Re: Speaking of Multiprocessing...

From	rickman <gnuarm@gmail.com>
Newsgroups	comp.arch.embedded
Subject	Re: Speaking of Multiprocessing...
Date	2017-03-24 22:10 -0400
Organization	A noiseless patient Spider
Message-ID	<ob4jdu$k87$1@dont-email.me> (permalink)
References	(5 earlier) <0if9dc9uphm9mrvjjstp1s30ah5fdcfd4i@4ax.com> <ob3la6$hgb$1@dont-email.me> <ob3n13$nhe$1@dont-email.me> <ob3nd5$ot2$1@dont-email.me> <ob4ek3$8v4$1@dont-email.me>

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On 3/24/2017 8:48 PM, David Brown wrote:
> 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.

I guess I didn't explain the full context.  But as I mentioned, it is 
simpler I think to include the swap memory instruction that will allow a 
test and set operation to be implemented atomically without disrupting 
any other functions.  It will use up an opcode, but it would be a simple 
one with the only difference from a normal memory write being the use of 
the read path.

-- 

Rick C

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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