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<div>Hi Matthieu,</div>
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<div>By default, ABT_rwlock() and ABT_mutex() waits in a busy-yield loop (keeps yielding until a flag is set). If that configuration is set, waiters will suspend, not yield.</div>
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<div>This behavior becomes default because of a historical reason (https://github.com/pmodels/argobots/pull/102). There's a performance trade-off between a simple mutex (current default) and a non-simple mutex, so we hesitate to change it silently.</div>
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<div>If this default behavior is not what the user expects, I will create a PR to change it (if so I'd be happy if you create a GitHub issue that briefly describes this issue). </div>
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<div>Thanks,</div>
<span>Shintaro</span><br>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> Dorier, Matthieu <mdorier@anl.gov><br>
<b>Sent:</b> Friday, July 23, 2021 10:34 AM<br>
<b>To:</b> discuss@lists.argobots.org <discuss@lists.argobots.org>; Iwasaki, Shintaro <siwasaki@anl.gov><br>
<b>Subject:</b> Re: condition variable and rwlock</font>
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Thanks, I'm curious about your comment about --disable-simple-mutex. What you describe is what I would expect rwlock to do by default... How do rwlock by default, then?<br>
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Thanks<br>
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Matthieu<span id="x_ms-outlook-android-cursor"></span><br>
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Get <a href="https://aka.ms/ghei36">Outlook for Android</a></div>
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<div id="x_divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" color="#000000" style="font-size:11pt"><b>From:</b> Iwasaki, Shintaro <siwasaki@anl.gov><br>
<b>Sent:</b> Friday, July 23, 2021 3:51:21 PM<br>
<b>To:</b> discuss@lists.argobots.org <discuss@lists.argobots.org>; Dorier, Matthieu <mdorier@anl.gov><br>
<b>Subject:</b> Re: condition variable and rwlock</font>
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Hello Matthieu,</div>
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Thanks for your question!</div>
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> Is there a way of using an Argobots condition variable with an rwlock instead of a mutex?<br>
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No. The user cannot combine ABT_rwlock with ABT_cond.</div>
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> my use-case is a structure that receives many read requests and a few writes, clearly justifying using a rwlock instead of a mutex, but I may want some of the reads to block until a write has happened, which means I need a condition variable
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<div>First, if readers encounter an rwlock locked by a writer, they will suspend until the writer releases the rwlock (if --disable-simple-mutex is set, which is not set by default).</div>
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<div>For this specific purpose(especially if "some", not "all" of the readers should block), what I came up with in my mind first is the following. It looks fine except for a seemingly complex structure that uses multiple synchronization objects.</div>
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<div>void reader() {</div>
<div> while (1) {</div>
<div> if (work_queue.is_empty() and I_AM_SOME_OF_READERS()) {</div>
<div> // 1. Internally ABT_self_suspend()-like mechanism needs</div>
<div> // to take a lock (even if it is in the readers' lock)</div>
<div> // since multiple readers might access the same data</div>
<div> // structure (for example, a user-maintained</div>
<div> // suspended ULT list).</div>
<div> // 2. Anyway this path is not performance sensitive</div>
<div> ABT_mutex_lock(mutex);</div>
<div> ABT_cond_wait(cond, mutex);</div>
<div> ABT_mutex_unlock(mutex);</div>
<div> // Now someone woke me up after pushing work.</div>
<div> }</div>
<div> ABT_rwlock_rdlock(rwlock);</div>
<div> if (!work_queue.is_empty())</div>
<div> ; // Do real work.</div>
<div> ABT_rwlock_unlock(rwlock);</div>
<div> }</div>
<div>}</div>
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<div>void writer() {</div>
<div> ABT_rwlock_wrlock(rwlock);</div>
<div> work_queue.push_work(work);</div>
<div> // You do not need to take a mutex to call ABT_cond_broadcast.</div>
<div> // It is fine even if there's no waiter.</div>
<div> ABT_cond_broadcast(cond);</div>
<div> ABT_rwlock_unlock(rwlock);</div>
<span>}</span><br>
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<span>I would welcome any suggestions! (For example, does ABT_rwlock_trywrlock() help?)<br>
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<span>Best,</span></div>
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<span>Shintaro</span></div>
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<div id="x_x_divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" color="#000000" style="font-size:11pt"><b>From:</b> Dorier, Matthieu via discuss <discuss@lists.argobots.org><br>
<b>Sent:</b> Friday, July 23, 2021 5:03 AM<br>
<b>To:</b> discuss@argobots.org <discuss@argobots.org><br>
<b>Cc:</b> Dorier, Matthieu <mdorier@anl.gov><br>
<b>Subject:</b> [argobots-discuss] condition variable and rwlock</font>
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<p class="x_x_x_MsoNormal"><span lang="EN-US" style="font-size:11.0pt">Hi,</span></p>
<p class="x_x_x_MsoNormal"><span lang="EN-US" style="font-size:11.0pt"> </span></p>
<p class="x_x_x_MsoNormal"><span lang="EN-US" style="font-size:11.0pt">I suspect the answer is no, but is there a way of using an Argobots condition variable with an rwlock instead of a mutex?</span></p>
<p class="x_x_x_MsoNormal"><span lang="EN-US" style="font-size:11.0pt">(my use-case is a structure that receives many read requests and a few writes, clearly justifying using a rwlock instead of a mutex, but I may want some of the reads to block until a write
has happened, which means I need a condition variable).</span></p>
<p class="x_x_x_MsoNormal"><span lang="EN-US" style="font-size:11.0pt">Thanks,</span></p>
<p class="x_x_x_MsoNormal"><span lang="EN-US" style="font-size:11.0pt"> </span></p>
<p class="x_x_x_MsoNormal"><span lang="EN-US" style="font-size:11.0pt">Matthieu </span>
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