mirror of
https://github.com/torvalds/linux.git
synced 2025-12-07 20:06:24 +00:00
d10bb374c4
Replace the bit by bit algorithm with one that generates 16 bits per
iteration on 32bit architectures and 32 bits on 64bit ones.
On my zen 5 this reduces the time for the tests (using the generic code)
from ~3350ns to ~1000ns.
Running the 32bit algorithm on 64bit x86 takes ~1500ns. It'll be slightly
slower on a real 32bit system, mostly due to register pressure.
The savings for 32bit x86 are much higher (tested in userspace). The
worst case (lots of bits in the quotient) drops from ~900 clocks to ~130
(pretty much independant of the arguments). Other 32bit architectures may
see better savings.
It is possibly to optimise for divisors that span less than
__LONG_WIDTH__/2 bits. However I suspect they don't happen that often and
it doesn't remove any slow cpu divide instructions which dominate the
result.
Typical improvements for 64bit random divides:
old new
sandy bridge: 470 150
haswell: 400 144
piledriver: 960 467 I think rdpmc is very slow.
zen5: 244 80
(Timing is 'rdpmc; mul_div(); rdpmc' with the multiply depending on the
first rdpmc and the second rdpmc depending on the quotient.)
Object code (64bit x86 test program): old 0x173 new 0x141.
Link: https://lkml.kernel.org/r/20251105201035.64043-9-david.laight.linux@gmail.com
Signed-off-by: David Laight <david.laight.linux@gmail.com>
Reviewed-by: Nicolas Pitre <npitre@baylibre.com>
Cc: Biju Das <biju.das.jz@bp.renesas.com>
Cc: Borislav Betkov <bp@alien8.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Li RongQing <lirongqing@baidu.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleinxer <tglx@linutronix.de>
Cc: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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.