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+BFQ (Budget Fair Queueing)
+==========================
+
+BFQ is a proportional-share I/O scheduler, with some extra
+low-latency capabilities. In addition to cgroups support (blkio or io
+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
+ groups (switching back to time distribution when needed to keep
+ throughput high).
+
+On average CPUs, the current version of BFQ can handle devices
+performing at most ~30K IOPS; at most ~50 KIOPS on faster CPUs. As a
+reference, 30-50 KIOPS correspond to very high bandwidths with
+sequential I/O (e.g., 8-12 GB/s if I/O requests are 256 KB large), and
+to 120-200 MB/s with 4KB random I/O. BFQ has not yet been tested on
+multi-queue devices.
+
+The table of contents follow. Impatients can just jump to Section 3.
+
+CONTENTS
+
+1. When may BFQ be useful?
+ 1-1 Personal systems
+ 1-2 Server systems
+2. How does BFQ work?
+3. What are BFQ's tunable?
+4. BFQ group scheduling
+ 4-1 Service guarantees provided
+ 4-2 Interface
+
+1. When may BFQ be useful?
+==========================
+
+BFQ provides the following benefits on personal and server systems.
+
+1-1 Personal systems
+--------------------
+
+Low latency for interactive applications
+
+Regardless of the actual background workload, BFQ guarantees that, for
+interactive tasks, the storage device is virtually as responsive as if
+it was idle. For example, even if one or more of the following
+background workloads are being executed:
+- one or more large files are being read, written or copied,
+- a tree of source files is being compiled,
+- one or more virtual machines are performing I/O,
+- a software update is in progress,
+- indexing daemons are scanning filesystems and updating their
+ databases,
+starting an application or loading a file from within an application
+takes about the same time as if the storage device was idle. As a
+comparison, with CFQ, NOOP or DEADLINE, and in the same conditions,
+applications experience high latencies, or even become unresponsive
+until the background workload terminates (also on SSDs).
+
+Low latency for soft real-time applications
+
+Also soft real-time applications, such as audio and video
+players/streamers, enjoy a low latency and a low drop rate, regardless
+of the background I/O workload. As a consequence, these applications
+do not suffer from almost any glitch due to the background workload.
+
+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,
+NOOP or DEADLINE.
+
+High throughput
+
+On hard disks, BFQ achieves up to 30% higher throughput than CFQ, and
+up to 150% higher throughput than DEADLINE and NOOP, with all the
+sequential workloads considered in our tests. With random workloads,
+and with all the workloads on flash-based devices, BFQ achieves,
+instead, about the same throughput as the other schedulers.
+
+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
+workload and regardless of the device parameters. From these bandwidth
+guarantees, it is possible to compute 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.
+
+1-2 Server systems
+------------------
+
+Most benefits for server systems follow from the same service
+properties as above. In particular, regardless of whether additional,
+possibly heavy workloads are being served, BFQ guarantees:
+
+. audio and video-streaming with zero or very low jitter and drop
+ rate;
+
+. fast retrieval of WEB pages and embedded objects;
+
+. real-time recording of data in live-dumping applications (e.g.,
+ packet logging);
+
+. responsiveness in local and remote access to a server.
+
+
+2. How does BFQ work?
+=====================
+
+BFQ is a proportional-share I/O scheduler, whose general structure,
+plus a lot of code, are borrowed from CFQ.
+
+- Each process doing I/O on a device is associated with a weight and a
+ (bfq_)queue.
+
+- BFQ grants exclusive access to the device, for a while, to one queue
+ (process) at a time, and implements this service model by
+ associating every queue with a budget, measured in number of
+ sectors.
+
+ - After a queue is granted access to the device, the budget of the
+ queue is decremented, on each request dispatch, by the size of the
+ request.
+
+ - The in-service queue is expired, i.e., its service is suspended,
+ only if one of the following events occurs: 1) the queue finishes
+ its budget, 2) the queue empties, 3) a "budget timeout" fires.
+
+ - The budget timeout prevents processes doing random I/O from
+ holding the device for too long and dramatically reducing
+ throughput.
