[maemo-developers] swap not a substitute for virtual memory

[From: Neal H. Walfield]

At Wed, 16 Apr 2008 13:18:29 -0400,
Ian Lawrence wrote:
> 
> Hi,
> >  The issue is more that the whole "Web 2.0" hasn't been planned to be run
> >  in 64MB of RAM.  There are sites out there where a single Flash object
> >  or JavaScript script consumes more memory.  Even 128MB is tight.
> But, I mean it is possible to increase the amount of memory available
> for a 770 using an MMC card, right?. I am not sure what the upper
> limit is but 128MB + 64MB of system memory seems feasible.

You are confused about what swap is.  Swap allows the memory manager
to page (save) anonymous memory to backing store (which in this case
is your disk).

If union of the working set of the active applications (the memory
they need to run with only a trivial amount of faults, i.e., page-ins)
exceeds the amount of *RAM*, then your system thrashes.  As disk is
several orders of magnitude slower than RAM, in such a case, your
system makes no useful progress and will appear to you to have frozen.

The advantage of swap then, is that if you are running e.g. Mozilla
and its working set is large, then the memory manager can page the
anonymous memory of background applications (as well as the anonymous
memory that Mozilla uses, which is hopefully inactive).

Without swap, the only data that the memory manager can page is that
associated with files.  So, program and library text, data files, etc.
The problem is that most programs don't use the disk format for the
data they use: on bringing it into memory, they convert it to a form
useful for in-memory operations.  Thus, a lot of the memory on your
system will be occupied by such computed data, which, without swap, is
locked in memory until it is explicitly freed.


For what it is worth, after I removed /usr/bin/metalayer-crawler, my
system went from being completely unusable for anything but being a
broken alarm clock to being relatively useful.