Re: Re: Swap Compression

["Jörn Engel" <joern@wohnheim.fh-wedel.de>]

On Fri, 25 April 2003 18:48:41 -0400, rmoser wrote:
> 
> Yeah, I had to mail it 3 times.  Lst time I figured it out.

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> As for the performance hit, the original idea of that very tiny format was
> to pack 6502 programs into 4k of code.  The expansion phase is very
> tight and very efficient, and on a ... anything... it will provide no problem.
> The swap-on-ram as long as it's not like 200 MB uncompressed SOR and
> 1 MB RAM will I think work great in the decompression phase.
> 
> Compression will take a little overhead.  I think if you use a boyer-moore
> fast string search algo for binary strings (yes you can do this), you can
> quickly compress the data.  It may be like.. just a guess... 10-30 times
> more overhead than the decompression phase.  So use it on at least a
> 10-30 mhz processor.  If I ever write the code, it won't be kernel; just the
> compression/decompression program (userspace).  Take the code and
> stuff it into the kernel if I do.  I'll at the point of the algo coming in to
> existence make another estimate.

A userspace program would be just fine. Send it to me and I'll convert
it to the kernel, putting it somewhere under lib/.
Do you have any problems with a GPL license to your code (necessary
for kernel port)?

> The real power in this is Swap on RAM, but setting that as having precidence
> over swap on disk (normal swaps) would decrease disk swap usage by
> supplying more RAM in RAM.  And of course swapping RAM to RAM is
> a lot faster.  I'm looking at this for PDA's but yes I will be running this on
> my desktop the day we see it.

Swapping RAM to RAM sounds interesting, but also quite complicated. As
a first step, I would try to compress the swap data before going to
disk, that should be relatively simple to do.

("I would" means, I will if I find the time for it.)

> Well, I could work on the compression code, mebbe I can put the tarball up
> here.  If I do I'd expect someone to add the code to swap to work with it--in
> kernel 2.4 at the very least (port it to dev kernel later!).  As a separate module.
> We don't want code that could be mean in the real swap driver. :)

Right. But for 2.4, there is no swap driver, that you can simply
enable or disable. I hacked up a patch, but so far, disabling swap
eats ~100k of memory every second, so that clearly needs more work.

Jörn

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