We have several latency-sensitive “pipeline”-style programs that have a measurable performance degredation when run on one Linux kernel versus another. In particular, we see better performance with the 2.6.9 CentOS 4.x (RHEL4) kernel, and worse performance with the 2.6.18 kernel from CentOS 5.x (RHEL5).
By “pipeline” program, I mean one that has multiple threads. The mutiple threads work on shared data. Between each thread, there is a queue. So thread A gets data, pushes into Qab, thread B pulls from Qab, does some processing, then pushes into Qbc, thread C pulls from Qbc, etc. The initial data is from the network (generated by a 3rd party).
We basically measure the time from when the data is received to when the last thread performs its task. In our application, we see an increase of anywhere from 20 to 50 microseconds when moving from CentOS 4 to CentOS 5.
I have used a few methods of profiling our application, and determined that the added latency on CentOS 5 comes from queue operations (in particular, popping).
However, I can improve performance on CentOS 5 (to be the same as CentOS 4) by using taskset to bind the program to a subset of the available cores.
So it appers to me, between CentOS 4 and 5, there was some change (presumably to the kernel) that caused threads to be scheduled differently (and this difference is suboptimal for our application).
While I can “solve” this problem with taskset (or in code via sched_setaffinity()), my preference is to not have to do this. I’m hoping there’s some kind of kernel tunable (or maybe collection of tunables) whose default was changed between versions.
Anyone have any experience with this? Perhaps some more areas to investigate?
Update: In this particular case, the issue was resolved by a BIOS update from the server vendor (Dell). I pulled my hair out quite a while on this one. Until I went back to the basics, and checked my vendor’s BIOS updates. Suspiciously, one of the updates said something like “improve performance in maximum performance mode”. Once I upgraded the BIOS, CentOS 5 was faster—generally speaking, but particularly in my queue tests, and actual production runs.
Hmm.. if the time taken for a pop() operation from a producer-consumer queue is making a significant difference to the overall performance of your app, I would suggest that the structure of your threads/workFlow is not optimal, somewhere . Unless there is a huge amount of contention on the queues, I would be surprised if any P-C queue push/pop on any modern OS would take more than a µS or so, even if the queue uses kernel locks in a classic ‘Computer Science 117 – how to make a bounded P-C queue with three semaphores’ manner.
Can you just absorb the functionality of the thread/s that do the least work into those that do the most, so reducing the number of push/pop per overall work item that flows through your system?