[argobots-discuss] condition variable and rwlock

[Phil Carns]

On 7/23/21 11:53 AM, Iwasaki, Shintaro via discuss wrote:
> Hi Matthieu,
>
> 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.


I'm confused :)


We use ABT_SCHED_BASIC_WAIT and ABT_POOL_FIFO_WAIT by default in Mochi 
(Margo).  Even so, if the default (simple) mutex is busy-yielding, that 
will effectively become a busy spin if the ES has no other units to 
execute, right?


If so, that wasn't our intention.  I thought that the ES would idle 
while trying to acquire an abt mutex if no other ULTs were runnable.  We 
might need to do some testing to better understand what's going on there.


thanks,

-Phil



>
> 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.
>
> 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).
>
> Thanks,
> Shintaro
> ------------------------------------------------------------------------
> *From:* Dorier, Matthieu <mdorier at anl.gov>
> *Sent:* Friday, July 23, 2021 10:34 AM
> *To:* discuss at lists.argobots.org <discuss at lists.argobots.org>; 
> Iwasaki, Shintaro <siwasaki at anl.gov>
> *Subject:* Re: condition variable and rwlock
> 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?
>
> Thanks
> Matthieu
>
> Get Outlook for Android <https://aka.ms/ghei36>
>
> ------------------------------------------------------------------------
> *From:* Iwasaki, Shintaro <siwasaki at anl.gov>
> *Sent:* Friday, July 23, 2021 3:51:21 PM
> *To:* discuss at lists.argobots.org <discuss at lists.argobots.org>; Dorier, 
> Matthieu <mdorier at anl.gov>
> *Subject:* Re: condition variable and rwlock
> Hello Matthieu,
>
> Thanks for your question!
>
> > Is there a way of using an Argobots condition variable with an 
> rwlock instead of a mutex?
> No. The user cannot combine ABT_rwlock with ABT_cond.
>
> > 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
>
> 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).
>
> 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.
>
> void reader() {
>   while (1) {
>     if (work_queue.is_empty() and I_AM_SOME_OF_READERS()) {
>       // 1. Internally ABT_self_suspend()-like mechanism needs
>       //    to take a lock (even if it is in the readers' lock)
>       //    since multiple readers might access the same data
>       //    structure (for example, a user-maintained
>       //    suspended ULT list).
>       // 2. Anyway this path is not performance sensitive
>       ABT_mutex_lock(mutex);
>       ABT_cond_wait(cond, mutex);
>       ABT_mutex_unlock(mutex);
>       // Now someone woke me up after pushing work.
>     }
>     ABT_rwlock_rdlock(rwlock);
>     if (!work_queue.is_empty())
>       ; // Do real work.
>     ABT_rwlock_unlock(rwlock);
>   }
> }
>
> void writer() {
>   ABT_rwlock_wrlock(rwlock);
>   work_queue.push_work(work);
>   // You do not need to take a mutex to call ABT_cond_broadcast.
>   // It is fine even if there's no waiter.
>   ABT_cond_broadcast(cond);
>   ABT_rwlock_unlock(rwlock);
> }
>
> I would welcome any suggestions! (For example, does 
> ABT_rwlock_trywrlock() help?)
>
> Best,
> Shintaro
> ------------------------------------------------------------------------
> *From:* Dorier, Matthieu via discuss <discuss at lists.argobots.org>
> *Sent:* Friday, July 23, 2021 5:03 AM
> *To:* discuss at argobots.org <discuss at argobots.org>
> *Cc:* Dorier, Matthieu <mdorier at anl.gov>
> *Subject:* [argobots-discuss] condition variable and rwlock
>
> Hi,
>
> I suspect the answer is no, but is there a way of using an Argobots 
> condition variable with an rwlock instead of a mutex?
>
> (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).
>
> Thanks,
>
> Matthieu
>
>
> _______________________________________________
> discuss mailing list
> discuss at lists.argobots.org
> https://lists.argobots.org/mailman/listinfo/discuss
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