What is "residual memory"? Can anyone explain?

[Oleg Goldshmidt]

Shlomi Fish <shlomif at shlomifish.org> writes:

> Hi all.
>
> I reported a bug in Amarok (
> https://bugs.kde.org/show_bug.cgi?id=288876 ) where it sometimes
> consumes over 10% of my RAM on startup, and it was closed with this
> comment:
>
> <<<< Well, there is the virtual memory but it is the residual memory
> use that matters. Yours us just 371M which is absolutely correct.
> FWIW: I suggest you read up some documentation on dynamic memory use
> in Linux.  In short: the more memory available, the more will be
> used, the system distributes this evenly to the running processes
> depending on their priority.
>
> Not a bug.
>>>>>
>
> Well, I don't understand it. What is "residual memory" in this
> context (as Google searches for it yields junk.), and why is still
> OK that Amarok consumes so much.

Well, there are two issues here. 

1. I suspect/assume that "residual memory" means "resident set"
   (someone confused "resident" with "residual"?). This is the total
   size of pages mapped in RAM, per process, without VM or process
   kernel data structures. This has been discussed here before, e.g.,

http://www.mail-archive.com/linux-il@cs.huji.ac.il/msg31797.html

2. What was it that you considered a bug? The screenshot of top in
   your bug report shows resident set size (a.k.a. RSS, RES field in
   top) of 387M, which is consistent with 12.9% of your total 3G of
   RAM. If this is your complaint (why the process uses so much RAM?) 
   then someone did not read your report attentively enough - I mean,
   the response can be (mis)construed as "you've looked at a wrong
   field".

   At the same time, it is not at all clear why they should consider
   it a bug. Out of curiosity, I started amarok on my F14 box and its
   RSS is 90M (without doing anything at all). There are other
   processes (quite a few) running on the machine right now, and this
   may explain why it is smaller than in your case (you did a "clean"
   experment, didn't you?). The app may allocate and preload/cache
   all sorts of stuff if it can - if there is memory available, why
   withhold it? This is what they told you, in different words.

   Finally, I did

   egrep "^Vm" /proc/<pid>/status

   and the app's heap is almost 10 times larger than RSS, i.e., stuff
   was allocated but not mapped. Maybe this is what they had in mind
   when they said that RSS is what matters. Another datum - the size
   of shared libraries used, is also significantly (50%) more than
   RSS. And if you note that RSS counts shared pages as belonging to
   the process (since they are mapped into the process's virtual
   memory space) it stops looking so badly.
  
Hope something is clearer than it used to be.

-- 
Oleg Goldshmidt | pub at goldshmidt.org