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Re: std::atomic<T>::is_lock_free()

From scott@slp53.sl.home (Scott Lurndal)
Subject Re: std::atomic<T>::is_lock_free()
Newsgroups comp.lang.c++
References (8 earlier) <qqn3ip$16u6$1@gioia.aioe.org> <b2FzF.150668$oT5.19859@fx47.iad> <qqn6uj$1m73$1@gioia.aioe.org> <1WWzF.121832$Ys1.31748@fx42.iad> <qqpr94$1cem$1@gioia.aioe.org>
Message-ID <dw_zF.18674$bC3.9879@fx08.iad> (permalink)
Organization UsenetServer - www.usenetserver.com
Date 2019-11-16 22:13 +0000

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"Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> writes:
>On 11/16/2019 10:07 AM, Scott Lurndal wrote:
>> "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> writes:
>>> On 11/15/2019 1:47 PM, Scott Lurndal wrote:
>>>> "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> writes:
>>>>> On 11/15/2019 12:31 AM, Bonita Montero wrote:
>>>>>>>>> The proposal provides lock-free query functions on individual
>>>>>>>>> objects rather than whole types to permit unavoidably misaligned
>>>>>>>>> atomic variables without penalizing the performance of aligned
>>>>>>>>> atomic variables.
>>>>>>
>>>>>>>> That's rather stupid since the language says that access to unaligned
>>>>>>>> data has an undefined behaviour. And there's not the least necessitiy
>>>>>>>> in using unaligned atomics.
>>>>>>
>>>>>>> You are JG, right?
>>>>>>
>>>>>> Tell me of the sense to use non-aligned atomics.
>>>>>
>>>>> Not sure what that even means. Are you talking about L2 cache alignment
>>>>> and proper padding? Its used to avoid false sharing.
>>>>
>>>> Why do you bother with BM?  It deliberatly removes attributes so
>>>> it's impossible to see who said what to whom.
>>>
>>> I think BM = JG. Its a sock puppet...
>>>
>>>> In any case, unaligned atomics are useless, and many architectures
>>>> won't support them (particularly in uncachable memory regions like
>>>> those mapped to PCI express bars (and PCI express endpoints -may- support atomic
>>>> operations...)).
>>>>
>>>> I suspect that most architectures avoid supporting unaligned atomics
>>>> because they might split cache lines which would make it impossible
>>>> (or very time consuming) to support atomic semantics for the access.
>>>>
>>>
>>> Well, I remember abusing unaligned atomics such that a word straddles a
>>> L2 cache line! Well, on x86 this should trigger a system wide bus lock
>>> for legacy reasons.
>> 
>> Yes, that's the "very time consuming" part.
>
>Indeed. A forced unaligned atomic access wrt hacking RCU was amortized 
>and used on a fairly rare basis. Think along the lines of read-"mostly" 
>usage patterns. Wrt RCU, we can collect garbage on a per-thread basis, 
>and when a threads garbage gets over a "threshold", it can decide to 
>execute a system wide membar, or something else. The asymmetric side of 
>things is to have a fast side, and a slow side... The fast are the 
>readers, the slow is the writers. Readers can run full steam ahead, and 
>do not interfere with writers.
>
>> 
>> We (3Leaf Systems) designed a chip in the mid 2000s that connected
>> to the processor via Hypertransport (Opteron) or QPI (Intel) and
>> extended the processor coherency domain over Infiniband[*] to another
>> instance of our chip (allowing creation of large shared memory
>> systems consisting of commodity 1u or 2u rack mount SMP  boxes).
>> 
>> One of the national labs was benchmarking the system by pounding on
>> a spinlock from all 128 cores (across 16 2-socket nodes).
>
>YIKES! Hammer time. ;^)
>
>> 
>> A little known characteristic of AMD and Intel
>> processors is that if a processor isn't making progress on acquiring
>> a highly contended cache-line, it will at some point decide to obtain the
>> system bus lock.
>> 
>> Needless to say, performance was shit (although the test was designed
>> to show this behavior and was not representative of the labs normal
>> workloads on the system).
>> 
>> [*] The chip had 2MByte of DRAM caching remote lines, a miss cost about
>>      800 ns round trip (using DDR IB) on the HT systems, and 500ns on QPI (using QDR IB).
>> 
>
>Did you try using hash tables just to experiment wrt taking pressure off 
>of a single lock? Even that does not scale all that great. However, its 
>better than a single funnel... ;^)

The purpose of the test was to see what happened when all cores contended
for the same line (running slow is expected, crashing (which we didn't) isn't).

