Messages in this thread | | | Date | Tue, 30 Jun 2015 15:12:24 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH RFC tip/core/rcu 0/5] Expedited grace periods encouraging normal ones |
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On Tue, Jun 30, 2015 at 03:00:15PM -0700, josh@joshtriplett.org wrote: > On Tue, Jun 30, 2015 at 02:48:05PM -0700, Paul E. McKenney wrote: > > Hello! > > > > This series contains some highly experimental patches that allow normal > > grace periods to take advantage of the work done by concurrent expedited > > grace periods. This can reduce the overhead incurred by normal grace > > periods by eliminating the need for force-quiescent-state scans that > > would otherwise have happened after the expedited grace period completed. > > It is not clear whether this is a useful tradeoff. Nevertheless, this > > series contains the following patches: > > While it makes sense to avoid unnecessarily delaying a normal grace > period if the expedited machinery has provided the necessary delay, I'm > also *deeply* concerned that this will create a new class of > nondeterministic performance issues. Something that uses RCU may > perform badly due to grace period latency, but then suddenly start > performing well because an unrelated task starts hammering expedited > grace periods. This seems particularly likely during boot, for > instance, where RCU grace periods can be a significant component of boot > time (when you're trying to boot to userspace in small fractions of a > second).
I will take that as another vote against. And for a reason that I had not yet come up with, so good show! ;-)
Thanx, Paul
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