Re: [Sheflug] memory usage in linux

[Seb James <seb@xxxxxxxxxxxx>]

On Mon, 2007-11-26 at 10:59 +0000, Alex Hudson wrote:
> On Mon, 2007-11-26 at 10:12 +0000, David Willington wrote:
> > I'm not in front of the machine at the moment so I can't cut and paste
> > output, but without anyone logged on the output of top shows 2Gb memory
> > being used, and that's without a graphical interface. I hope the problem
> > is that I don't understand how to interpret the figures I see, which is
> > why I'm trying to find a comprehensive explanation of how memory works and
> > how I interpret these 
> 
> In general, Linux will use all your memory - what isn't used for holding
> programs is used to cache the disk to speed up file access. So, it's
> rare for a machine to be on any serious length of time and not have most
> of it's RAM in use.
> 
> If you look at the output of free, off my laptop:
> 
> $ free
>            total       used       free     shared    buffers     cached
> Mem:     2061344    1063284     998060          0      46600     386384
> -/+ buffers/cache:   630300    1431044
> Swap:    2031608          0    2031608
> 
> You can see that my system is RAM-plentiful and hasn't been on that
> long, because although I have 2Gb of swap, none of it is in use. On a
> normal system, you'd expect some of the swap to be in use.
> 
> The "true" RAM free amount is the second figure in the "free" column:
> it's the amount of memory that is free plus the amount which is being
> used as a disk buffer (because that memory can be reclaimed at any time:
> if the data is needed again, you just read it off the disk again). So I
> have effectively ~1.4Gb RAM free. *However* - just because the RAM can
> be reclaimed doesn't really mean it's free: if I'm having to reclaim
> cache and read the data back off disc later, the machine is probably
> still going to be marginally slower.
> 
> If you're looking in ps, there are similar issues. There are two
> columns, VSZ (Virtual Size) and RSS (Resident Set Size). The true usage
> of any program is somewhere inbetween these two figures.
> 
> The virtual size is the complete amount of virtual memory in use by the
> program. This isn't a terribly interesting figure, because it includes
> kernel data, read-only and shared memory, so it could be quite huge even
> for a small program. Things like libc would get counted in that figure,
> and every program pretty much needs libc - plus, your system is probably
> using two or three copies of libc in the absolute worst case, not one
> for each program.
> 
> RSS is the amount of 'dirty' RAM in use by the program, and is the stuff
> not shared by other programs pretty much. However, this is very much a
> low water mark.
> 
> If a program is highly threaded, the RSS figure per-thread is pretty
> close to being accurate. If it's not threaded, and isn't sharing many
> libraries with other programs on the system, the VSZ is closer.
> 
> It's virtually impossible to get an absolute value for the RAM use
> simply because it depends on how you define "use".

I'll just add that there is more memory usage data in /proc/meminfo -
that's the original source of the figures output by the free command.
The ps command will get its memory usage figures from the process
directories in the /proc tree.

Seb


_______________________________________________
        Sheffield Linux User's Group
  http://www.sheflug.org.uk/mailfaq.html
 GNU - The choice of a complete generation