[CentOS] Good value for /proc/sys/vm/min_free_kbytes

Thu Dec 7 17:46:06 UTC 2006
Daniel de Kok <daniel-lists at taickim.com>


On Thu, 7 Dec 2006 08:03:54 -0800 (PST)
Martin Knoblauch <spamtrap at knobisoft.de> wrote:
>  what would be a good value for
> /proc/sys/vm/min_free_kbytes
>  on a Dual-CPU EM64T System with 8 GB of memory. Kernel is
> 2.6.9-42.0.3.ELsmp. I know that the default might be a bit low.
> Unfortunatelly the documentation is a bit weak in this area.
>  We are experiencing responsiveness problems (and higher than expected
> load) when the system is under combined memory+network+disk-IO stress.

Before changing this tunable, it is important to realize what it is
for. The free area is primarily useful to give the kernel enough memory
to act in situations where the swapper suddenly has to do a lot of work.
In other situations, you can make things worse, since you are
effectively giving less memory for applications. The following thread
may be interesting in this respect:


Use the usual suspects to nail down the bottleneck. It may well be the
case that the system has too little disk bandwidth, too little CPU
power, or too little memory to handle its load. But there's no way to
tell without more information. Touch vm_min_free bytes, if analysis
proves that e.g. kswapd has trouble to page out memory pages in a
timely fashion. A good value to start with is 4096KB if it is higher
than your current vm_min_free bytes, though I have seen others setting
it much higher.

-- Daniel