Messages in this thread | | | Date | Wed, 27 Jul 2022 01:15:45 +0100 | From | "Russell King (Oracle)" <> | Subject | Re: Linux 5.19-rc8 |
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On Tue, Jul 26, 2022 at 01:20:23PM -0700, Linus Torvalds wrote: > On Tue, Jul 26, 2022 at 12:44 PM Russell King (Oracle) > <linux@armlinux.org.uk> wrote: > > > > Overall, I would say it's pretty similar (some generic perform > > marginally better, some native perform marginally better) with the > > exception of find_first_bit() being much better with the generic > > implementation, but find_next_zero_bit() being noticably worse. > > The generic _find_first_bit() code is actually sane and simple. It > loops over words until it finds a non-zero one, and then does trivial > calculations on that last word. > > That explains why the generic code does so much better than your byte-wise asm. > > In contrast, the generic _find_next_bit() I find almost offensively > silly - which in turn explains why your byte-wide asm does better. > > I think the generic _find_next_bit() should actually do what the m68k > find_next_bit code does: handle the first special word itself, and > then just call find_first_bit() on the rest of it. > > And it should *not* try to handle the dynamic "bswap and/or bit sense > invert" thing at all. That should be just four different (trivial) > cases for the first word.
Here's the results for the native version converted to use word loads:
[ 37.319937] Start testing find_bit() with random-filled bitmap [ 37.330289] find_next_bit: 2222703 ns, 163781 iterations [ 37.339186] find_next_zero_bit: 2154375 ns, 163900 iterations [ 37.348118] find_last_bit: 2208104 ns, 163780 iterations [ 37.372564] find_first_bit: 17722203 ns, 16370 iterations [ 37.737415] find_first_and_bit: 358135191 ns, 32453 iterations [ 37.745420] find_next_and_bit: 1280537 ns, 73644 iterations [ 37.752143] Start testing find_bit() with sparse bitmap [ 37.759032] find_next_bit: 41256 ns, 655 iterations [ 37.769905] find_next_zero_bit: 4148410 ns, 327026 iterations [ 37.776675] find_last_bit: 48742 ns, 655 iterations [ 37.790961] find_first_bit: 7562371 ns, 655 iterations [ 37.797743] find_first_and_bit: 47366 ns, 1 iterations [ 37.804527] find_next_and_bit: 59924 ns, 1 iterations
which is generally faster than the generic version, with the exception of the sparse find_first_bit (generic was: [ 25.657304] find_first_bit: 7328573 ns, 656 iterations)
find_next_{,zero_}bit() in the sparse case are quite a bit faster than the generic code.
-- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!
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