documentation: add zram recompression documentation

If admin wants to measure writeback count in a certain period, he could

know it via /sys/block/zram0/bd_stat's 3rd column.

recompression

-------------

With CONFIG_ZRAM_MULTI_COMP, zram can recompress pages using alternative

(secondary) compression algorithms. The basic idea is that alternative

compression algorithm can provide better compression ratio at a price of

(potentially) slower compression/decompression speeds. Alternative compression

algorithm can, for example, be more successful compressing huge pages (those

that default algorithm failed to compress). Another application is idle pages

recompression - pages that are cold and sit in the memory can be recompressed

using more effective algorithm and, hence, reduce zsmalloc memory usage.

With CONFIG_ZRAM_MULTI_COMP, zram supports up to 4 compression algorithms:

one primary and up to 3 secondary ones. Primary zram compressor is explained

in "3) Select compression algorithm", secondary algorithms are configured

using recomp_algorithm device attribute.

Example:::

	#show supported recompression algorithms

	cat /sys/block/zramX/recomp_algorithm

	#1: lzo lzo-rle lz4 lz4hc [zstd]

	#2: lzo lzo-rle lz4 [lz4hc] zstd

Alternative compression algorithms are sorted by priority. In the example

above, zstd is used as the first alternative algorithm, which has priority

of 1, while lz4hc is configured as a compression algorithm with priority 2.

Alternative compression algorithm's priority is provided during algorithms

configuration:::

	#select zstd recompression algorithm, priority 1

	echo "algo=zstd priority=1" > /sys/block/zramX/recomp_algorithm

	#select deflate recompression algorithm, priority 2

	echo "algo=deflate priority=2" > /sys/block/zramX/recomp_algorithm

Another device attribute that CONFIG_ZRAM_MULTI_COMP enables is recompress,

which controls recompression.

Examples:::

	#IDLE pages recompression is activated by `idle` mode

	echo "type=idle" > /sys/block/zramX/recompress

	#HUGE pages recompression is activated by `huge` mode

	echo "type=huge" > /sys/block/zram0/recompress

	#HUGE_IDLE pages recompression is activated by `huge_idle` mode

	echo "type=huge_idle" > /sys/block/zramX/recompress

The number of idle pages can be significant, so user-space can pass a size

threshold (in bytes) to the recompress knob: zram will recompress only pages

of equal or greater size:::

	#recompress all pages larger than 3000 bytes

	echo "threshold=3000" > /sys/block/zramX/recompress

	#recompress idle pages larger than 2000 bytes

	echo "type=idle threshold=2000" > /sys/block/zramX/recompress

Recompression of idle pages requires memory tracking.

During re-compression for every page, that matches re-compression criteria,

ZRAM iterates the list of registered alternative compression algorithms in

order of their priorities. ZRAM stops either when re-compression was

successful (re-compressed object is smaller in size than the original one)

and matches re-compression criteria (e.g. size threshold) or when there are

no secondary algorithms left to try. If none of the secondary algorithms can

successfully re-compressed the page such a page is marked as incompressible,

so ZRAM will not attempt to re-compress it in the future.

This re-compression behaviour, when it iterates through the list of

registered compression algorithms, increases our chances of finding the

algorithm that successfully compresses a particular page. Sometimes, however,

it is convenient (and sometimes even necessary) to limit recompression to

only one particular algorithm so that it will not try any other algorithms.

This can be achieved by providing a algo=NAME parameter:::

	#use zstd algorithm only (if registered)

	echo "type=huge algo=zstd" > /sys/block/zramX/recompress

memory tracking

===============