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  1. 20 Apr, 2017 16 commits
  3. 19 Apr, 2017 24 commits
    • Bart Van Assche's avatar
      block: Optimize ioprio_best() · 9a87182c
      Bart Van Assche authored 
      Since ioprio_best() translates IOPRIO_CLASS_NONE into IOPRIO_CLASS_BE
and since lower numerical priority values represent a higher priority
a simple numerical comparison is sufficient.
Signed-off-by: default avatarBart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: default avatarAdam Manzanares <adam.manzanares@wdc.com>
Tested-by: default avatarAdam Manzanares <adam.manzanares@wdc.com>
Reviewed-by: default avatarChristoph Hellwig <hch@lst.de>
Cc: Matias Bjørling <m@bjorling.me>
Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      9a87182c
    • Bart Van Assche's avatar
      block: Inline blk_rq_set_prio() · 0be0dee6
      Bart Van Assche authored 
      Since only a single caller remains, inline blk_rq_set_prio(). Initialize
req->ioprio even if no I/O priority has been set in the bio nor in the
I/O context.
Signed-off-by: default avatarBart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: default avatarAdam Manzanares <adam.manzanares@wdc.com>
Tested-by: default avatarAdam Manzanares <adam.manzanares@wdc.com>
Reviewed-by: default avatarChristoph Hellwig <hch@lst.de>
Cc: Matias Bjørling <m@bjorling.me>
Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      0be0dee6
    • Bart Van Assche's avatar
      lightnvm: Use blk_init_request_from_bio() instead of open-coding it · 9460e280
      Bart Van Assche authored 
      This patch changes the behavior of the lightnvm driver as follows:
* REQ_FAILFAST_MASK is set for read-ahead requests.
* If no I/O priority has been set in the bio, the I/O priority is
  copied from the I/O context.
* The rq_disk member is initialized if bio->bi_bdev != NULL.
* The bio sector offset is copied into req->__sector instead of
  retaining the value -1 set by blk_mq_alloc_request().
* req->errors is initialized to zero.
Signed-off-by: default avatarBart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: default avatarChristoph Hellwig <hch@lst.de>
Cc: Matias Bjørling <m@bjorling.me>
Cc: Adam Manzanares <adam.manzanares@wdc.com>
Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      9460e280
    • Bart Van Assche's avatar
      null_blk: Use blk_init_request_from_bio() instead of open-coding it · 2644a3cc
      Bart Van Assche authored 
      This patch changes the behavior of the null_blk driver for the
LightNVM mode as follows:
* REQ_FAILFAST_MASK is set for read-ahead requests.
* If no I/O priority has been set in the bio, the I/O priority is
  copied from the I/O context.
* The rq_disk member is initialized if bio->bi_bdev != NULL.
* req->errors is initialized to zero.
Signed-off-by: default avatarBart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: default avatarChristoph Hellwig <hch@lst.de>
Cc: Matias Bjørling <m@bjorling.me>
Cc: Adam Manzanares <adam.manzanares@wdc.com>
Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      2644a3cc
    • Bart Van Assche's avatar
      block: Export blk_init_request_from_bio() · da8d7f07
      Bart Van Assche authored 
      Export this function such that it becomes available to block
drivers.
Signed-off-by: default avatarBart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: default avatarChristoph Hellwig <hch@lst.de>
Cc: Matias Bjørling <m@bjorling.me>
Cc: Adam Manzanares <adam.manzanares@wdc.com>
Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      da8d7f07
    • Arnd Bergmann's avatar
      lightnvm: assume 64-bit lba numbers · ef697902
      Arnd Bergmann authored 
      The driver uses both u64 and sector_t to refer to offsets, and assigns between the
two. This causes one harmless warning when sector_t is 32-bit:

drivers/lightnvm/pblk-rb.c: In function 'pblk_rb_write_entry_gc':
include/linux/lightnvm.h:215:20: error: large integer implicitly truncated to unsigned type [-Werror=overflow]
drivers/lightnvm/pblk-rb.c:324:22: note: in expansion of macro 'ADDR_EMPTY'

As the driver is already doing this inconsistently, changing the type
won't make it worse and is an easy way to avoid the warning.

