BFQ: v7r5

CHANGELOG

v7r5:
. IMPROVEMENT: Improve throughput boosting by idling the device
  only for processes that, in addition to perform sequential I/O,
  are I/O-bound (apart from weight-raised queues, for which idling
  is always performed to guarantee them a low latency).
. IMPROVEMENT: Improve throughput boosting by depriving processes
  that cooperate often of weight-raising.
. CODE IMPROVEMENT: Pass of improvement of the readability of both
  comments and actual code.

v7r4:
. BUGFIX. Modified the code so as to be robust against late detection of
  NCQ support for a rotational device.
. BUGFIX. Removed a bug that hindered the correct throughput distribution
  on flash-based devices when not every process had to receive the same
  fraction of the throughput. This fix entailed also a little efficiency
  improvement, because it implied the removal of a short function executed
  in a hot path.
. CODESTYLE IMPROVEMENT: removed quoted strings split across lines.

v7r3:
. IMPROVEMENT: Improved throughput boosting with NCQ-capable HDDs and
  random workloads. The mechanism that further boosts throghput with
  these devices and workloads is activated only in the cases where it
  does not cause any violation of throughput-distribution and latency
  guarantees.
. IMPROVEMENT: Generalized the computation of the parameters of the
  low-latency heuristic for interactive applications, so as to fit also
  slower storage devices. The purpose of this improvement is to preserve
  low-latency guarantees for interactive applications also on slower
  devices, such as portable hard disks, multimedia and SD cards.
. BUGFIX: Re-added MODULE_LICENSE macro.
. CODE IMPROVEMENTS: Small code cleanups; introduced a coherent naming
  scheme for all identifiers related to weight raising; refactored and
  optimized a few hot paths.

v7r2:
. BUGFIX/IMPROVEMENT. One of the requirements for an application to be
  deemed as soft real-time is that it issues its requests in batches, and
  stops doing I/O for a well-defined amount of time before issuing a new
  batch. Imposing this minimum idle time allows BFQ to filter out I/O-bound
  applications that may otherwise be incorrectly deemed as soft real-time
  (under the circumstances described in detail in the comments to the
  function bfq_bfqq_softrt_next_start()). Unfortunately, BFQ could however
  start counting this idle time from two different events: either from the
  expiration of the queue, if all requests of the queue had also been already
  completed when the queue expired, or, if the previous condition did not
  hold, from the first completion of one of the still outstanding requests.
  In the second case, an application had more chances to be deemed as soft
  real-time.
  Actually, there was no reason for this differentiated treatment. We
  addressed this issue by defining more precisely the above requirement for
  an application to be deemed as soft real-time, and changing the code
  consequently: a well-defined amount of time must elapse between the
  completion of *all the requests* of the current pending batch and the
  issuing of the first request of the next batch (this is, in the end, what
  happens with a true soft real-time application). This change further
  reduced false positives, and, as such, improved responsiveness and reduced
  latency for actual soft real-time applications.
. CODE IMPROVEMENT. We cleaned up the code a little bit and addressed
  some issues pointed out by the checkpatch.pl script.

v7r1:
. BUGFIX. Replace the old value used to approximate 'infinity', with
  the correct one to use in case times are compared through the macro
  time_is_before_jiffies(). In fact, this macro, designed to take
  wraparound issues into account, easily returns anomalous results if
  its argument is equal to the value that we used as an approximation
  of 'infinity', namely ((unsigned long) (-1)).  The consequence was
  that the logical expression used to determine whether a queue
  belongs to a soft real-time application often yielded an incorrect
  result. In the end, some application happened to be incorrectly
  deemed as soft real-time and hence weight-raised. This affected both
  throughput and latency guarantees.
. BUGFIX. Fixed a scriverner's error made in an attempt to use the
  above macro in a logical expression.
. IMPROVEMENT/BUGFIX. On the expiration of a queue, use a more general
  condition to allow a weight-raising period to start if the queue is
  soft real-time.  The previous condition could prevent an empty,
  soft-real time queue from being correctly deemed as soft real-time.
. IMPROVEMENT/MINOR BUGFIX. Use jiffies-comparison macros also in the
  following cases:
  . to establish whether an application initially deemed as interactive
    is now meeting the requirements for being classified as soft
    real-time;
  . to determine if a weight-raising period must be ended.
. CODE IMPROVEMENT. Change the type of the time quantities used in the
  weight-raising heuristics to unsigned long, as the type of the time
  (jiffies) is unsigned long.

