block, bfq: introduce the BFQ-v0 I/O scheduler as an extra scheduler

[linux-2.6-block.git] / Documentation / block / bfq-iosched.txt

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