Stop and read before you blindly try this tool
What is this tool good for:
- Learn about how the Linux page cache works, and what syscalls exist to interfere with its normal operation.
- Learn what hacks are necessary to intercept syscalls via their libc wrappers.
What this tool is not good for:
- Controlling how your page cache is used
- Why do you think some random tool you found on GitHub can do better than the Linux Kernel?
- Defending against cache thrashing
- Use cgroups to bound the amount of memory a process has. See below or search the internet, this is widely known, works reliably, and does not introduce performance penalties or potentially dangerous behavior like this tool does.
- Making a binary run faster
nocacheintercepts a bunch of syscalls and does lots of speculative work; it will slow down your binary.
So then why does this tool exist?
- It was written in 2012, when cgroups, containerization etc. were all new things. A decade+ on, they aren’t any more.
How to run a process and its children in a memory-bounded cgroup
Do this if you e.g. want to run a backup but don’t want your system to slow down due to page cache thrashing.
If you use systemd
If your distro uses systemd, this is very easy. Systemd allows to run a
process (and its subprocesses) in a “scope”, which is a cgroup, and you can
specify parameters that get translated to cgroup limits.
When I run my backups, I do:
$ systemd-run --scope --property=MemoryLimit=500M -- backup commandThe effect is that cache space stays bounded by an additional max 500MiB:
Before:
$ free -h
total used free shared buff/cache available
Mem: 7.5G 2.4G 1.3G 1.0G 3.7G 3.7G
Swap: 9.7G 23M 9.7G
During (notice how buff/cache only goes up by ~300MiB):
free -h
total used free shared buff/cache available
Mem: 7.5G 2.5G 1.0G 1.1G 4.0G 3.6G
Swap: 9.7G 23M 9.7G
How does this work?
Use systemd-cgls to list the cgroups systemd creates. On my system, the above
command creates a group called run-u467.scope in the system.slice parent
group; you can inspect its memory settings like this:
$ mount | grep cgroup | grep memory
cgroup on /sys/fs/cgroup/memory type cgroup (rw,nosuid,nodev,noexec latime,memory)
$ cat /sys/fs/cgroup/memory/system.slice/run-u467.scope/memory.limit_in_bytes
524288000
The hard way
Install cgroup-tools and be prepared to enter your root password to initially
create cgroups.
sudo env ppid=$$ sh -c '
cgcreate -g memory:backup ;
echo 500M > /sys/fs/cgroup/memory/backup/memory.limit_in_bytes ;
echo $ppid > /sys/fs/cgroup/memory/backup/tasks ;
'
After entering this, your shell is a member of that cgroup, and any new process spawned will belong to that cgroup, too, and inherit the memory limit. The cgroups created like this won’t be cleaned up automatically.
More info: https://www.kernel.org/doc/Documentation/cgroup-v1/memory.txt
nocache - minimize filesystem caching effects
The nocache tool tries to minimize the effect an application has on
the Linux file system cache. This is done by intercepting the open
and close system calls and calling posix_fadvise with the
POSIX_FADV_DONTNEED parameter. Because the library remembers which
pages (ie., 4K-blocks of the file) were already in file system cache
when the file was opened, these will not be marked as "don't need",
because other applications might need that, although they are not
actively used (think: hot standby).
Installation and Usage
Just type make. Then, prepend ./nocache to your command:
./nocache cp -a ~/ /mnt/backup/home-$(hostname)The command make install will install the shared library, man
pages and the nocache, cachestats and cachedel commands
under /usr/local. You can specify an alternate prefix by using
make install PREFIX=/usr.
Debian packages are available, see https://packages.qa.debian.org/n/nocache.html.
Please note that nocache will only build on a system that has
support for the posix_fadvise syscall and exposes it, too. This
should be the case on most modern Unices, but kfreebsd notably has no
support for this as of now.
