**diff options**

author | Paolo Valente <paolo.valente@linaro.org> | 2018-05-31 16:45:05 +0200 |
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committer | Jens Axboe <axboe@kernel.dk> | 2018-05-31 08:54:36 -0600 |

commit | 4029eef1be4c869ae4c1bdcdc0010a1f2a5b888f (patch) | |

tree | 163f41ca26b3d8b36c36bba805844a114f55bbb2 /block/bfq-iosched.c | |

parent | ac857e0d54c84edc1fdc578153f4f728c69c0e29 (diff) | |

download | linux-0-day-4029eef1be4c869ae4c1bdcdc0010a1f2a5b888f.tar.gz |

block, bfq: add description of weight-raising heuristics

A description of how weight raising works is missing in BFQ
sources. In addition, the code for handling weight raising is
scattered across a few functions. This makes it rather hard to
understand the mechanism and its rationale. This commits adds such a
description at the beginning of the main source file.
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Diffstat (limited to 'block/bfq-iosched.c')

-rw-r--r-- | block/bfq-iosched.c | 80 |

1 files changed, 56 insertions, 24 deletions

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c index dd527bab..e68d0a4 100644 --- a/block/bfq-iosched.c +++ b/block/bfq-iosched.c @@ -49,9 +49,39 @@ * * In particular, to provide these low-latency guarantees, BFQ * explicitly privileges the I/O of two classes of time-sensitive - * applications: interactive and soft real-time. This feature enables - * BFQ to provide applications in these classes with a very low - * latency. Finally, BFQ also features additional heuristics for + * applications: interactive and soft real-time. In more detail, BFQ + * behaves this way if the low_latency parameter is set (default + * configuration). This feature enables BFQ to provide applications in + * these classes with a very low latency. + * + * To implement this feature, BFQ constantly tries to detect whether + * the I/O requests in a bfq_queue come from an interactive or a soft + * real-time application. For brevity, in these cases, the queue is + * said to be interactive or soft real-time. In both cases, BFQ + * privileges the service of the queue, over that of non-interactive + * and non-soft-real-time queues. This privileging is performed, + * mainly, by raising the weight of the queue. So, for brevity, we + * call just weight-raising periods the time periods during which a + * queue is privileged, because deemed interactive or soft real-time. + * + * The detection of soft real-time queues/applications is described in + * detail in the comments on the function + * bfq_bfqq_softrt_next_start. On the other hand, the detection of an + * interactive queue works as follows: a queue is deemed interactive + * if it is constantly non empty only for a limited time interval, + * after which it does become empty. The queue may be deemed + * interactive again (for a limited time), if it restarts being + * constantly non empty, provided that this happens only after the + * queue has remained empty for a given minimum idle time. + * + * By default, BFQ computes automatically the above maximum time + * interval, i.e., the time interval after which a constantly + * non-empty queue stops being deemed interactive. Since a queue is + * weight-raised while it is deemed interactive, this maximum time + * interval happens to coincide with the (maximum) duration of the + * weight-raising for interactive queues. + * + * Finally, BFQ also features additional heuristics for * preserving both a low latency and a high throughput on NCQ-capable, * rotational or flash-based devices, and to get the job done quickly * for applications consisting in many I/O-bound processes. @@ -61,14 +91,14 @@ * all low-latency heuristics for that device, by setting low_latency * to 0. * - * BFQ is described in [1], where also a reference to the initial, more - * theoretical paper on BFQ can be found. The interested reader can find - * in the latter paper full details on the main algorithm, as well as - * formulas of the guarantees and formal proofs of all the properties. - * With respect to the version of BFQ presented in these papers, this - * implementation adds a few more heuristics, such as the one that - * guarantees a low latency to soft real-time applications, and a - * hierarchical extension based on H-WF2Q+. + * BFQ is described in [1], where also a reference to the initial, + * more theoretical paper on BFQ can be found. The interested reader + * can find in the latter paper full details on the main algorithm, as + * well as formulas of the guarantees and formal proofs of all the + * properties. With respect to the version of BFQ presented in these + * papers, this implementation adds a few more heuristics, such as the + * ones that guarantee a low latency to interactive and soft real-time + * applications, and a hierarchical extension based on H-WF2Q+. * * B-WF2Q+ is based on WF2Q+, which is described in [2], together with * H-WF2Q+, while the augmented tree used here to implement B-WF2Q+ @@ -218,21 +248,23 @@ static struct kmem_cache *bfq_pool; #define BFQ_RATE_SHIFT 16 /* - * By default, BFQ computes the duration of the weight raising for - * interactive applications automatically, using the following formula: - * duration = (R / r) * T, where r is the peak rate of the device, and - * R and T are two reference parameters. - * In particular, R is the peak rate of the reference device (see - * below), and T is a reference time: given the systems that are - * likely to be installed on the reference device according to its - * speed class, T is about the maximum time needed, under BFQ and + * When configured for computing the duration of the weight-raising + * for interactive queues automatically (see the comments at the + * beginning of this file), BFQ does it using the following formula: + * duration = (R / r) * T, + * where r is the peak rate of the device, and R + * and T are two reference parameters. In particular, + * R is the peak rate of the reference device (see below), and + * T is a reference time: given the systems that are likely + * to be installed on the reference device according to its speed + * class, T is about the maximum time needed, under BFQ and * while reading two files in parallel, to load typical large * applications on these systems (see the comments on - * max_service_from_wr below, for more details on how T is obtained). - * In practice, the slower/faster the device at hand is, the more/less - * it takes to load applications with respect to the reference device. - * Accordingly, the longer/shorter BFQ grants weight raising to - * interactive applications. + * max_service_from_wr below, for more details on how T is + * obtained). In practice, the slower/faster the device at hand is, + * the more/less it takes to load applications with respect to the + * reference device. Accordingly, the longer/shorter BFQ grants + * weight raising to interactive applications. * * BFQ uses four different reference pairs (R, T), depending on: * . whether the device is rotational or non-rotational; |