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Diffstat (limited to 'Documentation/block/bfq-iosched.rst')
| -rw-r--r-- | Documentation/block/bfq-iosched.rst | 71 |
1 files changed, 40 insertions, 31 deletions
diff --git a/Documentation/block/bfq-iosched.rst b/Documentation/block/bfq-iosched.rst index 0d237d402860..a0ff0eb11e7f 100644 --- a/Documentation/block/bfq-iosched.rst +++ b/Documentation/block/bfq-iosched.rst @@ -9,7 +9,7 @@ controllers), BFQ's main features are: - BFQ guarantees a high system and application responsiveness, and a low latency for time-sensitive applications, such as audio or video players; -- BFQ distributes bandwidth, and not just time, among processes or +- BFQ distributes bandwidth, not just time, among processes or groups (switching back to time distribution when needed to keep throughput high). @@ -111,7 +111,7 @@ Higher speed for code-development tasks If some additional workload happens to be executed in parallel, then BFQ executes the I/O-related components of typical code-development -tasks (compilation, checkout, merge, ...) much more quickly than CFQ, +tasks (compilation, checkout, merge, etc.) much more quickly than CFQ, NOOP or DEADLINE. High throughput @@ -127,9 +127,9 @@ Strong fairness, bandwidth and delay guarantees ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ BFQ distributes the device throughput, and not just the device time, -among I/O-bound applications in proportion their weights, with any +among I/O-bound applications in proportion to their weights, with any workload and regardless of the device parameters. From these bandwidth -guarantees, it is possible to compute tight per-I/O-request delay +guarantees, it is possible to compute a tight per-I/O-request delay guarantees by a simple formula. If not configured for strict service guarantees, BFQ switches to time-based resource sharing (only) for applications that would otherwise cause a throughput loss. @@ -199,7 +199,7 @@ plus a lot of code, are borrowed from CFQ. - On flash-based storage with internal queueing of commands (typically NCQ), device idling happens to be always detrimental - for throughput. So, with these devices, BFQ performs idling + to throughput. So, with these devices, BFQ performs idling only when strictly needed for service guarantees, i.e., for guaranteeing low latency or fairness. In these cases, overall throughput may be sub-optimal. No solution currently exists to @@ -212,7 +212,7 @@ plus a lot of code, are borrowed from CFQ. and to reduce their latency. The most important action taken to achieve this goal is to give to the queues associated with these applications more than their fair share of the device - throughput. For brevity, we call just "weight-raising" the whole + throughput. For brevity, we call it just "weight-raising" the whole sets of actions taken by BFQ to privilege these queues. In particular, BFQ provides a milder form of weight-raising for interactive applications, and a stronger form for soft real-time @@ -231,7 +231,7 @@ plus a lot of code, are borrowed from CFQ. responsive in detecting interleaved I/O (cooperating processes), that it enables BFQ to achieve a high throughput, by queue merging, even for queues for which CFQ needs a different - mechanism, preemption, to get a high throughput. As such EQM is a + mechanism, preemption, to get a high throughput. As such, EQM is a unified mechanism to achieve a high throughput with interleaved I/O. @@ -254,7 +254,7 @@ plus a lot of code, are borrowed from CFQ. - First, with any proportional-share scheduler, the maximum deviation with respect to an ideal service is proportional to the maximum budget (slice) assigned to queues. As a consequence, - BFQ can keep this deviation tight not only because of the + BFQ can keep this deviation tight, not only because of the accurate service of B-WF2Q+, but also because BFQ *does not* need to assign a larger budget to a queue to let the queue receive a higher fraction of the device throughput. @@ -327,7 +327,7 @@ applications. Unset this tunable if you need/want to control weights. slice_idle ---------- -This parameter specifies how long BFQ should idle for next I/O +This parameter specifies how long BFQ should idle for the next I/O request, when certain sync BFQ queues become empty. By default slice_idle is a non-zero value. Idling has a double purpose: boosting throughput and making sure that the desired throughput distribution is @@ -365,7 +365,7 @@ terms of I/O-request dispatches. To guarantee that the actual service order then corresponds to the dispatch order, the strict_guarantees tunable must be set too. -There is an important flipside for idling: apart from the above cases +There is an important flip side to idling: apart from the above cases where it is beneficial also for throughput, idling can severely impact throughput. One important case is random workload. Because of this issue, BFQ tends to avoid idling as much as possible, when it is not @@ -430,13 +430,13 @@ fifo_expire_async ----------------- This parameter is used to set the timeout of asynchronous requests. Default -value of this is 248ms. +value of this is 250ms. fifo_expire_sync ---------------- This parameter is used to set the timeout of synchronous requests. Default -value of this is 124ms. In case to favor synchronous requests over asynchronous +value of this is 125ms. In case to favor synchronous requests over asynchronous one, this value should be decreased relative to fifo_expire_async. low_latency @@ -475,7 +475,7 @@ max_budget Maximum amount of service, measured in sectors, that can be provided to a BFQ queue once it is set in service (of course within the limits -of the above timeout). According to what said in the description of +of the above timeout). According to what was said in the description of the algorithm, larger values increase the throughput in proportion to the percentage of sequential I/O requests issued. The price of larger values is that they coarsen the granularity of short-term bandwidth @@ -492,13 +492,6 @@ set max_budget to higher values than those to which BFQ would have set it with auto-tuning. An alternative way to achieve this goal is to just increase the value of timeout_sync, leaving max_budget equal to 0. -weights -------- - -Read-only parameter, used to show the weights of the currently active -BFQ queues. - - 4. Group scheduling with BFQ ============================ @@ -560,20 +553,36 @@ throughput sustainable with bfq, because updating the blkio.bfq.* stats is rather costly, especially for some of the stats enabled by CONFIG_BFQ_CGROUP_DEBUG. -Parameters to set ------------------ +Parameters +---------- -For each group, there is only the following parameter to set. +For each group, the following parameters can be set: -weight (namely blkio.bfq.weight or io.bfq-weight): the weight of the -group inside its parent. Available values: 1..10000 (default 100). The -linear mapping between ioprio and weights, described at the beginning -of the tunable section, is still valid, but all weights higher than -IOPRIO_BE_NR*10 are mapped to ioprio 0. + weight + This specifies the default weight for the cgroup inside its parent. + Available values: 1..1000 (default: 100). -Recall that, if low-latency is set, then BFQ automatically raises the -weight of the queues associated with interactive and soft real-time -applications. Unset this tunable if you need/want to control weights. + For cgroup v1, it is set by writing the value to `blkio.bfq.weight`. + + For cgroup v2, it is set by writing the value to `io.bfq.weight`. + (with an optional prefix of `default` and a space). + + The linear mapping between ioprio and weights, described at the beginning + of the tunable section, is still valid, but all weights higher than + IOPRIO_BE_NR*10 are mapped to ioprio 0. + + Recall that, if low-latency is set, then BFQ automatically raises the + weight of the queues associated with interactive and soft real-time + applications. Unset this tunable if you need/want to control weights. + + weight_device + This specifies a per-device weight for the cgroup. The syntax is + `minor:major weight`. A weight of `0` may be used to reset to the default + weight. + + For cgroup v1, it is set by writing the value to `blkio.bfq.weight_device`. + + For cgroup v2, the file name is `io.bfq.weight`. [1] |