Re: Memory reporting on CentOS Linux

On Fri, Aug 14, 2009 at 3:43 PM, Reid Thompson<reid(dot)thompson(at)ateb(dot)com> wrote:
> On Fri, 2009-08-14 at 14:00 -0400, Jeremy Carroll wrote:
>> I am confused about what the OS is reporting for memory usage on
>> CentOS 5.3 Linux. Looking at the resident memory size of the
>> processes. Looking at the resident size of all postgres processes, the
>> system should be using around 30Gb of physical ram. I know that it
>> states that it is using a lot of shared memory. My question is how to
>> I determine how much physical RAM postgres is using at any point in
>> time?
>>
>> This server has 24Gb of ram, and is reporting that 23GB is free for
>> use. See calculation below
>>
>> (Memory Total –  Used) + (Buffers + Cached) = Free Memory
>> (24675740 – 24105052) +  (140312 + 22825616) = 23,536,616 or ~23
>> Gigabytes
>>
> you're using cached swap in your calculation ( 22825616 )  swap is not
> RAM -- it's disk

As far as I know, cached is only on the next line as a formatting
convenience. It is unrelated to swap and not on disk. What could
"cached swap" possibly mean anyway?

Having said that, as a Linux user of many years, I have found that
per-process memory reporting is completely worthless. All you can
really do is look at the overall statistics for the box and try to get
some sense as to whether the box, over all, is struggling. Trying to
understand how individual processes are contributing to that is an
inexact science when it's not a complete waste of time.

...Robert