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SubjectNew Feature Idea: Compress swap file
How about, when using swap, it is compressed before it is written out?

It could be a two stage process, however, so as not to slow down performance.
Ie, a page needs to be swapped. First, it is written to a small, static
buffer(256k seems good. Should be dependent on physical ram size and page
size. Also might be useful to have it variable in size, like the disk
cache). If the machine is under heavy mem load, then eventually this buffer
will fill and need to be swapped. This is no change from previous behaviour.

However, when scheduling permits, the code can compress the buffer, and
optionally reread already swapped out, uncompressed pages to compress them.

This compress code could be optimized to only not compress executable pages,
as those don't always compress that well.

PS: The buffer would have low and high water marks, for both
swapping/compressing. Above the low_swap(size), it would start swapping as
time permitted. When above the high_swap, force swap until down to the
low_swap. When idle, do nothing. Below the low_compress(cpu usage), compress
the in-memory buffer. When below the high_compress, read in
uncompressed/swapped out pages into the buffer, but don't nescessarily
compress them yet(Cpu usage might suddenly spike back up, so wait and schedule
the compression in the low_compress loop).

Also, with this double buffer technique, Performance could increase, even if
compression is not used. Ie, buffer fills(but not overflows), and some pages
start to get swapped. However, they are not cleared from the buffer. If the
pages are then needed, then can be fetched from the buffer, without waiting
for the disk. This would, of course, also be applied to in memory compressed
pages.

And, one last thought. The buffer management would not need to actually copy
the pages in memory. Just remap them to its own control(this would almost
alleviate the need for a separate buffer).

Obviously, it is too late in 2.1 to have this for 2.2. But inclusion with 2.3
would be reasonable.

I don't know enough about compression/swap to start something like this, but
if I got some pointers, and guidance, I would be willing to help.

Adam.



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