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<p class="MsoNormal"><span style="font-size:11.0pt">Seems fine to me.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Dumb question: What (if anything) should be done to support platforms with a single threaded system. Assert if not immediately available? Optimistically spinlock in case they are using this on “elsewhere”
memory of some sort?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Pedantic: I probably wouldn’t call the Windows implementation futex based (
</span><a href="https://devblogs.microsoft.com/oldnewthing/20170601-00/?p=96265">https://devblogs.microsoft.com/oldnewthing/20170601-00/?p=96265</a> ). I wouldn’t go and call the Linux implementation WaitOnAddress based either.<span style="font-size:11.0pt"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:11.0pt">From:</span></b><span style="font-size:11.0pt"> libcxx-dev <libcxx-dev-bounces@lists.llvm.org>
<b>On Behalf Of </b>Olivier Giroux via libcxx-dev<br>
<b>Sent:</b> Tuesday, May 28, 2019 5:55 PM<br>
<b>To:</b> libcxx-dev@lists.llvm.org<br>
<b>Subject:</b> [EXTERNAL] [libcxx-dev] A proposed approach to std::atomic_wait for libcxx<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Hi everyone,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">To start the discussion of how libcxx should go about implementing this feature, I’ve prepared a cross-platform implementation of atomic_wait/atomic_notify_* for Mac/Linux/Windows/CUDA/unidentified-platform.
I don’t claim that it’s fully tuned for your platform, nor do I claim that it’s perfect for every possible use, but it should not be terribly bad for any use either. It has various knobs to turn paths On/Off, so you can choose a different path on each platform,
so long as it’s supported at all on that platform.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">You can find the implementation here:
<a href="https://urldefense.proofpoint.com/v2/url?u=https-3A__github.com_ogiroux_atomic-5Fwait_&d=DwMGaQ&c=I_0YwoKy7z5LMTVdyO6YCiE2uzI1jjZZuIPelcSjixA&r=y8mub81SfUi-UCZRX0Vl1g&m=52uONGiTLQpEB5Xuf3oJR6LnVXwuDGP-HiraUV4P5I8&s=_UMntk1qhxJm9tQFWUvq7zDZ3A9Pfse6ouh9047XIE8&e=">
https://github.com/ogiroux/atomic_wait/</a>.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">It has these strategies implemented:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">* Contention table. Used to optimize futex notify, or to hold CVs. Disable with __NO_TABLE.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">* Futex. Supported on Linux and Windows. For performance requires a table on Linux. Disable with __NO_FUTEX.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">* Condition variables. Supported on Linux and Mac. Requires table to function. Disable with __NO_CONDVAR.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">* Timed back-off. Supported on everything. Disable with __NO_SLEEP.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">* Spinlock. Supported on everything. Force with __NO_IDENT.
<i>Note:</i> <i>performance is too terrible to use</i>.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">The strategy is chosen this way, by platform:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">* Linux: default to futex (with table), fallback to futex (no table) -> CVs -> timed backoff -> spin.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">* Mac: default to CVs (table), fallback to timed backoff -> spin.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">* Windows: default to futex (no table), fallback to timed backoff -> spin.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">* CUDA: default to timed backoff, fallback to spin. (This is not all checked in in this tree.)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">* Unidentified platform: default to spin.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">The unidentified platform support could be better. For instance, we should probably assume that <thread> is implemented and use the sleeping/yielding facilities there. It should not fall all the way back to
__NO_IDENT, it should instead fall back to about where CUDA is expected to be.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">One of the main discussion points I’d like to drive with this, is the design of the contention management table, to go along the sharded lock table that backs __atomic_* and __c11_atomic_* built-ins. Ideally
this would be handled the same way, meaning that it should live in libatomic.a or your substitute, and be shared with other C++ standard libraries on your platform.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Please discuss!<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Sincerely,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Olivier<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
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