Testing
For testing purposes, I included two small tools:
cachedelcallsposix_fadvise(fd, 0, 0, POSIX_FADV_DONTNEED)on the file argument. Thus, if the file is not accessed by any other application, the pages will be eradicated from the fs cache. Specifying -n will repeat the syscall the given number of times which can be useful in some circumstances (see below).cachestatshas three modes: In quiet mode (-q), the exit status is 0 (success) if the file is fully cached. In normal mode, the number of cached vs. not-cached pages is printed. In verbose mode (-v), an actual map is printed out, where each page that is present in the cache is marked withx.
It looks like this:
$ cachestats -v ~/somefile.mp3
pages in cache: 85/114 (74.6%) [filesize=453.5K, pagesize=4K]
cache map:
0: |x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|
32: |x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|
64: |x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x|x| | | | | | | | | | | | |
96: | | | | | | | | | | | | | | | | | |x|
Also, you can use vmstat 1 to view cache statistics.
For debugging purposes, you can specify a filename that nocache should log
debugging messages to via the -D command line switch, e.g. use nocache -D /tmp/nocache.log …. Note that for simple testing the file /dev/stderr
might be a good choice.
Example run
Without nocache, the file will be fully cached when you copy it
somewhere:
$ ./cachestats ~/file.mp3
pages in cache: 154/1945 (7.9%) [filesize=7776.2K, pagesize=4K]
$ cp ~/file.mp3 /tmp
$ ./cachestats ~/file.mp3
pages in cache: 1945/1945 (100.0%) [filesize=7776.2K, pagesize=4K]
With nocache, the original caching state will be preserved.
$ ./cachestats ~/file.mp3
pages in cache: 154/1945 (7.9%) [filesize=7776.2K, pagesize=4K]
$ ./nocache cp ~/file.mp3 /tmp
$ ./cachestats ~/file.mp3
pages in cache: 154/1945 (7.9%) [filesize=7776.2K, pagesize=4K]
Limitations
The pre-loaded library tries really hard to catch all system calls
that open or close a file. This happens by "hijacking" the libc
functions that wrap the actual system calls. In some cases, this may
fail, for example because the application does some clever wrapping.
(That is the reason why __openat_2 is defined: GNU tar uses this
instead of a regular openat.)
However, since the actual fadvise calls are performed right before
the file descriptor is closed, this may not happen if they are left
open when the application exits, although the destructor tries to do
that.
There are timing issues to consider, as well. If you consider nocache cat <file>, in most (all?) cases the cache will not be restored. For
discussion and possible solutions see http://lwn.net/Articles/480930/.
My experience showed that in many cases you could "fix" this by doing
the posix_fadvise call twice. For both tools nocache and
cachedel you can specify the number using -n, like so:
$ nocache -n 2 cat ~/file.mp3
This actually only sets the environment variable NOCACHE_NR_FADVISE
to the specified value, and the shared library reads out this value.
If test number 3 in t/basic.t fails, then try increasing this number
until it works, e.g.:
$ env NOCACHE_NR_FADVISE=2 make test
One could also consider that the fact pages are kept mean the kernel considers they are hot, and decide the overhead of allocating one byte per page for mincore and the actual mincore calls are not worth it when the kernel actually does keep some pages when it wants to.
In this case you can either run nocache with -f or set the
NOCACHE_FLUSHALL environment variable to 1, e.g.:
$ nocache -f cat ~/file.mp3
$ env NOCACHE_FLUSHALL=1 make test
By default nocache will only keep track of file descriptors less than 2^20
that are opened by your application, in order to bound its memory
consumption. If you want to change this threshold, you can supply the
environment variable NOCACHE_MAX_FDS and set it to a higher (or lower) value.
It should specify a value one greater than the maximum file descriptor that
will be handled by nocache.
Acknowledgements
Most of the application logic is from Tobias Oetiker's patch for
rsync http://insights.oetiker.ch/linux/fadvise.html. Note however,
that rsync uses sockets, so if you try a nocache rsync, only
the local process will be intercepted.