Fixes: a4bd217b ("lightnvm: physical block device (pblk) target")
Signed-off-by: default avatarArnd Bergmann <arnd@arndb.de>
Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      ef697902
    • Christoph Hellwig's avatar
      block: make __blk_end_bidi_request private · d0fac025
      Christoph Hellwig authored 
      blk_insert_flush should be using __blk_end_request to start with.
Signed-off-by: default avatarChristoph Hellwig <hch@lst.de>
Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      d0fac025
    • Christoph Hellwig's avatar
      block: remove blk_end_request_cur · fa1a15c0
      Christoph Hellwig authored 
      This function is not used anywhere in the kernel.
Signed-off-by: default avatarChristoph Hellwig <hch@lst.de>
Reviewed-by: default avatarJohannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      fa1a15c0
    • Christoph Hellwig's avatar
      block: remove blk_end_request_err and __blk_end_request_err · 314fe91b
      Christoph Hellwig authored 
      Both functions are entirely unused.
Signed-off-by: default avatarChristoph Hellwig <hch@lst.de>
Reviewed-by: default avatarJohannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      314fe91b
    • Christoph Hellwig's avatar
      block: remove the osdblk driver · 10081552
      Christoph Hellwig authored 
      This was just a proof of concept user for the SCSI OSD library, and
never had any real users.
Signed-off-by: default avatarChristoph Hellwig <hch@lst.de>
Acked-by: default avatarBoaz Harrosh <ooo@electrozaur.com>
Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      10081552
    • Jan Kara's avatar
      block: Make writeback throttling defaults consistent for SQ devices · 8330cdb0
      Jan Kara authored 
      When CFQ is used as an elevator, it disables writeback throttling
because they don't play well together. Later when a different elevator
is chosen for the device, writeback throttling doesn't get enabled
again as it should. Make sure CFQ enables writeback throttling (if it
should be enabled by default) when we switch from it to another IO
scheduler.
Signed-off-by: default avatarJan Kara <jack@suse.cz>
Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      8330cdb0
    • Paolo Valente's avatar
      block, bfq: split bfq-iosched.c into multiple source files · ea25da48
      Paolo Valente authored 
      The BFQ I/O scheduler features an optimal fair-queuing
(proportional-share) scheduling algorithm, enriched with several
mechanisms to boost throughput and reduce latency for interactive and
real-time applications. This makes BFQ a large and complex piece of
code. This commit addresses this issue by splitting BFQ into three
main, independent components, and by moving each component into a
separate source file:
1. Main algorithm: handles the interaction with the kernel, and
decides which requests to dispatch; it uses the following two further
components to achieve its goals.
2. Scheduling engine (Hierarchical B-WF2Q+ scheduling algorithm):
computes the schedule, using weights and budgets provided by the above
component.
3. cgroups support: handles group operations (creation, destruction,
move, ...).
Signed-off-by: default avatarPaolo Valente <paolo.valente@linaro.org>
Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      ea25da48
    • Paolo Valente's avatar
      block, bfq: remove all get and put of I/O contexts · 6fa3e8d3
      Paolo Valente authored 
      When a bfq queue is set in service and when it is merged, a reference
to the I/O context associated with the queue is taken. This reference
is then released when the queue is deselected from service or
split. More precisely, the release of the reference is postponed to
when the scheduler lock is released, to avoid nesting between the
scheduler and the I/O-context lock. In fact, such nesting would lead
to deadlocks, because of other code paths that take the same locks in
the opposite order. This postponing of I/O-context releases does
complicate code.

This commit addresses these issue by modifying involved operations in
such a way to not need to get the above I/O-context references any
more. Then it also removes any get and release of these references.
Signed-off-by: default avatarPaolo Valente <paolo.valente@linaro.org>
Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      6fa3e8d3
    • Arianna Avanzini's avatar
      block, bfq: handle bursts of queue activations · e1b2324d
      Arianna Avanzini authored 
      Many popular I/O-intensive services or applications spawn or
reactivate many parallel threads/processes during short time
intervals. Examples are systemd during boot or git grep.  These
services or applications benefit mostly from a high throughput: the
quicker the I/O generated by their processes is cumulatively served,
the sooner the target job of these services or applications gets
completed. As a consequence, it is almost always counterproductive to
weight-raise any of the queues associated to the processes of these
services or applications: in most cases it would just lower the
throughput, mainly because weight-raising also implies device idling.

To address this issue, an I/O scheduler needs, first, to detect which
queues are associated with these services or applications. In this
respect, we have that, from the I/O-scheduler standpoint, these
services or applications cause bursts of activations, i.e.,
activations of different queues occurring shortly after each
other. However, a shorter burst of activations may be caused also by
the start of an application that does not consist in a lot of parallel
I/O-bound threads (see the comments on the function bfq_handle_burst
for details).

