# 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.

## 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. This can be done with `echo never > /sys/kernel/mm/redhat_transparent_hugepage/enabled` (runtime) or by adding “transparent_hugepage=never” to the kernel boot line in the “/etc/grub.conf” file (persistent).

On Debian/Ubuntu systems running 3.2 kernels the default value is “madvise” and does not need to be 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 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.