Re: Little program to test concurrency of .fetch_add and .compare_exchange_weak

From	David Brown <david.brown@hesbynett.no>
Newsgroups	comp.lang.c++
Subject	Re: Little program to test concurrency of .fetch_add and .compare_exchange_weak
Date	2021-12-08 09:08 +0100
Organization	A noiseless patient Spider
Message-ID	<sopp51$oeg$1@dont-email.me> (permalink)
References	(3 earlier) <7pNrJ.79952$6a3.76905@fx41.iad> <soo8au$26q$1@dont-email.me> <H0OrJ.110234$Z0a.15989@fx17.iad> <sooam4$j94$1@dont-email.me> <AoOrJ.102436$SW5.42759@fx45.iad>

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On 07/12/2021 20:14, Scott Lurndal wrote:
> Bonita Montero <Bonita.Montero@gmail.com> writes:
>> Am 07.12.2021 um 19:49 schrieb Scott Lurndal:
> 
>>
>>> How do you know?
>>
>> Because this would be slower since the lock-modifications
>> woudln't be done in the L1-caches but in far memory. That's
>> just a silly idea.
> 
> Hello, it's a cache-coherent multiprocessor.   You need to
> fetch it exclusively into the L1 first, so instead of sending the fetch
> (or invalidate if converting a shared line to owned),
> you send the atomic op and it gets handled atomically at
> the far end (e.g. LLC, PCI express device, SoC coprocessor)
> saving the interconnect (mesh, ring, whatever) bandwidth and
> the round-trip time between L1 and LLC and reducing contention
> for the line.
> 
> If it's already in the L1 cache, then the processor will
> automatically treat it as a near-atomic, this is expected
> to be a rare case with correctly designed atomic usage.
> 

This is such an obvious improvement that I am constantly amazed how long
it has taken to be implemented.  Using ordinary memory for atomic
operations, locks, etc., is massively inefficient compared to a
dedicated hardware solution.

I've used a multi-core embedded microcontroller with a semaphore block,
consisting of a number (16, IIRC) of individual semaphores.  Each of
these was made of two 16-bit parts - the lock tag and the value.  You
can only change the value if you have the lock, and you get the lock by
writing a non-zero tag when the tag is currently 0 (unlocked).  You
release it by writing your tag with the high bit set.  It is all very
simple, and extremely fast - no need to go through caches, snooping, or
any of that nonsense because it is dedicated and connected close to the
cpu's core buses.

Obviously in a "big" system you need to handle more than two cores (and
with the Z-Gen and CLX system, other bus masters), support larger
numbers of locks, and security is a rather different matter!  But the
principle of having dedicated hardware, memory mapped but not passing
through caches and slow external memory, is the same.

Atomic operations carried out by the core on memory in the L1 caches
will be fast as long as their are no conflicts, but you wouldn't bother
with atomics unless there /were/ a risk of conflict.  And then they get
slow.  With a "far atomics" solution, you should be able to get much
more consistent timings and efficient results.

(At least, that is my understanding of it, without having actually used
them!)

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Little program to test concurrency of .fetch_add and .compare_exchange_weak Bonita Montero <Bonita.Montero@gmail.com> - 2021-12-05 18:13 +0100
  Re: Little program to test concurrency of .fetch_add and .compare_exchange_weak Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-12-05 19:56 +0000
    Re: Little program to test concurrency of .fetch_add and .compare_exchange_weak "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2021-12-05 15:22 -0800
      Re: Little program to test concurrency of .fetch_add and .compare_exchange_weak Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-12-05 23:43 +0000
        Re: Little program to test concurrency of .fetch_add and .compare_exchange_weak "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2021-12-05 19:31 -0800
      Re: Little program to test concurrency of .fetch_add and .compare_exchange_weak Bonita Montero <Bonita.Montero@gmail.com> - 2021-12-06 06:26 +0100
    Re: Little program to test concurrency of .fetch_add and .compare_exchange_weak Bonita Montero <Bonita.Montero@gmail.com> - 2021-12-06 06:24 +0100
      Re: Little program to test concurrency of .fetch_add and .compare_exchange_weak Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-12-06 10:45 +0000
        Re: Little program to test concurrency of .fetch_add and .compare_exchange_weak Bonita Montero <Bonita.Montero@gmail.com> - 2021-12-06 13:09 +0100
  Re: Little program to test concurrency of .fetch_add and .compare_exchange_weak Bonita Montero <Bonita.Montero@gmail.com> - 2021-12-06 06:27 +0100
  Re: Little program to test concurrency of .fetch_add and .compare_exchange_weak scott@slp53.sl.home (Scott Lurndal) - 2021-12-06 18:02 +0000
    Re: Little program to test concurrency of .fetch_add and .compare_exchange_weak Bonita Montero <Bonita.Montero@gmail.com> - 2021-12-07 06:27 +0100
      Re: Little program to test concurrency of .fetch_add and .compare_exchange_weak scott@slp53.sl.home (Scott Lurndal) - 2021-12-07 18:06 +0000
        Re: Little program to test concurrency of .fetch_add and .compare_exchange_weak Bonita Montero <Bonita.Montero@gmail.com> - 2021-12-07 19:14 +0100
          Re: Little program to test concurrency of .fetch_add and .compare_exchange_weak scott@slp53.sl.home (Scott Lurndal) - 2021-12-07 18:49 +0000
            Re: Little program to test concurrency of .fetch_add and .compare_exchange_weak Bonita Montero <Bonita.Montero@gmail.com> - 2021-12-07 19:54 +0100
              Re: Little program to test concurrency of .fetch_add and .compare_exchange_weak scott@slp53.sl.home (Scott Lurndal) - 2021-12-07 19:14 +0000
                Re: Little program to test concurrency of .fetch_add and .compare_exchange_weak scott@slp53.sl.home (Scott Lurndal) - 2021-12-07 19:25 +0000
                Re: Little program to test concurrency of .fetch_add and .compare_exchange_weak Bonita Montero <Bonita.Montero@gmail.com> - 2021-12-08 09:50 +0100
                Re: Little program to test concurrency of .fetch_add and .compare_exchange_weak "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2021-12-08 00:59 -0800
                Re: Little program to test concurrency of .fetch_add and .compare_exchange_weak Bonita Montero <Bonita.Montero@gmail.com> - 2021-12-08 15:41 +0100
                Re: Little program to test concurrency of .fetch_add and .compare_exchange_weak scott@slp53.sl.home (Scott Lurndal) - 2021-12-08 15:48 +0000
                Re: Little program to test concurrency of .fetch_add and .compare_exchange_weak Bonita Montero <Bonita.Montero@gmail.com> - 2021-12-08 17:35 +0100
                Re: Little program to test concurrency of .fetch_add and .compare_exchange_weak Öö Tiib <ootiib@hot.ee> - 2021-12-08 10:57 -0800
                Re: Little program to test concurrency of .fetch_add and .compare_exchange_weak Manfred <noname@add.invalid> - 2021-12-09 00:27 +0100
                Re: Little program to test concurrency of .fetch_add and .compare_exchange_weak Juha Nieminen <nospam@thanks.invalid> - 2021-12-09 06:30 +0000
                Re: Little program to test concurrency of .fetch_add and .compare_exchange_weak David Brown <david.brown@hesbynett.no> - 2021-12-08 09:08 +0100

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