
Platform specific notes
------------------------

Transparent hugepages on Redhat Enterprise Linux 6
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

On RHEL6 Transparent Hugepage kernel support is enabled by default.
This is known to cause sporadic crashes of Varnish.

It is recommended to disable transparent hugepages on affected systems::

    $ echo "never" > /sys/kernel/mm/redhat_transparent_hugepage/enabled

On Debian/Ubuntu systems running 3.2 kernels the default value is "madvise" and does not need to changed.


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 to much memory per thread, which will make varnish fail
as soon as the number of threads (traffic) increases.

Reduce the maximum stack size by running::

    ulimit -s 256

in the Varnish startup script.

TCP keep-alive configuration
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

On some systems, Varnish is not able to set the TCP keep-alive values
per socket, and therefor the tcp_keepalive_* Varnish runtime
parameters are not available. On these platforms it can be benefitial
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 to 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.
