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

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Stephen Fuld wrote:
> On 5/13/2011 3:47 AM, Terje Mathisen wrote:
>
> snip
>
>> 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.
>
> I agree with your preference and would add another reason. There are
>  applications where a system wide unique, guaranteed increasing
> number is exactly what is needed. Think transaction numbers in some
> transaction systems. XADD provides this with minimal cost.

I know that such numbers are very useful, but I would really prefer to 
avoid the need for them, because they do end up as a global bottleneck.
>
>
>> 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.
>
> Yes. Although test and set is perhaps easier to do this with. But see
> below

Test&Set, in the form XCHG reg,[mem] is the original x86 sync primitive, 
the main problem is that it only provides a boolean gate, not something 
which can be used as-is.

[snip]
> itself. However, at the cost of that elegance, perhaps a reasonable
> compromise would be to handle the addition within the memory
> controller chip (or part of chip) instead of the RAM chips. This
> still avoids the need for bus locks/snoops. It would cost a second
> transfer at the RAM chips themselves, but since many/most RAM to
> controller interfaces are 64 bits wide, it makes the overflow problem
> small enough that you might be able to not implement anything special
> for it.

I'm currently assuming that any distributed RMW facility has to be 
implemented in the memory controllers...

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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