The patch titled
Subject: documentation: add recompression documentation
has been added to the -mm mm-unstable branch. Its filename is
documentation-add-recompression-documentation.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/documentation-add-recompression-documentation.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
Subject: documentation: add recompression documentation
Date: Sun, 9 Oct 2022 18:07:17 +0900
Document user-space visible device attributes that are enabled by
ZRAM_MULTI_COMP.
Link: https://lkml.kernel.org/r/20221009090720.1040633-6-senozhatsky@xxxxxxxxxxxx
Signed-off-by: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Cc: Nitin Gupta <ngupta@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
Documentation/admin-guide/blockdev/zram.rst | 55 ++++++++++++++++++
drivers/block/zram/zram_drv.c | 7 --
2 files changed, 57 insertions(+), 5 deletions(-)
--- a/Documentation/admin-guide/blockdev/zram.rst~documentation-add-recompression-documentation
+++ a/Documentation/admin-guide/blockdev/zram.rst
@@ -401,6 +401,61 @@ 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 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 value to the recompress knob, to filter out idle pages for
+recompression: zram will recompress only idle pages of equal or greater
+size:::
+
+ #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
===============
--- a/drivers/block/zram/zram_drv.c~documentation-add-recompression-documentation
+++ a/drivers/block/zram/zram_drv.c
@@ -1623,9 +1623,7 @@ out:
*
* Corresponding ZRAM slot should be locked.
*/
-static int zram_recompress(struct zram *zram,
- u32 index,
- struct page *page,
+static int zram_recompress(struct zram *zram, u32 index, struct page *page,
int size_watermark)
{
unsigned long handle_prev;
@@ -1708,8 +1706,7 @@ static int zram_recompress(struct zram *
static ssize_t recompress_store(struct device *dev,
struct device_attribute *attr,
- const char *buf,
- size_t len)
+ const char *buf, size_t len)
{
struct zram *zram = dev_to_zram(dev);
unsigned long nr_pages = zram->disksize >> PAGE_SHIFT;
_
Patches currently in -mm which might be from senozhatsky@xxxxxxxxxxxx are
zram-preparation-for-multi-zcomp-support.patch
zram-add-recompression-algorithm-sysfs-knob.patch
zram-factor-out-wb-and-non-wb-zram-read-functions.patch
zram-introduce-recompress-sysfs-knob.patch
documentation-add-recompression-documentation.patch
zram-add-recompression-algorithm-choice-to-kconfig.patch
zram-add-recompress-flag-to-read_block_state.patch
zram-correct-typos.patch
