This year I had the privilege of leading the Performance and Scalability micro-conference for Linux Plumbers. The goals and motivation behind organizing this track were threefold. First present relevant work-in-progress ideas that can improve performance in core kernel subsystems, and need some face to face discussion -- as such, this requires previous debate on lkml. Similarly, learn about real bottlenecks and issues people are running into. And finally, get to know more relevant academic (experimental) work going on in in both the kernel and system-level userland. As such, the sessions were grouped as follows:
(i) Fast Bounded-Concurrency Hash Tables. Samy Bahra introduced a novel non-blocking multi-reader/single writer hash table with strong forward progress guarantees for TSO. Because the common-case fastpath does not incur in barriers or atomic operations, this technique allows nearly perfect scaling. While his work is done in userspace, he sees potential for it in the kernel, such as the networking subsystem. In such situations, the use of RCU (readers being the common case) might also be used.
(ii) Improving Transactional Memory Performance with Queued Locking. While transactional memory works nicely in conflict-free setups, it ends up requiring common serialization otherwise. An option is to retry, however, when the amount of threads executing in the CR is larger than the amount of completed threads, you can get pileups. Tim Chen presented a solution based on applying a sort of 'aperture' and using principles based on MCS for faired queuing, where can be regulated based on metrics such as the number of threads in the critical region and abort rate.
(iii) How to Apply Mutation Testing to RCU. Iftekhar Ahmed from OSU, summarized his research in overcoming limitations of mutation testing to identify problems in RCU. As usual, working with Paul McKenney, they have been able to identify a number of mutants along with making use of rcutorture for specific periods of time. They generated ~3300 mutants from rcu and rcutorture is doing a good job identifying them. It would be interesting to see this applied along with fuzzy testing which has already uncovered several bugs in RCU in the past.
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| Scaling track -- LPC'15, Seattle. |
(iv) Unfair Queued Spinlocks and Transactional Locks. Waiman Long has been working on extending spinlocks and apply them to solve issues with transactional memory. He presented experiments based on rwlocks and transactional spinlock (new primitive) for transactional (reader) and non-transactional (writer) executions. This talk nicely complemented Tim Chen's previous presentation. He also touched on the qspinlock performance in virtualized environments and the challenges currently out there. As we already have code for this, it was much easier to discuss face to face. Consensus in the room was that kernel developers are not against improving pv spinlocks, but what is determined is that we will not accept a 3rd primitive.
(v) Do Virtual Machines Really Scale. Sanidhya Kashyap from GA Tech showed us the state of scalability in the cloud where there is a clear trend that services hit poor scalability after certain degrees of contention/core-count. These are LHP issues and vmexits/enters cause performance issues at high vcpu counts. He introduces oticket backed by performing multiple wakeups at once when granting the lock. Good feedback and suggestions to overcome some of the presented issues with the approach. This was an extra short BoF like of presentation, but there was quite a bit of interest, and the appropriate people were in the room.
(v) Do Virtual Machines Really Scale. Sanidhya Kashyap from GA Tech showed us the state of scalability in the cloud where there is a clear trend that services hit poor scalability after certain degrees of contention/core-count. These are LHP issues and vmexits/enters cause performance issues at high vcpu counts. He introduces oticket backed by performing multiple wakeups at once when granting the lock. Good feedback and suggestions to overcome some of the presented issues with the approach. This was an extra short BoF like of presentation, but there was quite a bit of interest, and the appropriate people were in the room.
Overall I would say that all three objectives were met and the quality of the sessions were high, thus meeting all expectations (if not, please email me for feedback ;-). In fact, there were some highly interesting and relevant presentations that, due to time constraints, had to be left out.
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