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Mon, 17 Oct 2022 19:50:25 -0700 (PDT) Date: Tue, 18 Oct 2022 11:50:21 +0900 From: Sergey Senozhatsky To: Andrew Morton , Minchan Kim Cc: Sergey Senozhatsky , Nitin Gupta , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCHv3 5/8] documentation: Add recompression documentation Message-ID: References: <20221009090720.1040633-1-senozhatsky@chromium.org> <20221009090720.1040633-6-senozhatsky@chromium.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20221009090720.1040633-6-senozhatsky@chromium.org> ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1666061437; a=rsa-sha256; cv=none; b=Y+azOpAUP9lu++n88truCeThArlqBdiBMAJe53xGYL+/23YziQSAuwytSmZotE/vadD8Vo oy/Mdd601+v+jGUy8y/PzWoN/gdw/kzmc/N2xVkr1wPIbTvvt7MjZ+qHa0CfK18IDhxVgj m2vq+yK+xr9UK7BvblWqArV4wVQftgo= ARC-Authentication-Results: i=1; imf27.hostedemail.com; dkim=pass header.d=chromium.org header.s=google header.b=Lvrit6Zx; dmarc=pass (policy=none) header.from=chromium.org; 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Signed-off-by: Sergey Senozhatsky --- Documentation/admin-guide/blockdev/zram.rst | 55 +++++++++++++++++++++ 1 file changed, 55 insertions(+) diff --git a/Documentation/admin-guide/blockdev/zram.rst b/Documentation/admin-guide/blockdev/zram.rst index c73b16930449..f09e778bc3dc 100644 --- a/Documentation/admin-guide/blockdev/zram.rst +++ b/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 (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 =============== -- 2.38.0.413.g74048e4d9e-goog