v7:
- IMPROVEMENT: In the presence of weight-raised queues and if the
  device is NCQ-enabled, device idling is now disabled for non-raised
  readers, i.e., for their associated sync queues. Hence a sync queue
  is expired immediately if it becomes empty, and a new queue is
  served.  As explained in detail in the papers about BFQ, not idling
  the device for sync queues when the latter become empty causes BFQ to
  assign higher timestamps to these queues when they get backlogged
  again, and hence to serve these queues less frequently. This fact,
  plus to the fact that, because of the immediate expiration itself,
  these queues get less service while they are granted access to the
  disk, reduces the relative rate at which the processes associated to
  these queues ask for requests from the I/O request pool. If the pool
  is saturated, as it happens in the presence of write hogs, reducing
  the above relative rate increases the probability that a request is
  available (soon) in the pool when a weight-raised process needs it.
  This change does seem to mitigate the typical starvation problems
  that occur in the presence of write hogs and NCQ, and hence to
  guarantee a higher application and system responsiveness in these
  hostile scenarios.
- IMPROVEMENT/BUGFIX: Introduced a new classification rule to the soft
  real-time heuristic, which takes into account also the isochronous
  nature of such applications. The computation of next_start has been
  fixed as well. Now it is correctly done from the time of the last
  transition from idle to backlogged; the next_start is therefore
  computed from the service received by the queue from its last
  transition from idle to backlogged. Finally, the code which
  preserved weight-raising for a soft real-time queue even with no
  idle->backlogged transition has been removed.
- IMPROVEMENT: Add a few jiffies to the reference time interval used to
  establish whether an application is greedy or not. This reference
  interval was, by default, HZ/125 seconds, which could generate false
  positives in the following two cases (especially if both cases occur):
  1) If HZ is so low that the duration of a jiffie is comparable to or
     higher than the above reference time interval. This happens, e.g.,
     on slow devices with HZ=100.
  2) If jiffies, instead of increasing at a constant rate, may stop
     increasing for some time, then suddenly 'jump' by several units to
     recover the lost increments. This seems to happen, e.g., in virtual
     machines.
  The added number of jiffies has been found experimentally. In particular,
  according to our experiments, adding this number of jiffies seems to make
  the filter quite precise also in embedded systems and KVM/QEMU virtual
  machines. Also contributed by
  Alexander Spyridakis <a.spyridakis@virtualopensystems.com>.
- IMPROVEMENT/BUGFIX: Keep disk idling also for NCQ-provided
  rotational devices, which boosts the throughput on NCQ-enabled
  rotational devices.
- BUGFIX: The budget-timeout condition in the bfq_rq_enqueued() function
  was checked only if the request is large enough to provoke an unplug. As
  a consequence, for a process always issuing small I/O requests the
  budget timeout was never checked. The queue associated to the process
  therefore expired only when its budget was exhausted, even if the
  queue had already incurred a budget timeout from a while.
  This fix lets a queue be checked for budget timeout at each request
  enqueue, and, if needed, expires the queue accordingly even if the
  request is small.
- BUGFIX: Make sure that weight-raising is resumed for a split queue,
  if it was merged when already weight-raised.
- MINOR BUGFIX: Let bfq_end_raising_async() correctly end weight-raising
  also for the queues belonging to the root group.
- IMPROVEMENT: Get rid of the some_coop_idle flag, which in its turn
  was used to decide whether to disable idling for an in-service
  shared queue whose seek mean decreased. In fact, disabling idling
  for such a queue turned out to be useless.
- CODE IMPROVEMENT: The bfq_bfqq_must_idle() function and the
  bfq_select_queue() function may not change the current in-service
  queue in various cases. We have cleaned up the involved conditions,
  by factoring out the common parts and getting rid of the useless
  ones.
- MINOR CODE IMPROVEMENT: The idle_for_long_time condition in the
  bfq_add_rq_rb() function should be evaluated only on an
  idle->backlogged transition. Now the condition is set to false
  by default, evaluating it only if the queue was not busy on a
  request insertion.
- MINOR CODE IMPROVEMENT: Added a comment describing the rationale
  behind the condition evaluated in the function
  bfq_bfqq_must_not_expire().

