测量无等待多线程java程序中的争用

Question

I have a wait-free implementation for binary search trees but I am not able to figure out concrete methods to measure thread contention. By contention, here I mean number of threads that try to access the same piece of memory at the same time.

So far, I have searched ThreadMXBean and ThreadInfo class, but as there are no locks involved, I haven't found any solution yet.

Answer 1

There is no way to measure the contention over "memory location" without prohibitive performance costs. Direct measurement (eg properly synchronized counter wrapping all the accesses) will introduce the artificial bottlenecks, which will blow up test reliability.

"Same time" is loosely defined on the scale you want to measure it, because only a single CPU "owns" the particular location in memory in a given time. The best you can do in this case it to measure the rate at which CPUs are dealing with memory conflicts, eg through the HW counters. Doing that requires the understanding of memory subsystem on a given platfom. Also, the HW counters attribute for machine (= CPU) state, not the memory state; in other words, you can estimate how many conflicts the particular instructions have experienced, not how many CPUs accessed the given memory location.

Answer 2

Trying the measure within the source of the contention is the wrong approach. What might be the reason for contention anyways?!

So, first of all, setup a benchmarking suite which runs typical access patterns on your data structure and graph the performance for different thread counts. Here is a nice example from nitro cache performance page .

If you scale almost linear: congrats, you are done!

If you don't scale linear, you need more insight. Now you need to profile the system as a whole and see what is the reason eg for CPU pipeline stalls. The best way is to use low-overhead tracing for this. On Linux you can use OProfile . OProfile has also Java support, which helps you to correlate the JITed machine code to your Java program.