In view of these facts, this commit introduces:
1) an heuristic to detect (only) bursts of queue activations caused by
   services or applications consisting in many parallel I/O-bound
   threads;
2) the prevention of device idling and weight-raising for the queues
   belonging to these bursts.
Signed-off-by: default avatarArianna Avanzini <avanzini.arianna@gmail.com>
Signed-off-by: default avatarPaolo Valente <paolo.valente@linaro.org>
Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      e1b2324d
    • Paolo Valente's avatar
      block, bfq: boost the throughput with random I/O on NCQ-capable HDDs · e01eff01
      Paolo Valente authored 
      This patch is basically the counterpart, for NCQ-capable rotational
devices, of the previous patch. Exactly as the previous patch does on
flash-based devices and for any workload, this patch disables device
idling on rotational devices, but only for random I/O. In fact, only
with these queues disabling idling boosts the throughput on
NCQ-capable rotational devices. To not break service guarantees,
idling is disabled for NCQ-enabled rotational devices only when the
same symmetry conditions considered in the previous patches hold.
Signed-off-by: default avatarPaolo Valente <paolo.valente@linaro.org>
Signed-off-by: default avatarArianna Avanzini <avanzini.arianna@gmail.com>
Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      e01eff01
    • Paolo Valente's avatar
      block, bfq: boost the throughput on NCQ-capable flash-based devices · bf2b79e7
      Paolo Valente authored 
      This patch boosts the throughput on NCQ-capable flash-based devices,
while still preserving latency guarantees for interactive and soft
real-time applications. The throughput is boosted by just not idling
the device when the in-service queue remains empty, even if the queue
is sync and has a non-null idle window. This helps to keep the drive's
internal queue full, which is necessary to achieve maximum
performance. This solution to boost the throughput is a port of
commits a68bbdd and f7d7b7a7 for CFQ.

As already highlighted in a previous patch, allowing the device to
prefetch and internally reorder requests trivially causes loss of
control on the request service order, and hence on service guarantees.
Fortunately, as discussed in detail in the comments on the function
bfq_bfqq_may_idle(), if every process has to receive the same
fraction of the throughput, then the service order enforced by the
internal scheduler of a flash-based device is relatively close to that
enforced by BFQ. In particular, it is close enough to let service
guarantees be substantially preserved.

Things change in an asymmetric scenario, i.e., if not every process
has to receive the same fraction of the throughput. In this case, to
guarantee the desired throughput distribution, the device must be
prevented from prefetching requests. This is exactly what this patch
does in asymmetric scenarios.
Signed-off-by: default avatarPaolo Valente <paolo.valente@linaro.org>
Signed-off-by: default avatarArianna Avanzini <avanzini.arianna@gmail.com>
Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      bf2b79e7
    • Arianna Avanzini's avatar
      block, bfq: reduce idling only in symmetric scenarios · 1de0c4cd
      Arianna Avanzini authored 
      A seeky queue (i..e, a queue containing random requests) is assigned a
very small device-idling slice, for throughput issues. Unfortunately,
given the process associated with a seeky queue, this behavior causes
the following problem: if the process, say P, performs sync I/O and
has a higher weight than some other processes doing I/O and associated
with non-seeky queues, then BFQ may fail to guarantee to P its
reserved share of the throughput. The reason is that idling is key
for providing service guarantees to processes doing sync I/O [1].

This commit addresses this issue by allowing the device-idling slice
to be reduced for a seeky queue only if the scenario happens to be
symmetric, i.e., if all the queues are to receive the same share of
the throughput.

[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.pdfSigned-off-by: default avatarArianna Avanzini <avanzini.arianna@gmail.com>
Signed-off-by: default avatarRiccardo Pizzetti <riccardo.pizzetti@gmail.com>
Signed-off-by: default avatarSamuele Zecchini <samuele.zecchini92@gmail.com>
Signed-off-by: default avatarPaolo Valente <paolo.valente@linaro.org>
Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      1de0c4cd
    • Arianna Avanzini's avatar
      block, bfq: add Early Queue Merge (EQM) · 36eca894
      Arianna Avanzini authored 
      A set of processes may happen to perform interleaved reads, i.e.,
read requests whose union would give rise to a sequential read pattern.
There are two typical cases: first, processes reading fixed-size chunks
of data at a fixed distance from each other; second, processes reading
variable-size chunks at variable distances. The latter case occurs for
example with QEMU, which splits the I/O generated by a guest into
multiple chunks, and lets these chunks be served by a pool of I/O
threads, iteratively assigning the next chunk of I/O to the first
available thread. CFQ denotes as 'cooperating' a set of processes that
are doing interleaved I/O, and when it detects cooperating processes,
it merges their queues to obtain a sequential I/O pattern from the union
of their I/O requests, and hence boost the throughput.

