Re: Speaking of Multiprocessing...

From	David Brown <david.brown@hesbynett.no>
Newsgroups	comp.arch.embedded
Subject	Re: Speaking of Multiprocessing...
Date	2017-03-25 01:48 +0100
Organization	A noiseless patient Spider
Message-ID	<ob4ek3$8v4$1@dont-email.me> (permalink)
References	(4 earlier) <ob2cck$cu5$1@dont-email.me> <0if9dc9uphm9mrvjjstp1s30ah5fdcfd4i@4ax.com> <ob3la6$hgb$1@dont-email.me> <ob3n13$nhe$1@dont-email.me> <ob3nd5$ot2$1@dont-email.me>

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On 24/03/17 19:12, rickman wrote:
> On 3/24/2017 2:06 PM, David Brown wrote:
>> On 24/03/17 18:36, rickman wrote:
>>> On 3/24/2017 3:07 AM, Robert Wessel wrote:
>>>> On Fri, 24 Mar 2017 01:58:26 -0400, rickman <gnuarm@gmail.com> wrote:
>>>>
>>>>> On 3/24/2017 1:37 AM, Robert Wessel wrote:
>>>>>> On Fri, 24 Mar 2017 01:05:02 -0400, rickman <gnuarm@gmail.com> wrote:
>>>>>>
>>>>>>> On 3/23/2017 11:43 PM, Robert Wessel wrote:
>>>>>>>> On Thu, 23 Mar 2017 18:38:13 -0400, rickman <gnuarm@gmail.com>
>>>>>>>> wrote:
>>
>> <snip>
>>
>>>
>>>>> There certainly is no reason for memory to be multiple cycle.  I think
>>>>> you are picturing various implementations where memory is slow
>>>>> compared
>>>>> to the CPU.  That's not a given.
>>>>
>>>>
>>>> No, but such a situation is sufficiently uncommon that you'd really
>>>> want to spec it up front.  Most cores these days don't even have
>>>> single cycle L1 caches.
>>>
>>> Again, assuming an architecture and requirements.  This conversation is
>>> not about designing the next generation of ARM CPU.
>>
>> It would be making architecture assumptions to require single-cycle
>> memory - saying that memory may be multiple cycle is the general case.
>>
>> (You might /want/ to make architecture assumptions here - I gave some
>> other suggestions of specific hardware for locking in another post.  The
>> solutions discussed here are for general memory.)
>
> We are having two different conversations here.   I am designing a CPU,
> you are talking about the theory of CPU design.

Aha!  That is some useful information to bring to the table.  You told 
us earlier about some things that you are /not/ doing, but not what you 
/are/ doing.  (Or if you did, I missed it.)

In that case, I would recommend making some dedicated hardware for your 
synchronisation primitives.  A simple method is one I described earlier 
- have a set of memory locations where you the upper half of each 32-bit 
entry is for a "thread id".  You can only write to the entry if the 
current upper half is 0, or it matches the thread id you are writing to 
it.  It should be straightforward to implement in an FPGA.

The disadvantage of this sort of solution is scaling - if your hardware 
supports 64 such semaphores, then that's all you've got.  A solution 
utilizing normal memory (such as TAS, CAS, LL/SC) lets user code make as 
many semaphores as it wants.  But you can always use the hardware ones 
to implement more software semaphores indirectly.

>>
>> If this is going to be efficient, it is unlikely that the core supports
>> reading and writing within the same instruction - LL/SC is usually a
>> better choice.
>
> As I have said before, I am designing a CPU in an FPGA.  I have FPGA
> features available to me that are different than what is available to
> chip designers.  I do not have the same constraints as chip designers.
>
> I was asking what is required in an instruction set to support
> multi-processors and I think the answer is that the memory swap
> instruction would do the job efficiently.
>

