Waiman, On Mon, 15 Jul 2013, Waiman Long wrote: > On 07/15/2013 06:31 PM, Thomas Gleixner wrote: > > On Fri, 12 Jul 2013, Waiman Long wrote: > > > Apparently, the regular read/write lock performs even better than > > > the queue read/write lock in some cases. This is probably due to the > > The regular rwlock performs better in most cases. This is the full > > list comparing both against the ticket lock. > > > > qrlock rwlock > > +20.7 +44.4 > > +30.1 +42.9 > > > > +56.3 +63.3 > > +52.9 +48.8 > > > > +54.4 +65.1 > > +49.2 +26.5 > > > > So you try to sell that qrwlock as a replacement for ticket spinlocks, > > while at the same time you omit the fact that we have an even better > > implementation (except for the last test case) already in the > > kernel. What's the point of this exercise? > > The main point is that the regular rwlock is not fair while the > queue rwlock is close to as fair as the ticket spinlock. The LWN > article http://lwn.net/Articles/364583/ mentioned about eliminating > rwlock altogether precisely because of this unfairness as it can > cause livelock in certain scenerio. I also saw slides to advise > again using rwlock because of this. I'm well aware of this. But that does not explain anything of what I asked. > > > + * has the following advantages: > > > + * 1. It is more deterministic. Even though there is a slight chance > > > of > > Why is it more deterministic than the existing implementation? > > Deterministic means that that a process can acquire a lock within a > reasonable time period without being starved for a long time. The qrwlock > grants lock in FIFO order in most cases. That is what I mean by being more > deterministic. That's exactly the kind of explanation we want to have in the code and the changelog. > > > > > + * stealing the lock if come at the right moment, the granting of > > > the > > > + * lock is mostly in FIFO order. > > > + * 2. It is faster in high contention situation. > > Again, why is it faster? > > The current rwlock implementation suffers from a thundering herd problem. > When many readers are waiting for the lock hold by a writer, they will all > jump in more or less at the same time when the writer releases the lock. > That is not the case with qrwlock. It has been shown in many cases that > avoiding this thundering herd problem can lead to better performance. That makes sense and wants to be documented as well. You could have avoided a lot of the discussion if you had included these details right away. > > > + * an increase in lock size is not an issue. > > So is it faster in the general case or only for the high contention or > > single thread operation cases? > > > > And you still miss to explain WHY it is faster. Can you please explain > > proper WHY it is faster and WHY we can't apply that technique you > > implemented for qrwlocks to writer only locks (aka spinlocks) with a > > smaller lock size? > > I will try to collect more data to justify the usefulness of qrwlock. And please provide a proper argument why we can't use the same technique for spinlocks. > > Aside of that, you are replacing all RW locks unconditionally by this > > new fangled thing, but did you actually run tests which look at other > > rwlock usage sites than the particular one you care about? > > Users have the choice of using the old rwlock or the queue rwlock by > selecting or unselecting the QUEUE_RWLOCK config parameter. I am not > forcing the unconditional replacement of rwlock by qrwlock. Looking at patch 2/2: +config ARCH_QUEUE_RWLOCK + def_bool y What's conditional about that? Where is the choice? > > You are optimizing for the high frequency writer case. And that's not > > the primary use case for rwlocks. That's the special use case for the > > jbd2 journal_state_lock which CANNOT be generalized for all other > > rwlock usage sites. > > It is true that this lock is kind of optimized for writers. For > reader heavy code, the performance may not be as good as the rwlock > for uncontended cases. However, I do believe that the fairness > attribute of the qrwlock far outweigh the slight performance > overhead of read lock/unlock. Furthermore, the lock/unlock sequence > contributes only a very tiny percentage of total CPU time in > uncontended cases. A slight increase may not really have a material > impact on performance. Again, as promised, I will try to collect > some more performance data for reader heavy usage cases. Yes, please. We really need this information and if it turns out, that it does not affect reader heavy sides, I have no objections against the technology itself. Thanks, tglx -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
