diff options
| -rw-r--r-- | block/bfq-iosched.c | 154 |
1 files changed, 106 insertions, 48 deletions
diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c index b97801ff3de0..208178478dc4 100644 --- a/block/bfq-iosched.c +++ b/block/bfq-iosched.c @@ -6442,15 +6442,25 @@ static bool bfq_bfqq_may_idle(struct bfq_queue *bfqq) * The value of the variable is computed considering that * idling is usually beneficial for the throughput if: * (a) the device is not NCQ-capable, or - * (b) regardless of the presence of NCQ, the request pattern - * for bfqq is I/O-bound (possible throughput losses - * caused by granting idling to seeky queues are mitigated - * by the fact that, in all scenarios where boosting - * throughput is the best thing to do, i.e., in all - * symmetric scenarios, only a minimal idle time is - * allowed to seeky queues). + * (b) regardless of the presence of NCQ, the device is rotational + * and the request pattern for bfqq is I/O-bound (possible + * throughput losses caused by granting idling to seeky queues + * are mitigated by the fact that, in all scenarios where + * boosting throughput is the best thing to do, i.e., in all + * symmetric scenarios, only a minimal idle time is allowed to + * seeky queues). + * + * Secondly, and in contrast to the above item (b), idling an + * NCQ-capable flash-based device would not boost the + * throughput even with intense I/O; rather it would lower + * the throughput in proportion to how fast the device + * is. Accordingly, the next variable is true if any of the + * above conditions (a) and (b) is true, and, in particular, + * happens to be false if bfqd is an NCQ-capable flash-based + * device. */ - idling_boosts_thr = !bfqd->hw_tag || bfq_bfqq_IO_bound(bfqq); + idling_boosts_thr = !bfqd->hw_tag || + (!blk_queue_nonrot(bfqd->queue) && bfq_bfqq_IO_bound(bfqq)); /* * The value of the next variable, @@ -6491,14 +6501,16 @@ static bool bfq_bfqq_may_idle(struct bfq_queue *bfqq) bfqd->wr_busy_queues == 0; /* - * There is then a case where idling must be performed not for - * throughput concerns, but to preserve service guarantees. To - * introduce it, we can note that allowing the drive to - * enqueue more than one request at a time, and hence + * There is then a case where idling must be performed not + * for throughput concerns, but to preserve service + * guarantees. + * + * To introduce this case, we can note that allowing the drive + * to enqueue more than one request at a time, and hence * delegating de facto final scheduling decisions to the - * drive's internal scheduler, causes loss of control on the + * drive's internal scheduler, entails loss of control on the * actual request service order. In particular, the critical - * situation is when requests from different processes happens + * situation is when requests from different processes happen * to be present, at the same time, in the internal queue(s) * of the drive. In such a situation, the drive, by deciding * the service order of the internally-queued requests, does @@ -6509,51 +6521,97 @@ static bool bfq_bfqq_may_idle(struct bfq_queue *bfqq) * the service distribution enforced by the drive's internal * scheduler is likely to coincide with the desired * device-throughput distribution only in a completely - * symmetric scenario where: (i) each of these processes must - * get the same throughput as the others; (ii) all these - * processes have the same I/O pattern (either sequential or - * random). In fact, in such a scenario, the drive will tend - * to treat the requests of each of these processes in about - * the same way as the requests of the others, and thus to - * provide each of these processes with about the same - * throughput (which is exactly the desired throughput - * distribution). In contrast, in any asymmetric scenario, - * device idling is certainly needed to guarantee that bfqq - * receives its assigned fraction of the device throughput - * (see [1] for details). + * symmetric scenario where: + * (i) each of these processes must get the same throughput as + * the others; + * (ii) all these processes have the same I/O pattern + (either sequential or random). + * In fact, in such a scenario, the drive will tend to treat + * the requests of each of these processes in about the same + * way as the requests of the others, and thus to provide + * each of these processes with about the same throughput + * (which is exactly the desired throughput distribution). In + * contrast, in any asymmetric scenario, device idling is + * certainly needed to guarantee that bfqq receives its + * assigned fraction of the device throughput (see [1] for + * details). + * + * We address this issue by controlling, actually, only the + * symmetry sub-condition (i), i.e., provided that + * sub-condition (i) holds, idling is not performed, + * regardless of whether sub-condition (ii) holds. In other + * words, only if sub-condition (i) holds, then idling is + * allowed, and the device tends to be prevented from queueing + * many requests, possibly of several processes. The reason + * for not controlling also sub-condition (ii) is that we + * exploit preemption to preserve guarantees in case of + * symmetric scenarios, even if (ii) does not hold, as + * explained in the next two paragraphs. + * + * Even if a queue, say Q, is expired when it remains idle, Q + * can still preempt the new in-service queue if the next + * request of Q arrives soon (see the comments on + * bfq_bfqq_update_budg_for_activation). If all queues and + * groups have the same weight, this form of preemption, + * combined with the hole-recovery heuristic described in the + * comments on function bfq_bfqq_update_budg_for_activation, + * are enough to preserve a correct bandwidth distribution in + * the mid term, even without idling. In fact, even if not + * idling allows the internal queues of the device to contain + * many requests, and thus to reorder requests, we can rather + * safely assume that the internal scheduler still preserves a + * minimum of mid-term fairness. The motivation for using + * preemption instead of idling is that, by not idling, + * service guarantees are preserved without minimally + * sacrificing throughput. In other words, both a high + * throughput and its desired distribution are obtained. + * + * More precisely, this preemption-based, idleless approach + * provides fairness in terms of IOPS, and not sectors per + * second. This can be seen with a simple example. Suppose + * that there are two queues with the same weight, but that + * the first queue receives requests of 8 sectors, while the + * second queue receives requests of 1024 sectors. In + * addition, suppose that each of the two queues contains at + * most one request at a time, which implies that each queue + * always remains idle after it is served. Finally, after + * remaining idle, each queue receives very quickly a new + * request. It follows that the two queues are served + * alternatively, preempting each other if needed. This + * implies that, although both queues have the same weight, + * the queue with large requests receives a service that is + * 1024/8 times as high as the service received by the other + * queue. * - * As for sub-condition (i), actually we check only whether - * bfqq is being weight-raised. In fact, if bfqq is not being - * weight-raised, we have that: - * - if the process associated with bfqq is not I/O-bound, then - * it is not either latency- or throughput-critical; therefore - * idling is not needed for bfqq; - * - if the process asociated with bfqq is I/O-bound, then - * idling is already granted with bfqq (see the comments on - * idling_boosts_thr). + * On the other hand, device idling is performed, and thus + * pure sector-domain guarantees are provided, for the + * following queues, which are likely to need stronger + * throughput guarantees: weight-raised queues, and queues + * with a higher weight than other queues. When such queues + * are active, sub-condition (i) is false, which triggers + * device idling. * - * We do not check sub-condition (ii) at all, i.e., the next - * variable is true if and only if bfqq is being - * weight-raised. We do not need to control sub-condition (ii) - * for the following reason: - * - if bfqq is being weight-raised, then idling is already - * guaranteed to bfqq by sub-condition (i); - * - if bfqq is not being weight-raised, then idling is - * already guaranteed to bfqq (only) if it matters, i.e., if - * bfqq is associated to a currently I/O-bound process (see - * the above comment on sub-condition (i)). + * According to the above considerations, the next variable is + * true (only) if sub-condition (i) holds. To compute the + * value of this variable, we not only use the return value of + * the function bfq_symmetric_scenario(), but also check + * whether bfqq is being weight-raised, because + * bfq_symmetric_scenario() does not take into account also + * weight-raised queues (see comments on + * bfq_weights_tree_add()). * * As a side note, it is worth considering that the above * device-idling countermeasures may however fail in the * following unlucky scenario: if idling is (correctly) - * disabled in a time period during which the symmetry - * sub-condition holds, and hence the device is allowed to + * disabled in a time period during which all symmetry + * sub-conditions hold, and hence the device is allowed to * enqueue many requests, but at some later point in time some * sub-condition stops to hold, then it may become impossible * to let requests be served in the desired order until all * the requests already queued in the device have been served. */ - asymmetric_scenario = bfqq->wr_coeff > 1; + asymmetric_scenario = bfqq->wr_coeff > 1 || + !bfq_symmetric_scenario(bfqd); /* * We have now all the components we need to compute the return |