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From: Sergey Senozhatsky <>
To: Minchan Kim <>,
	Andrew Morton <>
Cc: Nitin Gupta <>,
	Suleiman Souhlal <>,,,
	Sergey Senozhatsky <>
Subject: [PATCHv5 11/13] documentation: Add zram recompression documentation
Date: Wed,  9 Nov 2022 20:50:45 +0900	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

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

Signed-off-by: Sergey Senozhatsky <>
 Documentation/admin-guide/blockdev/zram.rst | 81 +++++++++++++++++++++
 1 file changed, 81 insertions(+)

diff --git a/Documentation/admin-guide/blockdev/zram.rst b/Documentation/admin-guide/blockdev/zram.rst
index 177a142c3146..d898b7ace33d 100644
--- a/Documentation/admin-guide/blockdev/zram.rst
+++ b/Documentation/admin-guide/blockdev/zram.rst
@@ -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.
+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.
+	#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
+	#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.
+	#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

  parent reply	other threads:[~2022-11-09 11:52 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-11-09 11:50 [PATCHv5 00/13] zram: Support multiple compression streams Sergey Senozhatsky
2022-11-09 11:50 ` [PATCHv5 01/13] zram: Preparation for multi-zcomp support Sergey Senozhatsky
2022-11-09 11:50 ` [PATCHv5 02/13] zram: Add recompression algorithm sysfs knob Sergey Senozhatsky
2022-11-09 11:50 ` [PATCHv5 03/13] zram: Factor out WB and non-WB zram read functions Sergey Senozhatsky
2022-11-09 11:50 ` [PATCHv5 04/13] zram: Introduce recompress sysfs knob Sergey Senozhatsky
2022-11-10 13:09   ` Nathan Chancellor
2022-11-10 14:31     ` Sergey Senozhatsky
2022-11-10 14:38       ` Sergey Senozhatsky
2022-11-10 15:18         ` Nathan Chancellor
2022-11-10 14:34   ` [PATCH] zram: we should always zero out err variable in recompress loop Sergey Senozhatsky
2022-11-14  2:14   ` [PATCH] zram: explicitly limit prio_max for static analyzers Sergey Senozhatsky
2022-11-15  0:41     ` Andrew Morton
2022-11-15  0:47       ` Sergey Senozhatsky
2022-11-09 11:50 ` [PATCHv5 05/13] zram: Add recompress flag to read_block_state() Sergey Senozhatsky
2022-11-09 11:50 ` [PATCHv5 06/13] zram: Clarify writeback_store() comment Sergey Senozhatsky
2022-11-09 11:50 ` [PATCHv5 07/13] zram: Use IS_ERR_VALUE() to check for zs_malloc() errors Sergey Senozhatsky
2022-11-09 11:50 ` [PATCHv5 08/13] zram: add size class equals check into recompression Sergey Senozhatsky
2022-11-09 11:50 ` [PATCHv5 09/13] zram: remove redundant checks from zram_recompress() Sergey Senozhatsky
2022-11-09 11:50 ` [PATCHv5 10/13] zram: Add algo parameter support to zram_recompress() Sergey Senozhatsky
2022-11-09 11:50 ` Sergey Senozhatsky [this message]
2022-11-09 11:50 ` [PATCHv5 12/13] zram: add incompressible writeback Sergey Senozhatsky
2022-11-09 11:50 ` [PATCHv5 13/13] zram: Add incompressible flag to read_block_state() Sergey Senozhatsky
2022-11-09 21:46 ` [PATCHv5 00/13] zram: Support multiple compression streams Minchan Kim

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