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Re: which program can test cpu speed

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On Fri, Oct 26, 2018 at 01:47:19PM -0500, David Wright wrote:
>On Fri 26 Oct 2018 at 11:04:48 (-0400), Michael Stone wrote:
>> FWIW, even the kernel doesn't use naive busy loops anymore on newer
>> hardware. (TSC or MWAIT is used, depending on what the processor
>> supports.)
>
>I've programmed a "busy loop" in the past and found that linux
>bogomips tracked the loop speed quite closely on a variety of machines
>from 486DX to 650MHz Pentium III (Coppermine). Nothing multiprocessor.
>
>When I say busy loop, I mean a loop like
>   FOR J=1 TO T
>     X=X+1
>   NEXT J
>where X is floating point and the language is an HP Basic clone on MSDOS.

That's basically all bogomips is:
		"	test %0,%0	\n"
		"	jz 3f		\n"
		"	jmp 1f		\n"

		".align 16		\n"
		"1:	jmp 2f		\n"

		".align 16		\n"
		"2:	dec %0		\n"
		"	jnz 2b		\n"
		"3:	dec %0		\n"
in a loop. (From https://github.com/torvalds/linux/blob/master/arch/x86/lib/delay.c)
(Note that other architectures are completely different and don't use 
the x86 assembly--one more way the bogomips numbers are meaningless. :) )

For a given CPU family there's a de-facto multiplier (essentially, how 
many instructions can be issued per cycle) and then within that family 
bogomips is directly proportional to clock speed. None of that works 
particularly well in the face of CPUs that change speed, and it's not 
particularly efficient given current desires to minimizing power 
consumption. So, current CPUs use the TSC (which, again for current 
CPUs, increments at a constant rate regardless of reduced clock speeds 
in power saving modes) possibly in concert with MWAIT (which lets the 
CPU idle for a bit to save power). In those cases the loop frequency 
calibration is just determining how the TSC counter relates to wall 
clock time, and has nothing at all to do with CPU performance--even the 
minimal "how fast can the CPU run these 5 instructions" 'benchmark' of 
classic bogomips.

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which program can test cpu speed Long Wind <longwind2@yahoo.com> - 2018-10-26 05:10 +0200
  Re: which program can test cpu speed Reco <recoverym4n@enotuniq.net> - 2018-10-26 08:00 +0200
    Re: which program can test cpu speed Reco <recoverym4n@enotuniq.net> - 2018-10-26 10:30 +0200
      Re: which program can test cpu speed arne <sp113438@telfort.nl> - 2018-10-26 10:50 +0200
        Re: which program can test cpu speed Reco <recoverym4n@enotuniq.net> - 2018-10-26 11:00 +0200
          Re: which program can test cpu speed arne <sp113438@telfort.nl> - 2018-10-26 11:10 +0200
    Re: which program can test cpu speed Long Wind <longwind2@yahoo.com> - 2018-10-26 10:30 +0200
    Re: which program can test cpu speed Michael Stone <mstone@debian.org> - 2018-10-26 16:00 +0200
      Re: which program can test cpu speed Reco <recoverym4n@enotuniq.net> - 2018-10-26 16:40 +0200
        Re: which program can test cpu speed Michael Stone <mstone@debian.org> - 2018-10-26 17:10 +0200
          Re: which program can test cpu speed David Wright <deblis@lionunicorn.co.uk> - 2018-10-26 20:50 +0200
            Re: which program can test cpu speed Michael Stone <mstone@debian.org> - 2018-10-26 21:20 +0200
        Re: which program can test cpu speed Pascal Hambourg <pascal@plouf.fr.eu.org> - 2018-10-26 20:00 +0200
          Re: which program can test cpu speed Reco <recoverym4n@enotuniq.net> - 2018-10-26 20:20 +0200
            Re: which program can test cpu speed Michael Stone <mstone@debian.org> - 2018-10-26 22:20 +0200
    Re: which program can test cpu speed mick crane <mick.crane@gmail.com> - 2018-10-26 16:30 +0200
      Re: which program can test cpu speed Reco <recoverym4n@enotuniq.net> - 2018-10-26 16:40 +0200
  Re: which program can test cpu speed Jörg-Volker Peetz <jvpeetz@web.de> - 2018-10-26 10:40 +0200
  Re: which program can test cpu speed Greg Wooledge <wooledg@eeg.ccf.org> - 2018-10-26 14:40 +0200
    Re: which program can test cpu speed Long Wind <longwind2@yahoo.com> - 2018-10-26 19:30 +0200
      Re: which program can test cpu speed Michael Stone <mstone@debian.org> - 2018-10-26 20:10 +0200
        Re: which program can test cpu speed Michael Stone <mstone@debian.org> - 2018-10-26 20:50 +0200

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