Fabio and I are a little bit worried about the fact that the problem of working in the time domain instead of the service domain is not being properly dealt with. Probably we did not express ourselves very clearly, so we will try to put in more practical terms. Using B-WF2Q+ in the time domain instead of using CFQ (Round-Robin) means introducing higher complexity than CFQ to get almost the same service properties of CFQ. With regard to fairness (long term) B-WF2Q+ in the time domain has exactly the same (un)fairness problems of CFQ. As far as bandwidth differentiation is concerned, it can be obtained with CFQ by just increasing the time slice (e.g., double weight => double slice). This has no impact on long term guarantees and certainly does not decrease the throughput. With regard to short term guarantees (request completion time), one of the properties of the reference ideal system of Wf2Q+ is that, assuming for simplicity that all the queues have the same weight, as the ideal system serves each queue at the same speed, shorter budgets are completed in a shorter time intervals than longer budgets. B-WF2Q+ guarantees O(1) deviation from this ideal service. Hence, the tight delay/jitter measured in our experiments with BFQ is a consequence of the simple (and probably still improvable) budget assignment mechanism of (the overall) BFQ. In contrast, if all the budgets are equal, as it happens if we use time slices, the resulting scheduler is exactly a Round-Robin, again as in CFQ (see [1]). Finally, with regard to completion time delay differentiation through weight differentiation, this is probably the only case in which B-WF2Q+ would perform better than CFQ, because, in case of CFQ, reducing the time slices may reduce the throughput, whereas increasing the time slice would increase the worst-case delay/jitter. In the end, BFQ succeeds in guaranteeing fairness (or in general the desired bandwidth distribution) because it works in the service domain (and this is probably the only way to achieve this goal), not because it uses WF2Q+ instead of Round-Robin. Similarly, it provides tight delay/jitter only because B-WF2Q+ is used in combination with a simple budget assignment (differentiation) mechanism (again in the service domain). [1] http://feanor.sssup.it/~fabio/linux/bfq/results.php -- ----------------------------------------------------------- | Paolo Valente | | | Algogroup | | | Dip. Ing. Informazione | tel: +39 059 2056318 | | Via Vignolese 905/b | fax: +39 059 2056199 | | 41100 Modena | | | home: http://algo.ing.unimo.it/people/paolo/ | ----------------------------------------------------------- _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/virtualization