Unfortunately, in the following frequent case, the mechanism
implemented in CFQ for detecting cooperating processes and merging
their queues is not responsive enough to handle also the fluctuating
I/O pattern of the second type of processes. Suppose that one process
of the second type issues a request close to the next request to serve
of another process of the same type. At that time the two processes
would be considered as cooperating. But, if the request issued by the
first process is to be merged with some other already-queued request,
then, from the moment at which this request arrives, to the moment
when CFQ controls whether the two processes are cooperating, the two
processes are likely to be already doing I/O in distant zones of the
disk surface or device memory.

CFQ uses however preemption to get a sequential read pattern out of
the read requests performed by the second type of processes too.  As a
consequence, CFQ uses two different mechanisms to achieve the same
goal: boosting the throughput with interleaved I/O.

This patch introduces Early Queue Merge (EQM), a unified mechanism to
get a sequential read pattern with both types of processes. The main
idea is to immediately check whether a newly-arrived request lets some
pair of processes become cooperating, both in the case of actual
request insertion and, to be responsive with the second type of
processes, in the case of request merge. Both types of processes are
then handled by just merging their queues.
Signed-off-by: default avatarArianna Avanzini <avanzini.arianna@gmail.com>
Signed-off-by: default avatarMauro Andreolini <mauro.andreolini@unimore.it>
Signed-off-by: default avatarPaolo Valente <paolo.valente@linaro.org>
Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      36eca894
    • Paolo Valente's avatar
      block, bfq: reduce latency during request-pool saturation · cfd69712
      Paolo Valente authored 
      This patch introduces an heuristic that reduces latency when the
I/O-request pool is saturated. This goal is achieved by disabling
device idling, for non-weight-raised queues, when there are weight-
raised queues with pending or in-flight requests. In fact, as
explained in more detail in the comment on the function
bfq_bfqq_may_idle(), this reduces the rate at which processes
associated with non-weight-raised queues grab requests from the pool,
thereby increasing the probability that processes associated with
weight-raised queues get a request immediately (or at least soon) when
they need one. Along the same line, if there are weight-raised queues,
then this patch halves the service rate of async (write) requests for
non-weight-raised queues.
Signed-off-by: default avatarPaolo Valente <paolo.valente@linaro.org>
Signed-off-by: default avatarArianna Avanzini <avanzini.arianna@gmail.com>
Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      cfd69712
    • Paolo Valente's avatar
      block, bfq: preserve a low latency also with NCQ-capable drives · bcd56426
      Paolo Valente authored 
      I/O schedulers typically allow NCQ-capable drives to prefetch I/O
requests, as NCQ boosts the throughput exactly by prefetching and
internally reordering requests.

Unfortunately, as discussed in detail and shown experimentally in [1],
this may cause fairness and latency guarantees to be violated. The
main problem is that the internal scheduler of an NCQ-capable drive
may postpone the service of some unlucky (prefetched) requests as long
as it deems serving other requests more appropriate to boost the
throughput.

This patch addresses this issue by not disabling device idling for
weight-raised queues, even if the device supports NCQ. This allows BFQ
to start serving a new queue, and therefore allows the drive to
prefetch new requests, only after the idling timeout expires. At that
time, all the outstanding requests of the expired queue have been most
certainly served.

[1] 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
Signed-off-by: default avatarPaolo Valente <paolo.valente@linaro.org>
Signed-off-by: default avatarArianna Avanzini <avanzini.arianna@gmail.com>
Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      bcd56426
    • Paolo Valente's avatar
      block, bfq: reduce I/O latency for soft real-time applications · 77b7dcea
      Paolo Valente authored 
      To guarantee a low latency also to the I/O requests issued by soft
real-time applications, this patch introduces a further heuristic,
which weight-raises (in the sense explained in the previous patch)
also the queues associated to applications deemed as soft real-time.

To be deemed as soft real-time, an application must meet two
requirements.  First, the application must not require an average
bandwidth higher than the approximate bandwidth required to playback
or record a compressed high-definition video. Second, the request
pattern of the application must be isochronous, i.e., after issuing a
request or a batch of requests, the application must stop issuing new
requests until all its pending requests have been completed. After
that, the application may issue a new batch, and so on.