v6r2:
- Fairness fix: the case of queue expiration for budget timeout is
  now correctly handled also for sync queues, thus allowing also
  the processes corresponding to these queues to be guaranteed their
  reserved share of the disk throughput.
- Fixed a bug that prevented group weights from being correctly
  set via the sysfs interface.
- Fixed a bug that cleared a previously-set group weight if the
  same value was re-inserted via the sysfs interface.
- Fixed an EQM bug that allowed a newly-started process to skip
  its initial weight-raising period if its queue was merged before
  its first request was inserted.
- Fixed a bug that preserved already-started weight-raising periods
  even if the low_latency tunable was disabled.
- The raising_max_time tunable now shows, more user-friendly, the
  maximum raising time in milliseconds.

v6r1:
- Fix use-after-free of queues in __bfq_bfqq_expire(). It may happen that
  a call to bfq_del_bfqq_busy() puts the last reference taken on a queue
  and frees it. Subsequent accesses to that same queue would result in a
  use-after-free. Make sure that a queue that has just been deleted from
  busy is no more touched.
- Use the uninitialized_var() macro when needed. It may happen that a
  variable is initialized in a function that is called by the function
  that defined it. Use the uninitialized_var() macro in these cases.

v6:
- Replacement of the cooperating-queue merging mechanism borrowed from
  CFQ with Early Queue Merge (EQM), a unified mechanism to get a
  sequential read pattern, and hence a high throughput, with any set of
  processes performing interleaved I/O. EQM also preserves low latency.
  (see http://algogroup.unimore.it/people/paolo/disk_sched/description.php
  for more details). Contributed by Mauro Andreolini and Arianna Avanzini.
  The code for detecting whether two queues have to be merged is a
  slightly modified version of the CFQ code for detecting whether two
  queues belong to cooperating processes and whether the service of a
  queue should be preempted to boost the throughput.
- Fix a bug that caused the peak rate of a disk to be computed as zero
  in case of multiple I/O errors. Subsequent estimations of the weight
  raising duration caused a division-by-zero error.

v5r1:
- BUG FIX: Fixed stall occurring when the active queue is moved to
  a different group while idling (this caused the idling timer to be
  cancelled and hence no new queue to be selected, and no new
  request to be dispatched).
- BUG FIX: Fixed wrong assignment of too high budgets to queues during
  the first few seconds after initialization.
- BUG FIX: Added proper locking to the function handling the "weights"
  tunable.

v5:
- Added an heuristic that, if the tunable raising_max_time is set to
  0, automatically computes the duration of the weight raising
  according to the estimated peak rate of the device. This enables
  flash-based devices to reach maximum throughput as soon as possible,
  without sacrificing latency.

v4:
- Throughput-boosting for flash-based devices: improved version of commits
  a68bbdd and f7d7b7a, which boosts the throughput while still preserving
  latency guarantees for interactive and soft real-time applications.
- Better identification of NCQ-capable disks: port of commit e459dd0.

v3-r4:
- Bugfixes
  * Removed an important memory leak: under some circumstances the process references
    to a queue were not decremented correctly, which prevented unused shared bfq_queue
    to be correctly deallocated.
  * Fixed various errors related to hierarchical scheduling:
	* Removed an error causing tasks to be attached to the bfqio cgroup
	  controller even when BFQ was not the active scheduler
	* Corrected wrong update of the budgets from the leaf to the root upon
	  forced selection of a service tree or a bfq_queue
	* Fixed the way how active leaf entities are moved to the root group before
	  the group entity is deactivated when a cgroup is destroyed
- Throughput-boosting improvement for cooperating queues: close detection is now based
  on a fixed threshold instead of the queue's average seek. This is a port of one of
  the changes in the CFQ commit 3dde36d by Corrado Zoccolo.

v3-r3:
- Bugfix: removed an important error causing occasional kernel panics when
  moving a process to a new cgroup. The panic occurred if:
  1) the queue associated to the process was idle when the process was moved
     and
  2) a new disk request was inserted into the queue just after the move.
- Further latency improvement through a better treatment of low-bandwidth
  async queues.

v3-r2:
- Bugfix: added a forgotten condition that prevents weights of low-bw async
  queues from being raised when low_latency is off.
- Latency improvement: low-bw async queues are now better identified.

v3-r1:
- Fixed an important request-dispatch bug causing occasional IO hangs.
- Added a new mechanism to reduce the latency of low-bw async queues.
  This reduces the latency of also the sync queues synchronized with
  the above async queues.
- Fixed a minor bug in iocontext locking (port of commits 9b50902 and 3181faa
  from CFQ).

v3:

- Improved low-latency mechanisms, including a more accurate criterion to
  distinguish between greedy-but-seeky and soft real-time applications.
  Interactive applications now enjoy noticeably lower latencies.

