patches/old/documentation-add-recompression-documentation-fix.patch

From: Sergey Senozhatsky <senozhatsky@chromium.org>
Subject: Re: documentation: Add recompression documentation
Date: Tue, 18 Oct 2022 11:50:21 +0900

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

Link: https://lkml.kernel.org/r/Y04UbTZh3fJYPhvJ@google.com
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 Documentation/admin-guide/blockdev/zram.rst |   55 ++++++++++++++++++
 1 file changed, 55 insertions(+)

--- a/Documentation/admin-guide/blockdev/zram.rst~documentation-add-recompression-documentation-fix
+++ a/Documentation/admin-guide/blockdev/zram.rst
@@ -456,6 +456,61 @@ size:::
 Recompression is mostly focused on idle pages (except for huge pages
 recompression), so it works better in conjunction with memory tracking.
 
+recompression
+-------------
+
+With CONFIG_ZRAM_MULTI_COMP, zram can recompress idle/huge pages using
+alternative (secondary) compression algorithm. 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 will setup two compression algorithms
+per-CPU: primary and secondary ones. Primary zram compressor is explained
+in "3) Select compression algorithm", the secondary algorithm is configured
+in a similar way, using recomp_algorithm device attribute:
+
+Examples::
+
+	#show supported recompression algorithms
+	cat /sys/block/zramX/recomp_algorithm
+	zstd [lzo]
+
+	#select zstd recompression algorithm
+	echo zstd > /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 idle > /sys/block/zramX/recompress
+
+	#HUGE pages recompression is activated by `huge` mode
+	echo huge > /sys/block/zram0/recompress
+
+	#HUGE_IDLE pages recompression is activated by `huge_idle` mode
+	echo huge_idle > /sys/block/zramX/recompress
+
+The number of idle pages can be significant, so user-space can pass a size
+watermark (in bytes) value to the recompress knob, to filter out idle pages
+for recompression: zram will recompress only idle pages of equal or greater
+size (bytes):::
+
+	#recompress idle pages larger than 3000 bytes
+	echo 3000 > /sys/block/zramX/recompress
+
+	#recompress idle pages larger than 2000 bytes
+	echo 2000 > /sys/block/zramX/recompress
+
+Recompression is mostly focused on idle pages (except for huge pages
+recompression), so it works better in conjunction with memory tracking.
+
 memory tracking
 ===============
 
_