Commits on Mar 3, 2022

1. Implement the BRAVO biased reader/writer lock …

   The goal of the BRAVO biased rwlock is to avoid readers taking the reader
   lock and thus contending for the atomic variable. Instead, readers raise a
   flag in an array to signal that they "took the lock" to any future writer.
   A writer takes the underlying atomic lock and waits for all readers to
   complete. While a writer has the lock, readers wait for the atomic rwlock.
   
   The hash table in PaRSEC is a prime example for a use-case: writers are
   extremely rare since the resizing happens rarely and the max size is capped.
   However, every thread locking a bucket also takes a reader-lock.
   We can thus avoid the contention on the global lock for most of the
   application run.
   
   The original proposal used a global hash table for all locks
   (https://arxiv.org/abs/1810.01553) but we use one array per lock.
   We know the number of threads in PaRSEC and can use fixed offsets, with
   padding to prevent cache line sharing (64B per thread). If an unknown
   thread takes the lock it goes straight to the atomic rwlock (unfortuntely,
   this includes the MPI progress thread at the moment).
   
   Signed-off-by: Joseph Schuchart <schuchart@icl.utk.edu>

   

   devreal committed Mar 3, 2022
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