# Platform specific notes

On some platforms it is necessary to adjust the operating system before running Varnish on it. The systems and steps known to us are described in this section.

## On Linux, use tmpfs for the workdir

Varnish uses mapped files for shared memory, for which performance depends on writes not blocking. On Linux, however, write throttling implemented by some file systems (which is generally useful in other scenarios) can interact badly with the way Varnish works and cause lockups or performance impacts. To avoid such problems, it is recommended to use a `tmpfs` “virtual memory file system” as the workdir.

To ensure `tmpfs` is used, check the following:

Determine the workdir. If you use a specific `-n` option to `varnishd` or set the `VARNISH_DEFAULT_N` variable, it is that value. Otherwise run `varnishd -x options`, which outputs the workdir default.

Run `df *workdir*`. If it reports `tmpfs` as the file system in the first column, no additional action is necessary.

Otherwise, consider creating a `tmpfs` mountpoint at workdir, or configure workdir on an existing `tmpfs`.

The `tmpfs` for workdir should be mounted with Transparent Hugepage disabled. Consider mounting the working directory with the `huge=never` mount option if that is not the default.

Note: Very valid reasons exist for not following this recommendation, if you know what you are doing.

## workdir can not be mounted `noexec`

Varnish compiles VCL to a shared object and needs to load it at runtime. So the workdir can not reside on a file system mounted with `noexec`.

## Lift locked memory limits

For the same reason as explained above, varnish tries to lock shared memory in RAM. Therefore, the locked memory limit should ideally be set to unlimited or sufficiently high to accommodate all mapped files. The specific minimum required value is dynamic, depending among other factors on the number of VCLs loaded and backends configured. As a rule of thumb, it should be a generous multiple of the size of workdir when varnish is running.

See VSM: Shared Memory Logging and Statistics for details.

## Transparent Hugepage on Linux

On certain Linux distributions Transparent Hugepage (THP) kernel support is enabled by default. This is known to cause instabilities of Varnish.

By default, Varnish tries to disable the THP feature, but does not fail if it can’t. The `linux` Jail offers to optionally enable, disable or ignore THP.

Alternatively, THP can be disabled system-wide. If Varnish is the only significant service running on this system, this can be done during runtime with:

    echo never > /sys/kernel/mm/transparent_hugepage/enabled

The setting can be also be persisted in the bootloader configuration by adding `transparent_hugepage=never` to the kernel command line.

## OpenVZ

It is possible, but not recommended for high performance, to run Varnish on virtualised hardware. Reduced disk and network -performance will reduce the performance a bit so make sure your system has good IO performance.

If you are running on 64bit OpenVZ (or Parallels VPS), you must reduce the maximum stack size before starting Varnish.

The default allocates too much memory per thread, which will make Varnish fail as soon as the number of threads (traffic) increases.

Reduce the maximum stack size by adding `ulimit -s 256` before starting Varnish in the init script.

## TCP keep-alive configuration

On some Solaris, FreeBSD and OS X systems, Varnish is not able to set the TCP keep-alive values per socket, and therefore the tcp_keepalive_ Varnish runtime parameters are not available. On these platforms it can be beneficial to tune the system wide values for these in order to more reliably detect remote close for sessions spending long time on waitinglists. This will help free up resources faster.

Systems that does not support TCP keep-alive values per socket include:

- Solaris releases prior to version 11

- FreeBSD releases prior to version 9.1

- OS X releases prior to Mountain Lion

On platforms with the necessary socket options the defaults are set to:

- tcp_keepalive_time = 600 seconds

- tcp_keepalive_probes = 5

- tcp_keepalive_intvl = 5 seconds

Note that Varnish will only apply these run-time parameters so long as they are less than the system default value.