View source for Guidelines and HOWTOs/Debugging/Shared Memory Usage in KDE

It is unfortunate that a lot of people don't understand the UNIX memory model. Unfortunately this is helped by tools like ps which are not able to provide accurate information about memory usage. In UNIX a process uses basically three kinds of memory segments: '''shared memory segments''', '''code segments''' and '''data segments'''.

'''Shared memory''' is used by shared libraries. This memory is shared
by all processes which use a certain library. Unfortunately there is no
easy way to determine how much shared memory is used by how many processes.
So a process can use 10Mb of shared memory, but you don't know whether this
memory is shared with 1, 2 or 10 processes. So if you have 10 processes who
each use 10Mb of shared memory this actually requires 10Mb in the best case
and 100Mb in the worst case.

'''Code segments''' contain the actual executable code of your program.
This memory is shared by all processes of this same program. If you start
your program 5 times, it needs to load the code segment of your program
only once.

'''Data segments''' contain the data of your program. This kind of memory
is very important because the data segments of a process are not shared
with other processes. Starting the same program 5 times makes that the data
segments are 5 times in memory.

The size reported by ps is typically just the numbers for shared, code and
data added. This is not a very accurate representation of the memory usage
of an application.

KDE applications tend to be reported as quite large because the numbers
reported include the size of the shared memory segments. This size is
added to the size of each kde application while in practice the shared
memory segments appear in memory only once. This is rather illusive,
imagine how the output of ps would look like if it included the size of
the UNIX kernel for each process!

Instead of looking at the output of ps you get a better idea of the actual
memory usage of an application by looking at the output of
 cat /proc/<pid-of-process>/status.

== Further Information ==

More information about this topic can be found in the article about [http://ktown.kde.org/~seli/memory/analysis.html memory usage analysis].