[PATCH 0/3] compressed in-memory swapping take4

[Nitin Gupta]

Hi,

Project home: http://compcache.googlecode.com

It allows creating a RAM based block device which acts as swap disk.
Pages swapped to this device are compressed and stored in memory itself.
This is a big win over swapping to slow hard-disk which are typically used
as swap disk. For flash, these suffer from wear-leveling issues when used
as swap disk - so again its helpful. For swapless systems, it allows more
apps to run.

* Changelog: take4 vs take3
xvmalloc changes:
 - Fixed regression in take3 that caused ramzswap write failures.
   This happened due to error in find_block() where we did not do
   explicit cast to 'unsigned long' when checking for bits set in
   bitmap. Now changed it to use kernel build-in test_bit().
 - Fix divide by zero error in proc read function.
ramzswap changes:
 - Forward write requests to backing swap device if allocation for
   compressed page fails.
 - Code cleanups.

(Please also see testing notes below).

* Changelog: take3 vs take2
xvmalloc changes:
 - Use kernel defined macros and constants in xvmalloc and remove
   equivalent defines for ALIGN, roundup etc.
 - Use kernel bitops (set_bit, clear_bit)
 - Moved it to drivers/block since its not clear if it has any other
   user.
ramzswap changes:
 - All instances of compcache renamed to ramzswap.
   Also renamed module to ramzswap
 - Renamed "backing_dev" parameter to "backing_swap"
 - Documentation changes to reflect above changes.
 - Remove "table index" from object header (4 bytes). This will be
   needed when memory defragmentation is implemented. So, avoid this
   (small) overhead for now.

* Changelog: take2 vs initial revision:
xvmalloc changes:
 - Use Linux kernel coding style for xvmalloc
 - Collapse all individual flag test/set/get to generic {test_set_get}_flag
 - Added BLOCK_NEXT() macro to reach next contiguous block
 - Other minor cleanups - no functional changes
compcache block device code:
 - compcache core changes due to change in xvmalloc interface names

* Testing notes:
 - Multiple cycles of 'scan' benchmark available at:
http://linux-mm.org/PageReplacementTesting
It does scans of anonymous mapped memory, both cyclic and use once.

Config:
Arch: x86 and x64
CPUs: 1/2, RAM: 512MB
backing swap: 768MB, ramzswap memlimit: 76MB (15% of RAM).

Continuously run 'scan' till it triggers 200K R/W operations on ramzswap.
Any incompressible pages were correctly forwarded to backing swap device.
cmd: ./scan 450 20 # scan over 450MB, 20 times.

 - Links to more performance numbers, use cases can be found at:
http://lkml.org/lkml/2009/3/17/116

Thanks to Ed Tomlinson for reporting bug in 'take3' patches
and to reviewers of previous versions.

Thanks,
Nitin

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