As for the second requirement, it is critical to require also that,
after all the pending requests of the application have been completed,
an adequate minimum amount of time elapses before the application
starts issuing new requests. This prevents also greedy (i.e.,
I/O-bound) applications from being incorrectly deemed, occasionally,
as soft real-time. In fact, if *any amount of time* is fine, then even
a greedy application may, paradoxically, meet both the above
requirements, if: (1) the application performs random I/O and/or the
device is slow, and (2) the CPU load is high. The reason is the
following.  First, if condition (1) is true, then, during the service
of the application, the throughput may be low enough to let the
application meet the bandwidth requirement.  Second, if condition (2)
is true as well, then the application may occasionally behave in an
apparently isochronous way, because it may simply stop issuing
requests while the CPUs are busy serving other processes.

To address this issue, the heuristic leverages the simple fact that
greedy applications issue *all* their requests as quickly as they can,
whereas soft real-time applications spend some time processing data
after each batch of requests is completed. In particular, the
heuristic works as follows. First, according to the above isochrony
requirement, the heuristic checks whether an application may be soft
real-time, thereby giving to the application the opportunity to be
deemed as such, only when both the following two conditions happen to
hold: 1) the queue associated with the application has expired and is
empty, 2) there is no outstanding request of the application.

Suppose that both conditions hold at time, say, t_c and that the
application issues its next request at time, say, t_i. At time t_c the
heuristic computes the next time instant, called soft_rt_next_start in
the code, such that, only if t_i >= soft_rt_next_start, then both the
next conditions will hold when the application issues its next
request: 1) the application will meet the above bandwidth requirement,
2) a given minimum time interval, say Delta, will have elapsed from
time t_c (so as to filter out greedy application).

The current value of Delta is a little bit higher than the value that
we have found, experimentally, to be adequate on a real,
general-purpose machine. In particular we had to increase Delta to
make the filter quite precise also in slower, embedded systems, and in
KVM/QEMU virtual machines (details in the comments on the code).

If the application actually issues its next request after time
soft_rt_next_start, then its associated queue will be weight-raised
for a relatively short time interval. If, during this time interval,
the application proves again to meet the bandwidth and isochrony
requirements, then the end of the weight-raising period for the queue
is moved forward, and so on. Note that an application whose associated
queue never happens to be empty when it expires will never have the
opportunity to be deemed as soft real-time.
Signed-off-by: default avatarPaolo Valente <paolo.valente@linaro.org>
Signed-off-by: default avatarArianna Avanzini <avanzini.arianna@gmail.com>
Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      77b7dcea
    • Paolo Valente's avatar
      block, bfq: improve responsiveness · 44e44a1b
      Paolo Valente authored 
      This patch introduces a simple heuristic to load applications quickly,
and to perform the I/O requested by interactive applications just as
quickly. To this purpose, both a newly-created queue and a queue
associated with an interactive application (we explain in a moment how
BFQ decides whether the associated application is interactive),
receive the following two special treatments:

1) The weight of the queue is raised.

2) The queue unconditionally enjoys device idling when it empties; in
fact, if the requests of a queue are sync, then performing device
idling for the queue is a necessary condition to guarantee that the
queue receives a fraction of the throughput proportional to its weight
(see [1] for details).

For brevity, we call just weight-raising the combination of these
two preferential treatments. For a newly-created queue,
weight-raising starts immediately and lasts for a time interval that:
1) depends on the device speed and type (rotational or
non-rotational), and 2) is equal to the time needed to load (start up)
a large-size application on that device, with cold caches and with no
additional workload.

Finally, as for guaranteeing a fast execution to interactive,
I/O-related tasks (such as opening a file), consider that any
interactive application blocks and waits for user input both after
starting up and after executing some task. After a while, the user may
trigger new operations, after which the application stops again, and
so on. Accordingly, the low-latency heuristic weight-raises again a
queue in case it becomes backlogged after being idle for a
sufficiently long (configurable) time. The weight-raising then lasts
for the same time as for a just-created queue.

According to our experiments, the combination of this low-latency
heuristic and of the improvements described in the previous patch
allows BFQ to guarantee a high application responsiveness.