+
+ - Actually, as in CFQ, a queue associated with a process issuing
+ sync requests may not be expired immediately when it empties. In
+ contrast, BFQ may idle the device for a short time interval,
+ giving the process the chance to go on being served if it issues
+ a new request in time. Device idling typically boosts the
+ throughput on rotational devices, if processes do synchronous
+ and sequential I/O. In addition, under BFQ, device idling is
+ also instrumental in guaranteeing the desired throughput
+ fraction to processes issuing sync requests (see the description
+ of the slice_idle tunable in this document, or [1, 2], for more
+ details).
+
+ - With respect to idling for service guarantees, if several
+ processes are competing for the device at the same time, but
+ all processes (and groups, after the following commit) have
+ the same weight, then BFQ guarantees the expected throughput
+ distribution without ever idling the device. Throughput is
+ thus as high as possible in this common scenario.
+
+ - If low-latency mode is enabled (default configuration), BFQ
+ executes some special heuristics to detect interactive and soft
+ real-time applications (e.g., video or audio players/streamers),
+ 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
+ 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
+ applications.
+
+ - BFQ automatically deactivates idling for queues born in a burst of
+ queue creations. In fact, these queues are usually associated with
+ the processes of applications and services that benefit mostly
+ from a high throughput. Examples are systemd during boot, or git
+ grep.
+
+ - As CFQ, BFQ merges queues performing interleaved I/O, i.e.,
+ performing random I/O that becomes mostly sequential if
+ merged. Differently from CFQ, BFQ achieves this goal with a more
+ reactive mechanism, called Early Queue Merge (EQM). EQM is so
+ 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
+ unified mechanism to achieve a high throughput with interleaved
+ I/O.
+
+ - Queues are scheduled according to a variant of WF2Q+, named
+ B-WF2Q+, and implemented using an augmented rb-tree to preserve an
+ O(log N) overall complexity. See [2] for more details. B-WF2Q+ is
+ also ready for hierarchical scheduling. However, for a cleaner
+ logical breakdown, the code that enables and completes
+ hierarchical support is provided in the next commit, which focuses
+ exactly on this feature.
+
+ - B-WF2Q+ guarantees a tight deviation with respect to an ideal,
+ perfectly fair, and smooth service. In particular, B-WF2Q+
+ guarantees that each queue receives a fraction of the device
+ throughput proportional to its weight, even if the throughput
+ fluctuates, and regardless of: the device parameters, the current
+ workload and the budgets assigned to the queue.
+
+ - The last, budget-independence, property (although probably
+ counterintuitive in the first place) is definitely beneficial, for
+ the following reasons:
+
+ - 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
+ 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.
+
+ - Second, BFQ is free to choose, for every process (queue), the
+ budget that best fits the needs of the process, or best
+ leverages the I/O pattern of the process. In particular, BFQ
+ updates queue budgets with a simple feedback-loop algorithm that
+ allows a high throughput to be achieved, while still providing
+ tight latency guarantees to time-sensitive applications. When
+ the in-service queue expires, this algorithm computes the next
+ budget of the queue so as to:
+
+ - Let large budgets be eventually assigned to the queues
+ associated with I/O-bound applications performing sequential
+ I/O: in fact, the longer these applications are served once
+ got access to the device, the higher the throughput is.
+
+ - Let small budgets be eventually assigned to the queues
+ associated with time-sensitive applications (which typically
+ perform sporadic and short I/O), because, the smaller the
+ budget assigned to a queue waiting for service is, the sooner
+ B-WF2Q+ will serve that queue (Subsec 3.3 in [2]).
+
+- If several processes are competing for the device at the same time,
+ but all processes and groups have the same weight, then BFQ
+ guarantees the expected throughput distribution without ever idling
+ the device. It uses preemption instead. Throughput is then much
+ higher in this common scenario.
+
+- ioprio classes are served in strict priority order, i.e.,
+ lower-priority queues are not served as long as there are
+ higher-priority queues. Among queues in the same class, the
+ bandwidth is distributed in proportion to the weight of each
+ queue. A very thin extra bandwidth is however guaranteed to
+ the Idle class, to prevent it from starving.
+
+
+3. What are BFQ's tunable?
