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234fdd2815
There are two separate checks in the bounds checker, the first one being a special case of the second. As this function is performance critical due to checking access to any eb member, reducing the size can slightly improve performance. On a release build on x86_64 the helper is completely inlined so the function call overhead is also gone. There was a report of 5% performance drop on metadata heavy workload, that disappeared after disabling asserts. The most significant part of that is the bounds checker. https://lore.kernel.org/linux-btrfs/20200724164147.39925-1-josef@toxicpanda.com/ After the analysis, the optimized code removes the worst overhead which is the function call and the performance was restored. https://lore.kernel.org/linux-btrfs/20200730110943.GE3703@twin.jikos.cz/ 1. baseline, asserts on, setget check on run time: 46s run time with perf: 48s 2. asserts on, comment out setget check run time: 44s run time with perf: 47s So this is confirms the 5% difference 3. asserts on, optimized seget check run time: 44s run time with perf: 47s The optimizations are reducing the number of ifs to 1 and inlining the hot path. Low-level stuff, gets the performance back. Patch below. 4. asserts off, no setget check run time: 44s run time with perf: 45s This verifies that asserts other than the setget check have negligible impact on performance and it's not harmful to keep them on. Analysis where the performance is lost: * check_setget_bounds is short function, but it's still a function call, changing the flow of instructions and given how many times it's called the overhead adds up * there are two conditions, one to check if the range is completely outside (member_offset > eb->len) or partially inside (member_offset + size > eb->len) Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com> Signed-off-by: David Sterba <dsterba@suse.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.