[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.pdfSigned-off-by: default avatarPaolo Valente <paolo.valente@linaro.org>
Signed-off-by: default avatarArianna Avanzini <avanzini.arianna@gmail.com>
Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      44e44a1b
    • Paolo Valente's avatar
      block, bfq: add more fairness with writes and slow processes · c074170e
      Paolo Valente authored 
      This patch deals with two sources of unfairness, which can also cause
high latencies and throughput loss. The first source is related to
write requests. Write requests tend to starve read requests, basically
because, on one side, writes are slower than reads, whereas, on the
other side, storage devices confuse schedulers by deceptively
signaling the completion of write requests immediately after receiving
them. This patch addresses this issue by just throttling writes. In
particular, after a write request is dispatched for a queue, the
budget of the queue is decremented by the number of sectors to write,
multiplied by an (over)charge coefficient. The value of the
coefficient is the result of our tuning with different devices.

The second source of unfairness has to do with slowness detection:
when the in-service queue is expired, BFQ also controls whether the
queue has been "too slow", i.e., has consumed its last-assigned budget
at such a low rate that it would have been impossible to consume all
of this budget within the maximum time slice T_max (Subsec. 3.5 in
[1]). In this case, the queue is always (over)charged the whole
budget, to reduce its utilization of the device. Both this overcharge
and the slowness-detection criterion may cause unfairness.

First, always charging a full budget to a slow queue is too coarse. It
is much more accurate, and this patch lets BFQ do so, to charge an
amount of service 'equivalent' to the amount of time during which the
queue has been in service. As explained in more detail in the comments
on the code, this enables BFQ to provide time fairness among slow
queues.

Secondly, because of ZBR, a queue may be deemed as slow when its
associated process is performing I/O on the slowest zones of a
disk. However, unless the process is truly too slow, not reducing the
disk utilization of the queue is more profitable in terms of disk
throughput than the opposite. A similar problem is caused by logical
block mapping on non-rotational devices. For this reason, this patch
lets a queue be charged time, and not budget, only if the queue has
consumed less than 2/3 of its assigned budget. As an additional,
important benefit, this tolerance allows BFQ to preserve enough
elasticity to still perform bandwidth, and not time, distribution with
little unlucky or quasi-sequential processes.

Finally, for the same reasons as above, this patch makes slowness
detection itself much less harsh: a queue is deemed slow only if it
has consumed its budget at less than half of the peak rate.

[1] 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
Signed-off-by: default avatarPaolo Valente <paolo.valente@linaro.org>
Signed-off-by: default avatarArianna Avanzini <avanzini.arianna@gmail.com>
Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      c074170e
    • Paolo Valente's avatar
      block, bfq: modify the peak-rate estimator · ab0e43e9
      Paolo Valente authored 
      Unless the maximum budget B_max that BFQ can assign to a queue is set
explicitly by the user, BFQ automatically updates B_max. In
particular, BFQ dynamically sets B_max to the number of sectors that
can be read, at the current estimated peak rate, during the maximum
time, T_max, allowed before a budget timeout occurs. In formulas, if
we denote as R_est the estimated peak rate, then B_max = T_max ∗
R_est. Hence, the higher R_est is with respect to the actual device
peak rate, the higher the probability that processes incur budget
timeouts unjustly is. Besides, a too high value of B_max unnecessarily
increases the deviation from an ideal, smooth service.

Unfortunately, it is not trivial to estimate the peak rate correctly:
because of the presence of sw and hw queues between the scheduler and
the device components that finally serve I/O requests, it is hard to
say exactly when a given dispatched request is served inside the
device, and for how long. As a consequence, it is hard to know
precisely at what rate a given set of requests is actually served by
the device.

On the opposite end, the dispatch time of any request is trivially
available, and, from this piece of information, the "dispatch rate"
of requests can be immediately computed. So, the idea in the next
function is to use what is known, namely request dispatch times
(plus, when useful, request completion times), to estimate what is
unknown, namely in-device request service rate.

The main issue is that, because of the above facts, the rate at
which a certain set of requests is dispatched over a certain time
interval can vary greatly with respect to the rate at which the
same requests are then served. But, since the size of any
intermediate queue is limited, and the service scheme is lossless
(no request is silently dropped), the following obvious convergence
property holds: the number of requests dispatched MUST become
closer and closer to the number of requests completed as the
observation interval grows. This is the key property used in
this new version of the peak-rate estimator.
Signed-off-by: default avatarPaolo Valente <paolo.valente@linaro.org>
Signed-off-by: default avatarArianna Avanzini <avanzini.arianna@gmail.com>
Signed-off-by: default avatarJens Axboe <axboe@fb.com>
      ab0e43e9
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