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Re: What are the various alternate uses for the PC's LSB ?

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Andy "Krazy" Glew wrote:
> However, LL/SC is a mechanism that allows a lot of different
> instruction sequences to be made atomic. You want
> atomic-increment-with-saturation at [0,255] ... ? LL/SC can do that
> for you.

_Most_ of these can be handled equally well with traditional CAS
(CMPXCHG*) semantics, it is only the ABA problem which is obviously
easier to avoid with LL/SC.

(earlier you wrote:)
> Let alone a machine where you want to export the atomic RMW to the
> interconnect, so that you can do NYU Ultracomputer style fetch and op
> combining.  Or, heck, even the simple form of RMW coalescing that
> most GPUs provide.

Personally I prefer algorithms which can use XADD, i.e. atomic
fetch_and_add(), specifically because they can provide guaranteed
forward progress, with minimum bus uglyness.

It should also be relatively easy (for hw) to propagate an XADD
operation across a mesh and handle it at the location of the target memory.

It does mean that you have to allow the XADD - updated counter to wrap 
at whatever the natural size is, and handle any masking/special 
processing in the surrounding code. Saturation does not work in this 
scenario, since there's no easy way to get immediately back out of 
saturation:

  253+, 254+, 255+, 255+, 255+, 255-, 254-, 253

(A workaround would be to detect that the current increment caused an 
overflow, then immediately decrement by that amount, but this can 
quickly get into race conditions against other threads. Having one or 
two secondary counters to record the amount of over/underflow could 
work, but that would make it much more complicated to read the current 
value. :-()

Terje

-- 
- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"

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Re: What are the various alternate uses for the PC's LSB ? "Paul A. Clayton" <paaronclayton@gmail.com> - 2011-05-12 08:49 -0700
  Re: What are the various alternate uses for the PC's LSB ? whygee <yg@yg.yg> - 2011-05-13 02:01 +0200
    Re: What are the various alternate uses for the PC's LSB ? "Andy \"Krazy\" Glew" <andy@SPAM.comp-arch.net> - 2011-05-12 20:32 -0700
      Re: What are the various alternate uses for the PC's LSB ? Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-05-13 12:47 +0200
        Re: What are the various alternate uses for the PC's LSB ? Stephen Fuld <SFuld@alumni.cmu.edu.invalid> - 2011-05-13 08:44 -0700
          Re: What are the various alternate uses for the PC's LSB ? Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-05-13 19:53 +0200
            Re: What are the various alternate uses for the PC's LSB ? Stephen Fuld <SFuld@alumni.cmu.edu.invalid> - 2011-05-13 11:36 -0700
              Re: What are the various alternate uses for the PC's LSB ? Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-05-13 22:26 +0200
            Re: What are the various alternate uses for the PC's LSB ? "Andy \"Krazy\" Glew" <andy@SPAM.comp-arch.net> - 2011-05-14 17:17 -0700
              Re: What are the various alternate uses for the PC's LSB ? Terje Mathisen <"terje.mathisen at tmsw.no"> - 2011-05-16 14:22 +0200
                Re: What are the various alternate uses for the PC's LSB ? "Andy \"Krazy\" Glew" <andy@SPAM.comp-arch.net> - 2011-05-16 07:38 -0700
          Re: What are the various alternate uses for the PC's LSB ? "Andy \"Krazy\" Glew" <andy@SPAM.comp-arch.net> - 2011-05-14 17:14 -0700
      Re: What are the various alternate uses for the PC's LSB ? rpw3@rpw3.org (Rob Warnock) - 2011-05-13 07:45 -0500

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