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

[From: Allen Brown]

The subject line is incorrect.  Swap is the portion of virtual
memory which is not physical.  What you should have said is
"swap not a substitute for *physical* memory".
-- 
Allen Brown
http://brown.armoredpenguin.com/~abrown

> 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.
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