[mythtv-users] Will moving to DDR-400 memory from DDR-333 improve performance?

[Michael Heironimus]

On Mon, Oct 30, 2006 at 09:13:45PM -0500, Brian J. Murrell wrote:
> On Mon, 2006-30-10 at 16:47 -0500, Chris Pinkham wrote:
> > USER       PID %CPU %MEM   VSZ  RSS TTY      STAT START   TIME COMMAND
> > mythtv    2074  0.1 11.8 248100 30244 pts/2  S    Oct28   3:27 mythbackend
>                            ^^^^^^ ^^^^^
> Uhm, these are in KiB you realize yes?
> > 
> >              total       used       free     shared    buffers     cached
> > Mem:        255208     245296       9912          0      31932     111540
>               ^^^^^^     ^^^^^^       ^^^^
> As are these
> 
> > OK, so we're using ~30MB there for mythbackend.
> 
> No, you are using 248MB.  Only 30 of it is in actual RAM though.  That
> doesn't mean that the process is not not using 248MB.  It's just doesn't
> need to access all 248MB at the exact moment in time that you took that
> process snapshot.  But take comfort in knowing that at points in time
> portions of that 30MB that are in RAM will need to be swapped out to
> disk and what is in swap will need to come into RAM.  This causes lots
> of disk trashing which of course means recording and playback burps and
> so forth.

VSZ is based on the number of virtual pages that have been allocated to
the process. VM pages can be backed by a number of things. For example:

- physical memory

- swap space (this is what you're thinking)

- mmap'd files

- mmap'd devices

- shared memory segments (which only exist once but are counted toward
  virtual set size in every attached process)

- nothing at all (if it hasn't been written it's not really there)

So no, that ~218MB difference between VSZ and RSS does not necessarily
mean that 218MB of this process is paged out to disk and subject to
recall.

-- 
Michael Heironimus