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The near mode fallback algorithm consists of a left/right scan of the bnobt. This algorithm has very poor breakdown characteristics under worst case free space fragmentation conditions. If a suitable extent is far enough from the locality hint, each allocation may scan most or all of the bnobt before it completes. This causes pathological behavior and extremely high allocation latencies. While locality is important to near mode allocations, it is not so important as to incur pathological allocation latency to provide the asolute best available locality for every allocation. If the allocation is large enough or far enough away, there is a point of diminishing returns. As such, we can bound the overall operation by including an iterative cntbt lookup in the broader search. The cntbt lookup is optimized to immediately find the extent with best locality for the given size on each iteration. Since the cntbt is indexed by extent size, the lookup repeats with a variably aggressive increasing search key size until it runs off the edge of the tree. This approach provides a natural balance between the two algorithms for various situations. For example, the bnobt scan is able to satisfy smaller allocations such as for inode chunks or btree blocks more quickly where the cntbt search may have to search through a large set of extent sizes when the search key starts off small relative to the largest extent in the tree. On the other hand, the cntbt search more deterministically covers the set of suitable extents for larger data extent allocation requests that the bnobt scan may have to search the entire tree to locate. Signed-off-by: Brian Foster <bfoster@redhat.com> Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com> |
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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.