Linux kernel mirror (for testing)
git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
kernel
os
linux
zram: update recompression documentation
Emphasize usage of the `priority` parameter for recompression and explain
why `algo` parameter can lead to unexpected behavior and thus is not
recommended.
Link: https://lkml.kernel.org/r/20260311084312.1766036-5-senozhatsky@chromium.org
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Brian Geffon <bgeffon@google.com>
Cc: gao xu <gaoxu2@honor.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
authored by
Sergey Senozhatsky
and committed by
Andrew Morton
be5f13d9
5004a27e
+18
-22
+18
-22
Documentation/admin-guide/blockdev/zram.rst
+18
-22
Documentation/admin-guide/blockdev/zram.rst
···
462
462
recompression
463
463
-------------
464
464
465
-
With CONFIG_ZRAM_MULTI_COMP, zram can recompress pages using alternative
465
+
With `CONFIG_ZRAM_MULTI_COMP`, zram can recompress pages using alternative
466
466
(secondary) compression algorithms. The basic idea is that alternative
467
467
compression algorithm can provide better compression ratio at a price of
468
468
(potentially) slower compression/decompression speeds. Alternative compression
···
471
471
recompression - pages that are cold and sit in the memory can be recompressed
472
472
using more effective algorithm and, hence, reduce zsmalloc memory usage.
473
473
474
-
With CONFIG_ZRAM_MULTI_COMP, zram supports up to 4 compression algorithms:
474
+
With `CONFIG_ZRAM_MULTI_COMP`, zram supports up to 4 compression algorithms:
475
475
one primary and up to 3 secondary ones. Primary zram compressor is explained
476
476
in "3) Select compression algorithm", secondary algorithms are configured
477
477
using recomp_algorithm device attribute.
···
495
495
#select deflate recompression algorithm, priority 2
496
496
echo "algo=deflate priority=2" > /sys/block/zramX/recomp_algorithm
497
497
498
-
Another device attribute that CONFIG_ZRAM_MULTI_COMP enables is recompress,
498
+
Another device attribute that `CONFIG_ZRAM_MULTI_COMP` enables is `recompress`,
499
499
which controls recompression.
500
500
501
501
Examples:::
502
502
503
503
#IDLE pages recompression is activated by `idle` mode
504
-
echo "type=idle" > /sys/block/zramX/recompress
504
+
echo "type=idle priority=1" > /sys/block/zramX/recompress
505
505
506
506
#HUGE pages recompression is activated by `huge` mode
507
-
echo "type=huge" > /sys/block/zram0/recompress
507
+
echo "type=huge priority=2" > /sys/block/zram0/recompress
508
508
509
509
#HUGE_IDLE pages recompression is activated by `huge_idle` mode
510
-
echo "type=huge_idle" > /sys/block/zramX/recompress
510
+
echo "type=huge_idle priority=1" > /sys/block/zramX/recompress
511
511
512
512
The number of idle pages can be significant, so user-space can pass a size
513
513
threshold (in bytes) to the recompress knob: zram will recompress only pages
514
514
of equal or greater size:::
515
515
516
516
#recompress all pages larger than 3000 bytes
517
-
echo "threshold=3000" > /sys/block/zramX/recompress
517
+
echo "threshold=3000 priority=1" > /sys/block/zramX/recompress
518
518
519
519
#recompress idle pages larger than 2000 bytes
520
-
echo "type=idle threshold=2000" > /sys/block/zramX/recompress
520
+
echo "type=idle threshold=2000 priority=1" > \
521
+
/sys/block/zramX/recompress
521
522
522
523
It is also possible to limit the number of pages zram re-compression will
523
524
attempt to recompress:::
524
525
525
-
echo "type=huge_idle max_pages=42" > /sys/block/zramX/recompress
526
+
echo "type=huge_idle priority=1 max_pages=42" > \
527
+
/sys/block/zramX/recompress
528
+
529
+
It is advised to always specify `priority` parameter. While it is also
530
+
possible to specify `algo` parameter, so that `zram` will use algorithm's
531
+
name to determine the priority, it is not recommended, since it can lead to
532
+
unexpected results when the same algorithm is configured with different
533
+
priorities (e.g. different parameters). `priority` is the only way to
534
+
guarantee that the expected algorithm will be used.
526
535
527
536
During re-compression for every page, that matches re-compression criteria,
528
537
ZRAM iterates the list of registered alternative compression algorithms in
···
541
532
no secondary algorithms left to try. If none of the secondary algorithms can
542
533
successfully re-compressed the page such a page is marked as incompressible,
543
534
so ZRAM will not attempt to re-compress it in the future.
544
-
545
-
This re-compression behaviour, when it iterates through the list of
546
-
registered compression algorithms, increases our chances of finding the
547
-
algorithm that successfully compresses a particular page. Sometimes, however,
548
-
it is convenient (and sometimes even necessary) to limit recompression to
549
-
only one particular algorithm so that it will not try any other algorithms.
550
-
This can be achieved by providing a `algo` or `priority` parameter:::
551
-
552
-
#use zstd algorithm only (if registered)
553
-
echo "type=huge algo=zstd" > /sys/block/zramX/recompress
554
-
555
-
#use zstd algorithm only (if zstd was registered under priority 1)
556
-
echo "type=huge priority=1" > /sys/block/zramX/recompress
557
535
558
536
memory tracking
559
537
===============