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authorPaolo Valente <paolo.valente@linaro.org>2017-04-12 18:23:11 +0200
committerJens Axboe <axboe@fb.com>2017-04-19 08:30:26 -0600
commitc074170e65995706be78e8c57ed2017c638d5464 (patch)
tree380cebbd58df1c0ecfb033e1b19755cf829e2530 /mm
parentab0e43e9cea047873599bc8041cd6278781fd4e0 (diff)
downloadlinux-c074170e65995706be78e8c57ed2017c638d5464.tar.bz2
block, bfq: add more fairness with writes and slow processes
This patch deals with two sources of unfairness, which can also cause high latencies and throughput loss. The first source is related to write requests. Write requests tend to starve read requests, basically because, on one side, writes are slower than reads, whereas, on the other side, storage devices confuse schedulers by deceptively signaling the completion of write requests immediately after receiving them. This patch addresses this issue by just throttling writes. In particular, after a write request is dispatched for a queue, the budget of the queue is decremented by the number of sectors to write, multiplied by an (over)charge coefficient. The value of the coefficient is the result of our tuning with different devices. The second source of unfairness has to do with slowness detection: when the in-service queue is expired, BFQ also controls whether the queue has been "too slow", i.e., has consumed its last-assigned budget at such a low rate that it would have been impossible to consume all of this budget within the maximum time slice T_max (Subsec. 3.5 in [1]). In this case, the queue is always (over)charged the whole budget, to reduce its utilization of the device. Both this overcharge and the slowness-detection criterion may cause unfairness. First, always charging a full budget to a slow queue is too coarse. It is much more accurate, and this patch lets BFQ do so, to charge an amount of service 'equivalent' to the amount of time during which the queue has been in service. As explained in more detail in the comments on the code, this enables BFQ to provide time fairness among slow queues. Secondly, because of ZBR, a queue may be deemed as slow when its associated process is performing I/O on the slowest zones of a disk. However, unless the process is truly too slow, not reducing the disk utilization of the queue is more profitable in terms of disk throughput than the opposite. A similar problem is caused by logical block mapping on non-rotational devices. For this reason, this patch lets a queue be charged time, and not budget, only if the queue has consumed less than 2/3 of its assigned budget. As an additional, important benefit, this tolerance allows BFQ to preserve enough elasticity to still perform bandwidth, and not time, distribution with little unlucky or quasi-sequential processes. Finally, for the same reasons as above, this patch makes slowness detection itself much less harsh: a queue is deemed slow only if it has consumed its budget at less than half of the peak rate. [1] P. Valente and M. Andreolini, "Improving Application Responsiveness with the BFQ Disk I/O Scheduler", Proceedings of the 5th Annual International Systems and Storage Conference (SYSTOR '12), June 2012. Slightly extended version: http://algogroup.unimore.it/people/paolo/disk_sched/bfq-v1-suite- results.pdf Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Arianna Avanzini <avanzini.arianna@gmail.com> Signed-off-by: Jens Axboe <axboe@fb.com>
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