+==========================
+
+The tunables back_seek-max, back_seek_penalty, fifo_expire_async and
+fifo_expire_sync below are the same as in CFQ. Their description is
+just copied from that for CFQ. Some considerations in the description
+of slice_idle are copied from CFQ too.
+
+per-process ioprio and weight
+-----------------------------
+
+Unless the cgroups interface is used, weights can be assigned to
+processes only indirectly, through I/O priorities, and according to
+the relation: weight = (IOPRIO_BE_NR - ioprio) * 10.
+
+slice_idle
+----------
+
+This parameter specifies how long BFQ should idle for 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
+respected (see the description of how BFQ works, and, if needed, the
+papers referred there).
+
+As for throughput, idling can be very helpful on highly seeky media
+like single spindle SATA/SAS disks where we can cut down on overall
+number of seeks and see improved throughput.
+
+Setting slice_idle to 0 will remove all the idling on queues and one
+should see an overall improved throughput on faster storage devices
+like multiple SATA/SAS disks in hardware RAID configuration.
+
+So depending on storage and workload, it might be useful to set
+slice_idle=0. In general for SATA/SAS disks and software RAID of
+SATA/SAS disks keeping slice_idle enabled should be useful. For any
+configurations where there are multiple spindles behind single LUN
+(Host based hardware RAID controller or for storage arrays), setting
+slice_idle=0 might end up in better throughput and acceptable
+latencies.
+
+Idling is however necessary to have service guarantees enforced in
+case of differentiated weights or differentiated I/O-request lengths.
+To see why, suppose that a given BFQ queue A must get several I/O
+requests served for each request served for another queue B. Idling
+ensures that, if A makes a new I/O request slightly after becoming
+empty, then no request of B is dispatched in the middle, and thus A
+does not lose the possibility to get more than one request dispatched
+before the next request of B is dispatched. Note that idling
+guarantees the desired differentiated treatment of queues only in
+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
+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
+beneficial also for throughput. As a consequence of this behavior, and
+of further issues described for the strict_guarantees tunable,
+short-term service guarantees may be occasionally violated. And, in
+some cases, these guarantees may be more important than guaranteeing
+maximum throughput. For example, in video playing/streaming, a very
+low drop rate may be more important than maximum throughput. In these
+cases, consider setting the strict_guarantees parameter.
+
+strict_guarantees
+-----------------
+
+If this parameter is set (default: unset), then BFQ
+
+- always performs idling when the in-service queue becomes empty;
+
+- forces the device to serve one I/O request at a time, by dispatching a
+ new request only if there is no outstanding request.
+
+In the presence of differentiated weights or I/O-request sizes, both
+the above conditions are needed to guarantee that every BFQ queue
+receives its allotted share of the bandwidth. The first condition is
+needed for the reasons explained in the description of the slice_idle
+tunable. The second condition is needed because all modern storage
+devices reorder internally-queued requests, which may trivially break
+the service guarantees enforced by the I/O scheduler.
+
+Setting strict_guarantees may evidently affect throughput.
+
+back_seek_max
+-------------
+
+This specifies, given in Kbytes, the maximum "distance" for backward seeking.
+The distance is the amount of space from the current head location to the
+sectors that are backward in terms of distance.
+
+This parameter allows the scheduler to anticipate requests in the "backward"
+direction and consider them as being the "next" if they are within this
+distance from the current head location.
+
+back_seek_penalty
+-----------------
+
+This parameter is used to compute the cost of backward seeking. If the
+backward distance of request is just 1/back_seek_penalty from a "front"
+request, then the seeking cost of two requests is considered equivalent.
+
+So scheduler will not bias toward one or the other request (otherwise scheduler
+will bias toward front request). Default value of back_seek_penalty is 2.
+
+fifo_expire_async
+-----------------
+
+This parameter is used to set the timeout of asynchronous requests. Default
+value of this is 248ms.
+
+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
+one, this value should be decreased relative to fifo_expire_async.
+
+low_latency
+-----------
+
+This parameter is used to enable/disable BFQ's low latency mode. By
+default, low latency mode is enabled. If enabled, interactive and soft
+real-time applications are privileged and experience a lower latency,
+as explained in more detail in the description of how BFQ works.
