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Each zspage maintains ->inuse counter which keeps track of the number of objects stored in the zspage. The ->inuse counter also determines the zspage's "fullness group" which is calculated as the ratio of the "inuse" objects to the total number of objects the zspage can hold (objs_per_zspage). The closer the ->inuse counter is to objs_per_zspage, the better. Each size class maintains several fullness lists, that keep track of zspages of particular "fullness". Pages within each fullness list are stored in random order with regard to the ->inuse counter. This is because sorting the zspages by ->inuse counter each time obj_malloc() or obj_free() is called would be too expensive. However, the ->inuse counter is still a crucial factor in many situations. For the two major zsmalloc operations, zs_malloc() and zs_compact(), we typically select the head zspage from the corresponding fullness list as the best candidate zspage. However, this assumption is not always accurate. For the zs_malloc() operation, the optimal candidate zspage should have the highest ->inuse counter. This is because the goal is to maximize the number of ZS_FULL zspages and make full use of all allocated memory. For the zs_compact() operation, the optimal source zspage should have the lowest ->inuse counter. This is because compaction needs to move objects in use to another page before it can release the zspage and return its physical pages to the buddy allocator. The fewer objects in use, the quicker compaction can release the zspage. Additionally, compaction is measured by the number of pages it releases. This patch reworks the fullness grouping mechanism. Instead of having two groups - ZS_ALMOST_EMPTY (usage ratio below 3/4) and ZS_ALMOST_FULL (usage ration above 3/4) - that result in too many zspages being included in the ALMOST_EMPTY group for specific classes, size classes maintain a larger number of fullness lists that give strict guarantees on the minimum and maximum ->inuse values within each group. Each group represents a 10% change in the ->inuse ratio compared to neighboring groups. In essence, there are groups for zspages with 0%, 10%, 20% usage ratios, and so on, up to 100%. This enhances the selection of candidate zspages for both zs_malloc() and zs_compact(). A printout of the ->inuse counters of the first 7 zspages per (random) class fullness group: class-768 objs_per_zspage 16: fullness 100%: empty fullness 99%: empty fullness 90%: empty fullness 80%: empty fullness 70%: empty fullness 60%: 8 8 9 9 8 8 8 fullness 50%: empty fullness 40%: 5 5 6 5 5 5 5 fullness 30%: 4 4 4 4 4 4 4 fullness 20%: 2 3 2 3 3 2 2 fullness 10%: 1 1 1 1 1 1 1 fullness 0%: empty The zs_malloc() function searches through the groups of pages starting with the one having the highest usage ratio. This means that it always selects a zspage from the group with the least internal fragmentation (highest usage ratio) and makes it even less fragmented by increasing its usage ratio. The zs_compact() function, on the other hand, begins by scanning the group with the highest fragmentation (lowest usage ratio) to locate the source page. The first available zspage is selected, and then the function moves downward to find a destination zspage in the group with the lowest internal fragmentation (highest usage ratio). Link: https://lkml.kernel.org/r/20230304034835.2082479-3-senozhatsky@chromium.org Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org> Acked-by: Minchan Kim <minchan@kernel.org> Cc: Yosry Ahmed <yosryahmed@google.com> 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 Restructured Text 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.