On Thu, 2013-01-03 at 11:45 -0500, Steven Rostedt wrote: > On Thu, 2013-01-03 at 08:10 -0800, Eric Dumazet wrote: > > > > But then would the problem even exist? If the lock is on its own cache > > > line, it shouldn't cause a performance issue if other CPUs are spinning > > > on it. Would it? > > > > Not sure I understand the question. > > > > I'll explain my question better. > > I thought the whole point of Rik's patches was to solve a performance > problem caused by contention on a lock that shares a cache line with > data. > > In the ideal case, locks wont be contented, and are taken and released > quickly (being from the RT world, I know this isn't true :-( ). In this > case, it's also advantageous to keep the lock on the same cache line as > the data that's being updated. This way, the process of grabbing the > lock also pulls in the data that you will soon be using. > > But then the problem occurs when you have a bunch of other CPUs trying > to take this lock in a tight spin. Every time the owner of the lock > touches the data, the other CPUs doing a LOCK read on the spinlock will > cause bus contention on the owner CPU as the data shares the cache and > needs to be synced. As the owner CPU just touched the cache line that is > under a tight loop of LOCK reads on other CPUs. By adding the delays, > the CPU with the lock doesn't stall at every update of the data > protected by the lock. > > Thus, if monitor/mwait is ideal only for locks on its own cache line, > then they are pointless for the locks that are causing the issue we are > trying to fix.
I think you misunderstood the monitor/mwait usage I was speaking of - Only for MCS type lock, where each cpu spins on it own busy/locked bit. Of course, if we use a ticket spinlock with no additional storage, we have to spin without making any memory reference, and thats Rick's patch using this idea : http://www.cs.rochester.edu/research/synchronization/pseudocode/ss.html#ticket -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
