83a2c42...
by
Paolo Bonzini <email address hidden>
urcu-qsbr: avoid useless futex wakeups and burning CPU for long grace periods
I noticed that urcu makes exactly _one_ attempt at using futexes
to avoid busy looping on synchronize_rcu. The attached patch instead
switches from busy waiting to futexes after RCU_QS_ACTIVE_ATTEMPTS.
To limit the amount of system calls, reading threads remember whether
they already had a quiescent state in this grace period; if so they were
already removed from the list, and can avoid signaling the futex.
Performance measured with rcutorture (nreaders: 10, nupdaters: 1,
duration: 10, median of nine runs):
RCU_QS_ACTIVE_ATTEMPTS == 100, no patch n_updates = 292
RCU_QS_ACTIVE_ATTEMPTS == 1, no patch n_updates = 290
RCU_QS_ACTIVE_ATTEMPTS == 100, with patch n_updates = 408
RCU_QS_ACTIVE_ATTEMPTS == 1, with patch n_updates = 404
(the first two cases are obviously the same; the only change is
when the futex is used, but over many calls there is no difference).
This patch matches the update to the Promela model.
8c4f494...
by
Paolo Bonzini <email address hidden>
test api cleanup: remove unused primitives
[ Mathieu: the rationale for this is that we can always add back that
code if every needed. Removing leftover GPLv2 test code is an
incentive to create the appropriate library-wide LGPL/MIT-style
abstractions.]
