[LU-9228] Hard TBF Token Compensation under congestion

Related — Mon, 26 Mar 2018 04:22:15 +0000

During TBF evaluation, we find that when the sum of I/O bandwidth requirements for all classes exceeds the system capacity, the classes with same rate limits get less bandwidth than preconfigured evenly.
The reason is as follow. Under heavy load on a congested server, it will result in some missed deadlines for some classes. The calculated tokens may larger than 1 during dequeuing. In the original implementation, all classes are equally handled to simply discard exceeding tokens.
Thus, a Hard Token Compensation (HTC) strategy is proposed. A class can be configured with HTC feature by the rule it matches. This feature means that requests in this kind of class queues have high real-time requirements and that the bandwidth assignment must be satisfied as good as possible. When deadline misses happen, the class keeps the deadline unchanged and the time residue (the remainder of elapsed time divided by 1/r) is compensated to the next round. This ensures that the next idle I/O thread will always select this class to serve until all accumulated exceeding tokens are handled or there are no pending requests in the class queue.

A new command format is added to enable HTC feature for a rule:

start $ruleName jobid={dd.0} rate=100 realtime=1

Whamcloud Community JIRA

[LU-9228] Hard TBF Token Compensation under congestion