Hello, Shaohua. On Wed, Nov 23, 2016 at 05:15:18PM -0800, Shaohua Li wrote: > > Hmm... I'm not sure thinktime is the best measure here. Think time is > > used by cfq mainly to tell the likely future behavior of a workload so > > that cfq can take speculative actions on the prediction. However, > > given that the implemented high limit behavior tries to provide a > > certain level of latency target, using the predictive thinktime to > > regulate behavior might lead to too unpredictable behaviors. > > Latency just reflects one side of the IO. Latency and think time haven't any > relationship. For example, a cgroup dispatching 1 IO per second can still have > high latency. If we only take latency account, we will think the cgroup is > busy, which is not justified. Yes, the two are indepndent metrics; however, whether a cgroup is considered idle or not affects whether blk-throttle will adhere to the latency target or not. Thinktime is a magic number which can be good but whose behavior can be very difficult to predict from outside the black box. What I was trying to say was that putting in thinktime here can greatly weaken the configured latency target in unobvious ways. > > Moreover, I don't see why we need to bother with predictions anyway. > > cfq needed it but I don't think that's the case for blk-throtl. It > > can just provide idle threshold where a cgroup which hasn't issued an > > IO over that threshold is considered idle. That'd be a lot easier to > > understand and configure from userland while providing a good enough > > mechanism to prevent idle cgroups from clamping down utilization for > > too long. > > We could do this, but it will only work for very idle workload, eg, the > workload is completely idle. If workload dispatches IO sporadically, this will > likely not work. The average think time is more precise for predication. But we can increase sharing by upping the target latency. That should be the main knob - if low, the user wants stricter service guarantee at the cost of lower overall utilization; if high, the workload can deal with higher latency and the system can achieve higher overall utilization. I think the idle detection should be an extra mechanism which can be used to ignore cgroup-disk combinations which are staying idle for a long time. Thanks. -- tejun -- To unsubscribe from this list: send the line "unsubscribe linux-block" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
