Memory not freed?

[gaf at blu.org (Jerry Feldman)]

Again, generalizing. 
1. A program forks a copy of itself.
 That's actually what a fork() does except on most modern VM systems rather 
than a copy of itself, it just sets up the appropriate maps. The pages are 
marked copy-on-write. The text segment remains fully shared because that is 
read only. The data segment (including the heap) remain shared until the 
parent or child write into them, causing a copy.

2. process forks a new program, which is not already running.
    This is a fork-exec. 2 separate calls. A whole new set of memory is 
allocated for the new image. Remember, the fork() causes the second process 
to be run, and the exec() causes a new image to be loaded for that process. 
Normally what is done is that the appropriate pages are mapped. You do not 
want to load any pages into either memory or into backing store (eg. swap 
area) unless necessary. There are some very different strategies, and I am 
not exactly sure what Linux does in detail. But, as far as the user is 
concerned a completely new image is loaded.

3. process forks a new program, which IS already running. 
    Again, this is a fork()-exec(). The text segment is normally shared. 
The data segment is most likely not shared, but the data segment pages 
could be mapped until they are written to. I don't know exactly how Linux 
does it.

4. The shells. The shells normally do some processing before actually 
execing the command or program. Remember on a fork(), the child process 
inherits open file descriptors, some signal settings and sme other 
resources. The shell must perform a certain amount of cleanup.  It must 
also set up any pipes that the user has created, so I am adding this as the 
fourth case.

But, going back to the original question, Linux as well as some Unixes 
aggresssively use free memory for buffering and other things.
 
"Derek D. Martin" wrote:

> From a user's perspective, 
> Really, I think there are 3 cases of the fork scenario you describe:
> 
>  - process forks a copy of itself
>  - process forks a new program, which is not already running
>  - process forks a new program, which IS already running
-- 
Jerry Feldman <gaf at blu.org>
Boston Linux and Unix user group
http://www.blu.org


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