+
+timeout_sync
+------------
+
+Maximum amount of device time that can be given to a task (queue) once
+it has been selected for service. On devices with costly seeks,
+increasing this time usually increases maximum throughput. On the
+opposite end, increasing this time coarsens the granularity of the
+short-term bandwidth and latency guarantees, especially if the
+following parameter is set to zero.
+
+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
+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
+and latency guarantees.
+
+The default value is 0, which enables auto-tuning: BFQ sets max_budget
+to the maximum number of sectors that can be served during
+timeout_sync, according to the estimated peak rate.
+
+weights
+-------
+
+Read-only parameter, used to show the weights of the currently active
+BFQ queues.
+
+
+wr_ tunables
+------------
+
+BFQ exports a few parameters to control/tune the behavior of
+low-latency heuristics.
+
+wr_coeff
+
+Factor by which the weight of a weight-raised queue is multiplied. If
+the queue is deemed soft real-time, then the weight is further
+multiplied by an additional, constant factor.
+
+wr_max_time
+
+Maximum duration of a weight-raising period for an interactive task
+(ms). If set to zero (default value), then this value is computed
+automatically, as a function of the peak rate of the device. In any
+case, when the value of this parameter is read, it always reports the
+current duration, regardless of whether it has been set manually or
+computed automatically.
+
+wr_max_softrt_rate
+
+Maximum service rate below which a queue is deemed to be associated
+with a soft real-time application, and is then weight-raised
+accordingly (sectors/sec).
+
+wr_min_idle_time
+
+Minimum idle period after which interactive weight-raising may be
+reactivated for a queue (in ms).
+
+wr_rt_max_time
+
+Maximum weight-raising duration for soft real-time queues (in ms). The
+start time from which this duration is considered is automatically
+moved forward if the queue is detected to be still soft real-time
+before the current soft real-time weight-raising period finishes.
+
+wr_min_inter_arr_async
+
+Minimum period between I/O request arrivals after which weight-raising
+may be reactivated for an already busy async queue (in ms).
+
+
+4. Group scheduling with BFQ
+============================
+
+BFQ supports both cgroup-v1 and cgroup-v2 io controllers, namely blkio
+and io. In particular, BFQ supports weight-based proportional
+share.
+
+4-1 Service guarantees provided
+-------------------------------
+
+With BFQ, proportional share means true proportional share of the
+device bandwidth, according to group weights. For example, a group
+with weight 200 gets twice the bandwidth, and not just twice the time,
+of a group with weight 100.
+
+BFQ supports hierarchies (group trees) of any depth. Bandwidth is
+distributed among groups and processes in the expected way: for each
+group, the children of the group share the whole bandwidth of the
+group in proportion to their weights. In particular, this implies
+that, for each leaf group, every process of the group receives the
+same share of the whole group bandwidth, unless the ioprio of the
+process is modified.
+
+The resource-sharing guarantee for a group may partially or totally
+switch from bandwidth to time, if providing bandwidth guarantees to
+the group lowers the throughput too much. This switch occurs on a
+per-process basis: if a process of a leaf group causes throughput loss
+if served in such a way to receive its share of the bandwidth, then
+BFQ switches back to just time-based proportional share for that
+process.
+
+4-2 Interface
+-------------
+
+To get proportional sharing of bandwidth with BFQ for a given device,
+BFQ must of course be the active scheduler for that device.
+
+Within each group directory, the names of the files associated with
+BFQ-specific cgroup parameters and stats begin with the "bfq."
+prefix. So, with cgroups-v1 or cgroups-v2, the full prefix for
+BFQ-specific files is "blkio.bfq." or "io.bfq." For example, the group
+parameter to set the weight of a group with BFQ is blkio.bfq.weight
+or io.bfq.weight.
+
+Parameters to set
+-----------------
+
+For each group, there is only the following parameter to 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.
+
+
+[1] P. Valente, A. Avanzini, "Evolution of the BFQ Storage I/O
+ Scheduler", Proceedings of the First Workshop on Mobile System
+ Technologies (MST-2015), May 2015.
+ http://algogroup.unimore.it/people/paolo/disk_sched/mst-2015.pdf
+
+[2] 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