

And much more...

Support

To complement the open source community, Hoytech offers services related to vmtouch:

Advanced feature development

Support contracts

Training sessions

Please contact Doug Hoyte for more information.

Examples

Example 1

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

Example 2

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 seconds

None of it. Now let's bring part of it into memory with tail:

$ tail -n 10000 big-dataset.txt > /dev/null

Now 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 seconds

vmtouch tells us that 4950 pages at the end of the file are now resident in memory.

Example 3

Let's touch the rest of /big-dataset.txt/ and bring it 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

Example 4

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

Example 5

Daemonise and lock all files in a directory into physical memory:

vmtouch -dl /var/www/htdocs/critical/

What other people are saying

People have found lots of uses for vmtouch over the years. Here are a few links in no particular order:

Articles

Real-world sightings

Instagram

Discussion about instagram's usage of vmtouch:

Stack-overflow and friends

OS packages/ports

Non-english

Misc

Other tools

There are also lots of mentions on twitter using the #vmtouch hash-tag

Have another link? Please let me know!

Author

vmtouch is copyright (c) 2009-2017 Doug Hoyte and contributors.

Contributors are listed in CHANGES.