Emphasize usage of the `priority` parameter for recompression and explain why `algo` parameter can lead to unexpected behavior and thus is not recommended. Signed-off-by: Sergey Senozhatsky --- Documentation/admin-guide/blockdev/zram.rst | 40 ++++++++++----------- 1 file changed, 18 insertions(+), 22 deletions(-) diff --git a/Documentation/admin-guide/blockdev/zram.rst b/Documentation/admin-guide/blockdev/zram.rst index 451fa00d3004..967b58c3aad2 100644 --- a/Documentation/admin-guide/blockdev/zram.rst +++ b/Documentation/admin-guide/blockdev/zram.rst @@ -462,7 +462,7 @@ know it via /sys/block/zram0/bd_stat's 3rd column. recompression ------------- -With CONFIG_ZRAM_MULTI_COMP, zram can recompress pages using alternative +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 @@ -471,7 +471,7 @@ 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: +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. @@ -495,34 +495,43 @@ configuration::: #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, +Another device attribute that `CONFIG_ZRAM_MULTI_COMP` enables is `recompress`, which controls recompression. Examples::: #IDLE pages recompression is activated by `idle` mode - echo "type=idle" > /sys/block/zramX/recompress + echo "type=idle priority=1" > /sys/block/zramX/recompress #HUGE pages recompression is activated by `huge` mode - echo "type=huge" > /sys/block/zram0/recompress + echo "type=huge priority=2" > /sys/block/zram0/recompress #HUGE_IDLE pages recompression is activated by `huge_idle` mode - echo "type=huge_idle" > /sys/block/zramX/recompress + echo "type=huge_idle priority=1" > /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 + echo "threshold=3000 priority=1" > /sys/block/zramX/recompress #recompress idle pages larger than 2000 bytes - echo "type=idle threshold=2000" > /sys/block/zramX/recompress + echo "type=idle threshold=2000 priority=1" > \ + /sys/block/zramX/recompress It is also possible to limit the number of pages zram re-compression will attempt to recompress::: - echo "type=huge_idle max_pages=42" > /sys/block/zramX/recompress + echo "type=huge_idle priority=1 max_pages=42" > \ + /sys/block/zramX/recompress + +It is advised to always specify `priority` parameter. While it is also +possible to specify `algo` parameter, so that `zram` will use algorithm's +name to determine the priority, it is not recommended, since it can lead to +unexpected results when the same algorithm is configured with different +priorities (e.g. different parameters). `priority` is the only way to +guarantee that the expected algorithm will be used. During re-compression for every page, that matches re-compression criteria, ZRAM iterates the list of registered alternative compression algorithms in @@ -533,19 +542,6 @@ 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` or `priority` parameter::: - - #use zstd algorithm only (if registered) - echo "type=huge algo=zstd" > /sys/block/zramX/recompress - - #use zstd algorithm only (if zstd was registered under priority 1) - echo "type=huge priority=1" > /sys/block/zramX/recompress - memory tracking =============== -- 2.53.0.473.g4a7958ca14-goog