Thread

Speaking of Multiprocessing... rickman <gnuarm@gmail.com> - 2017-03-23 18:38 -0400
  Re: Speaking of Multiprocessing... Tim Wescott <seemywebsite@myfooter.really> - 2017-03-23 18:19 -0500
    Re: Speaking of Multiprocessing... Don Y <blockedofcourse@foo.invalid> - 2017-03-23 16:26 -0700
      Re: Speaking of Multiprocessing... Tim Wescott <seemywebsite@myfooter.really> - 2017-03-23 18:47 -0500
        Re: Speaking of Multiprocessing... Don Y <blockedofcourse@foo.invalid> - 2017-03-23 17:49 -0700
          Re: Speaking of Multiprocessing... upsidedown@downunder.com - 2017-03-24 09:48 +0200
            Re: Speaking of Multiprocessing... Don Y <blockedofcourse@foo.invalid> - 2017-03-24 01:22 -0700
        Re: Speaking of Multiprocessing... Clifford Heath <no.spam@please.net> - 2017-03-24 21:04 +1100
          Re: Speaking of Multiprocessing... Robert Wessel <robertwessel2@yahoo.com> - 2017-03-24 22:01 -0500
            Re: Speaking of Multiprocessing... Clifford Heath <no.spam@please.net> - 2017-03-25 18:49 +1100
    Re: Speaking of Multiprocessing... Grant Edwards <invalid@invalid.invalid> - 2017-03-24 14:36 +0000
  Re: Speaking of Multiprocessing... Robert Wessel <robertwessel2@yahoo.com> - 2017-03-23 22:43 -0500
    Re: Speaking of Multiprocessing... rickman <gnuarm@gmail.com> - 2017-03-24 01:05 -0400
      Re: Speaking of Multiprocessing... Robert Wessel <robertwessel2@yahoo.com> - 2017-03-24 00:37 -0500
        Re: Speaking of Multiprocessing... rickman <gnuarm@gmail.com> - 2017-03-24 01:58 -0400
          Re: Speaking of Multiprocessing... Robert Wessel <robertwessel2@yahoo.com> - 2017-03-24 02:07 -0500
            Re: Speaking of Multiprocessing... rickman <gnuarm@gmail.com> - 2017-03-24 13:36 -0400
              Re: Speaking of Multiprocessing... David Brown <david.brown@hesbynett.no> - 2017-03-24 19:06 +0100
                Re: Speaking of Multiprocessing... rickman <gnuarm@gmail.com> - 2017-03-24 14:12 -0400
                Re: Speaking of Multiprocessing... David Brown <david.brown@hesbynett.no> - 2017-03-25 01:48 +0100
                Re: Speaking of Multiprocessing... rickman <gnuarm@gmail.com> - 2017-03-24 22:10 -0400
                Re: Speaking of Multiprocessing... David Brown <david.brown@hesbynett.no> - 2017-03-25 15:25 +0100
                Re: Speaking of Multiprocessing... rickman <gnuarm@gmail.com> - 2017-03-25 15:27 -0400
                Re: Speaking of Multiprocessing... upsidedown@downunder.com - 2017-03-25 22:43 +0200
                Re: Speaking of Multiprocessing... rickman <gnuarm@gmail.com> - 2017-03-25 17:06 -0400
                Re: Speaking of Multiprocessing... David Brown <david.brown@hesbynett.no> - 2017-03-26 17:28 +0200
                Re: Speaking of Multiprocessing... rickman <gnuarm@gmail.com> - 2017-03-26 13:27 -0400
              Re: Speaking of Multiprocessing... Dimiter_Popoff <dp@tgi-sci.com> - 2017-03-25 00:42 +0200
                Re: Speaking of Multiprocessing... rickman <gnuarm@gmail.com> - 2017-03-24 19:22 -0400
                Re: Speaking of Multiprocessing... Rob Gaddi <rgaddi@highlandtechnology.invalid> - 2017-03-24 16:33 -0700
                Re: Speaking of Multiprocessing... Robert Wessel <robertwessel2@yahoo.com> - 2017-03-24 22:47 -0500
                Re: Speaking of Multiprocessing... Dimiter_Popoff <dp@tgi-sci.com> - 2017-03-25 12:40 +0200
                Re: Speaking of Multiprocessing... Robert Wessel <robertwessel2@yahoo.com> - 2017-03-26 00:42 -0500
  Re: Speaking of Multiprocessing... David Brown <david.brown@hesbynett.no> - 2017-03-24 09:17 +0100
    Re: Speaking of Multiprocessing... Tom Gardner <spamjunk@blueyonder.co.uk> - 2017-03-24 09:28 +0000
      Re: Speaking of Multiprocessing... David Brown <david.brown@hesbynett.no> - 2017-03-24 11:19 +0100
        Re: Speaking of Multiprocessing... Tom Gardner <spamjunk@blueyonder.co.uk> - 2017-03-24 11:06 +0000
          Re: Speaking of Multiprocessing... David Brown <david.brown@hesbynett.no> - 2017-03-24 12:21 +0100
        Re: Speaking of Multiprocessing... Tom Gardner <spamjunk@blueyonder.co.uk> - 2017-03-24 16:09 +0000
          Re: Speaking of Multiprocessing... David Brown <david.brown@hesbynett.no> - 2017-03-24 17:33 +0100
            Re: Speaking of Multiprocessing... Tom Gardner <spamjunk@blueyonder.co.uk> - 2017-03-24 17:52 +0000

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