> I'm trying to understand if this is expected or not, and if there is Without careful tuning, outliers around a couple of hundred ms are definitely expected in general (not *necessarily*, depending on workload) as a result of garbage collection pauses. The impact will be worsened a bit if you are running under high CPU load (or even maxing it out with stress) because post-pause, if you are close to max CPU usage you will take considerably longer to "catch up".
Personally, I would just log each response time and feed it to gnuplot or something. It should be pretty obvious whether or not the latencies are due to periodic pauses. If you are concerned with eliminating or reducing outliers, I would: (1) Make sure that when you're benchmarking, that you're putting Cassandra under a reasonable amount of load. Latency benchmarks are usually useless if you're benchmarking against a saturated system. At least, start by achieving your latency goals at 25% or less CPU usage, and then go from there if you want to up it. (2) One can affect GC pauses, but it's non-trivial to eliminate the problem completely. For example, the length of frequent young-gen pauses can typically be decreased by decreasing the size of the young generation, leading to more frequent shorter GC pauses. But that instead causes more promotion into the old generation, which will result in more frequent very long pauses (relative to normal; they would still be infrequent relative to young gen pauses) - IF your workload is such that you are suffering from fragmentation and eventually seeing Cassandra fall back to full compacting GC:s (stop-the-world) for the old generation. I would start by adjusting young gen so that your frequent pauses are at an acceptable level, and then see whether or not you can sustain that in terms of old-gen. Start with this in any case: Run Cassandra with -XX:+PrintGC -XX:+PrintGCDetails -XX:+PrintGCTimeStamps -XX:+PrintGCDateStamps -- / Peter Schuller (@scode, http://worldmodscode.wordpress.com)
