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Fehnocache minimize caching effects GitHub

What is this tool good for:

What this tool is not good for:

So then why does this tool exist?

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 slowdown due to page cache thrashing.

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:

The effect is that cache space stays bounded by an additional max 500MiB:

Before:

During (notice how buff/cache only goes up by ~300MiB):

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:

Install cgroup-tools and be prepared to enter your root password to initiallycreate cgroups.

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

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).

Just type make. Then, prepend ./nocache to your command:

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.

For testing purposes, I included two small tools:

It looks like this:

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.

Without nocache, the file will be fully cached when you copy itsomewhere:

With nocache, the original caching state will be preserved.

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 , 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:

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.:

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 theNOCACHE_FLUSHALL environment variable to 1, e.g.:

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.

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.

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