- Switch to the simpler one-request-dispatch-at-a-time scheme as in CFQ.

- Ported cooperating-queues merging from CFQ (6d048f5, 1afba04,
  d9e7620, a36e71f, 04dc6e7, 26a2ac0, 3ac6c9f, f2d1f0a, 83096eb,
  2e46e8b, df5fe3e, b3b6d04, e6c5bc7, c0324a0, f04a642, 8682e1f,
  b9d8f4c, 2f7a2d8, ae54abe, e9ce335, 39c01b2, d02a2c0, c10b61f).
  Contributed by Arianna Avanzini. Queues of processes performing IO
  on interleaved, yet contiguous disk zones are merged to boost the
  throughput. Some little optimizations to get a more stable throughput
  have been added to the original CFQ version.

- Added static fallback queue for extreme OOM conditions (porting of
  CFQ commits d5036d7, 6118b70, b706f64, 32f2e80). Port contributed by
  Francesco Allertsen.

- Ported CFQ commits b0b78f8, 40bb54d, 30996f4, dddb745, ad5ebd2, cf7c25c;
  mainly code cleanup and fix of minor bugs. Port contributed by
  Francesco Allertsen.

v2:

- An issue that may cause little throughput loss on fast disks has been solved.
  BFQ-v1 and CFQ may suffer from this problem.
- The disk-idling timeout has been better tuned to further file latency
  (especially for the idle- or light-loaded-disk scenarios).
- One of the parameters of the low-latency heuristics has been tuned a little
  bit more, so as to reduce the probability that a disk-bound process may
  hamper the reduction of the latency of interactive and soft real-time
  applications.

  - Same low-latency guarantees with and without NCQ.

  - Latency for interactive applications about halved with respect to BFQ-v1.

  - When the low_latency tunable is set, also soft real-time applications
    now enjoy reduced latency.

  - A very little minimum bandwidth is now guaranteed to the
    Idle IO-scheduling class also when the other classes are
    backlogged, just to prevent them from starving.

v1:

This is a new version of BFQ with respect to the versions you can
find on Fabio's site: http://feanor.sssup.it/~fabio/linux/bfq.
Here is what we changed with respect to the previous versions:

1) re-tuned the budget feedback mechanism: it is now slighlty more
biased toward assigning high budgets, to boost the aggregated
throughput more, and more quickly as new processes are started

2) introduced more tolerance toward seeky queues (I verified that the
phenomena described below used to occur systematically):

   2a: if a queue is expired after having received very little
       service, then it is not punished as a seeky queue, even if it
       occurred to consume that little service too slowly; the
       rationale is that, if the new active queue has been served for
       a too short time interval, then its possible sequential
       accesses may not yet prevail on the initial latencies for
       moving the disk head on the first sector requested

   2b: the waiting time (disk idling) of a queue detected as seeky as
       a function of the position of the requests it issued is reduced
       to a very low value only after the queue has consumed a minimum
       fraction of the assigned budget; this prevents processes
       generating (partly) seeky workloads from being too ill-treated

   2c: if a queue has consumed 'enough' budget upon a budget timeout, then,
       even if it did not consume all of its budget, that queue is not punished
       as any seeky queue; the rationale is that, depending on the disk zones,
       a queue may be served at a lower rate than the estimated peak rate.

   Changes 2a and 2b have been critical in lowering latencies, whereas
   change 2c, in addition to change 1, helped a lot increase the disk
   throughput.

3) slightly changed the peak rate estimator: a low-pass filter is now
used instead of just keeping the highest rate sampled; the rationale
is that the peak rate of a disk should be quite stable, so the filter
should converge more or less smoothly to the right value; it seemed to
correctly catch the peak rate with all disks we used

4) added the low latency mechanism described in detail in
http://algogroup.unimore.it/people/paolo/disk_sched/description.php.

Signed-off-by: Chet Kener <Cl3Kener@gmail.com>