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 :ref:`ref-vsm` for details.

.. _platform-thp:

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 :ref:ref-varnishd-opt_j 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.

.. XXX:Maybe a sample-command of using/setting/changing these values? benc