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documentation: add zram recompression documentation

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Document user-space visible device attributes that are enabled by
ZRAM_MULTI_COMP.

Link: https://lkml.kernel.org/r/20221109115047.2921851-12-senozhatsky@chromium.org
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Acked-by: Minchan Kim <minchan@kernel.org>
Cc: Alexey Romanov <avromanov@sberdevices.ru>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Nitin Gupta <ngupta@vflare.org>
Cc: Suleiman Souhlal <suleiman@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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Sergey Senozhatsky 2022-11-09 20:50:45 +09:00 committed by Andrew Morton
parent a55cf9648d
commit 443dd79806
1 changed files with 81 additions and 0 deletions
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81
Documentation/admin-guide/blockdev/zram.rst
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@ -401,6 +401,87 @@ budget in next setting is user's job.
If admin wants to measure writeback count in a certain period, they 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
===============