Actual lab codes were much better structured from a parallelism standpoint.

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std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-12 10:54 +0100
  Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-12 11:24 +0100
    Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-12 14:11 +0100
      Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-12 14:39 +0100
        Re: std::atomic<T>::is_lock_free() Bo Persson <bo@bo-persson.se> - 2019-11-12 16:46 +0100
          Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-12 17:40 +0100
            Re: std::atomic<T>::is_lock_free() Bo Persson <bo@bo-persson.se> - 2019-11-12 20:23 +0100
              Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-12 15:21 -0800
              Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-14 13:09 -0800
              Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-13 06:34 +0100
                Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-13 22:00 -0800
                Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 07:26 +0100
                Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-13 22:33 -0800
                Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-13 22:33 -0800
                Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 07:51 +0100
                Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 07:50 +0100
                Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-13 23:14 -0800
                Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 10:43 +0100
  Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-12 15:14 -0800
    Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 13:11 +0100
  Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-14 07:53 -0800
    Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 16:57 +0100
      Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-14 08:16 -0800
        Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 17:26 +0100
          Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 17:28 +0100
          Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-14 08:30 -0800
            Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 17:33 +0100
              Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-14 13:41 -0800
                Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-15 09:31 +0100
                Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-15 12:59 -0800
                Re: std::atomic<T>::is_lock_free() scott@slp53.sl.home (Scott Lurndal) - 2019-11-15 21:47 +0000
                Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-15 13:56 -0800
                Re: std::atomic<T>::is_lock_free() scott@slp53.sl.home (Scott Lurndal) - 2019-11-16 18:07 +0000
                Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-16 13:55 -0800
                Re: std::atomic<T>::is_lock_free() scott@slp53.sl.home (Scott Lurndal) - 2019-11-16 22:13 +0000
                Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-16 18:40 -0800
                Re: std::atomic<T>::is_lock_free() James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-17 18:39 -0500
                Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-17 20:59 -0800
                Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-17 21:00 -0800
                Re: std::atomic<T>::is_lock_free() James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-18 08:25 -0500
                Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-18 18:24 -0800
                Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-15 13:59 -0800
                Re: std::atomic<T>::is_lock_free() Ian Collins <ian-news@hotmail.com> - 2019-11-16 11:08 +1300
                Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-15 15:39 -0800
                Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-16 07:22 +0100
                Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-16 08:06 -0800
                Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-16 17:13 +0100
                Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-16 08:26 -0800
                Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-16 19:27 +0100
                Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-16 10:37 -0800
                Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-16 19:47 +0100
                Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-16 11:34 -0800
                Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-17 09:17 +0100
                Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-17 01:52 -0800
                Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-17 11:23 +0100
                Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-17 03:22 -0800
                Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-17 14:20 +0100
                Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-17 06:39 -0800
                Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-17 15:49 +0100
                Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-17 22:54 -0800
                Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-18 08:03 +0100
                Re: std::atomic<T>::is_lock_free() Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-11-28 15:39 -0800
                Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-28 16:33 -0800
                Re: std::atomic<T>::is_lock_free() Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-01-10 22:48 -0800
                Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2020-01-11 00:24 -0800
                Re: std::atomic<T>::is_lock_free() Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-01-14 07:16 -0800
                Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2020-01-14 08:55 -0800
                Re: std::atomic<T>::is_lock_free() Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-01-21 02:01 -0800
                Re: std::atomic<T>::is_lock_free() James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-01-21 06:00 -0800
                Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-17 15:58 +0100
                Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-17 21:01 -0800
                Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-18 07:00 +0100
                Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-17 22:45 -0800
                Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-18 07:52 +0100
                Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-18 08:10 +0100
                Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-17 23:23 -0800
                Re: std::atomic<T>::is_lock_free() Ian Collins <ian-news@hotmail.com> - 2019-11-18 20:26 +1300
                Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-17 23:05 -0800
                Re: std::atomic<T>::is_lock_free() Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-11-28 15:44 -0800
                Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-28 16:43 -0800
                Re: std::atomic<T>::is_lock_free() Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-01-11 06:41 -0800
        Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-14 13:33 -0800
  Re: std::atomic<T>::is_lock_free() Melzzzzz <Melzzzzz@zzzzz.com> - 2019-11-14 19:08 +0000
    Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 20:12 +0100
      Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-14 13:35 -0800
  Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-14 13:32 -0800
    Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-14 13:37 -0800
    Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-15 09:32 +0100
      Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-15 13:06 -0800
  Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-14 02:29 -0800
    Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 11:45 +0100

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