On Saturday 20 June 2009 23:56:51 Jonas Gorski wrote: > 2009/6/20 Michael Buesch <m...@bu3sch.de>: > > On Saturday 20 June 2009 00:09:56 Jonas Gorski wrote: > >> > If we do _not_ gain performance, it certainly is not a good idea to > >> > waste space. > >> > >> I would be very surprised if we wouldn't. Every kernel emulated > >> floating point operation results in an exception and the appropriate > >> handling (fpu emulation is ugly), while soft-float should stay > >> completely in user space. > >> > >> Also, the mips fpu emulator itself suggests that. > >> Quoting linux/arch/mips/math-emu/cp1emu.c: > >> > * Note if you know that you won't have an fpu, then you'll get much > >> > * better performance by compiling with -msoft-float! > >> > >> To get some numbers: Perhaps could somebody test with 'sample' from > >> libmpcdec the difference in speed between in-kernel emulation and > >> soft-float? https://dev.openwrt.org/ticket/4715 suggests that the > >> library depends heavily on floating point when not using fixed point > >> math. > > > > No you completely got me wrong. I am talking about the performance gain in > > real life. > > userspace soft float certainly _is_ faster than kernel float. But _if_ > > userspace > > soft float is bigger in size than kernel float, it probably is not worth > > the space tradeoff, > > because floating point is hardly used on a router. > > I apology, I really did misunderstand you. > > > Did somebody test this: > > * Image with kernel FP emu disabled and userspace softemu enabled. > > * Image with kernel FP emu enabled and userspace softemu disabled. > > > > Which one is smaller? > > Disabling the kernel fpu emu isn't intended in linux, so I had to hack > the emulation away, mainly by removing any fpu references in > arch/mips/kernel/ until it compiled. > I don't know if it really works, I don't have a second device for testing. > > I used the brcm47xx/default profile for testing, making distclean > before compiling. > > With kernel-fpuemu and no softfloat: > 2560000 openwrt-brcm47xx-squashfs.trx > Without fpuemu and with softfloat: > 2625536 openwrt-brcm47xx-squashfs.trx > > So the image with soft-float instead of the in-kernel fpu emulation > uses one block more. For me this would be an acceptable increase.
So what about the following. We introduce another option in the kernel config to disable fpuemu. This way the user can select either useremu, kernelemu or no emu at all. This probably is a win-win situation then. Because if I do care about space, I can turn off _both_ emulators. And you, who do care about performance, can set the openwrt default to kernelemu-off useremu-on. -- Greetings, Michael. _______________________________________________ openwrt-devel mailing list openwrt-devel@lists.openwrt.org https://lists.openwrt.org/mailman/listinfo/openwrt-devel
