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Target slot selection for recompression is just a simple iteration over zram->table entries (stored pages) from slot 0 to max slot. Given that zram->table slots are written in random order and are not sorted by size, a simple iteration over slots selects suboptimal targets for recompression. This is not a problem if we recompress every single zram->table slot, but we never do that in reality. In reality we limit the number of slots we can recompress (via max_pages parameter) and hence proper slot selection becomes very important. The strategy is quite simple, suppose we have two candidate slots for recompression, one of size 48 bytes and one of size 2800 bytes, and we can recompress only one, then it certainly makes more sense to pick 2800 entry for recompression. Because even if we manage to compress 48 bytes objects even further the savings are going to be very small. Potential savings after good re-compression of 2800 bytes objects are much higher. This patch reworks slot selection and introduces the strategy described above: among candidate slots always select the biggest ones first. For that the patch introduces zram_pp_ctl (post-processing) structure which holds NUM_PP_BUCKETS pp buckets of slots. Slots are assigned to a particular group based on their sizes - the larger the size of the slot the higher the group index. This, basically, sorts slots by size in liner time (we still perform just one iteration over zram->table slots). When we select slot for recompression we always first lookup in higher pp buckets (those that hold the largest slots). Which achieves the desired behavior. TEST ==== A very simple demonstration: zram is configured with zstd, and zstd with dict as a recompression stream. A limited (max 4096 pages) recompression is performed then, with a log of sizes of slots that were recompressed. You can see that patched zram selects slots for recompression in significantly different manner, which leads to higher memory savings (see column #2 of mm_stat output). BASE ---- *** initial state of zram device /sys/block/zram0/mm_stat 1750994944 504491413 514203648 0 514203648 1 0 34204 34204 *** recompress idle max_pages=4096 /sys/block/zram0/mm_stat 1750994944 504262229 514953216 0 514203648 1 0 34204 34204 Sizes of selected objects for recompression: ... 45 58 24 226 91 40 24 24 24 424 2104 93 2078 2078 2078 959 154 ... PATCHED ------- *** initial state of zram device /sys/block/zram0/mm_stat 1750982656 504492801 514170880 0 514170880 1 0 34204 34204 *** recompress idle max_pages=4096 /sys/block/zram0/mm_stat 1750982656 503716710 517586944 0 514170880 1 0 34204 34204 Sizes of selected objects for recompression: ... 3680 3694 3667 3590 3614 3553 3537 3548 3550 3542 3543 3537 ... Note, pp-slots are not strictly sorted, there is a PP_BUCKET_SIZE_RANGE variation of sizes within particular bucket. [senozhatsky@chromium.org: do not skip the first bucket] Link: https://lkml.kernel.org/r/20241001085634.1948384-1-senozhatsky@chromium.org Link: https://lkml.kernel.org/r/20240917021020.883356-4-senozhatsky@chromium.org Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org> Cc: Minchan Kim <minchan@kernel.org> Cc: Dan Carpenter <dan.carpenter@linaro.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.