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CFQ (Complete Fairness Queueing)
===============================

The main aim of CFQ scheduler is to provide a fair allocation of the disk
I/O bandwidth for all the processes which requests an I/O operation.

CFQ maintains the per process queue for the processes which request I/O
operation(synchronous requests). In case of asynchronous requests, all the
requests from all the processes are batched together according to their
process's I/O priority.

CFQ ioscheduler tunables
========================

slice_idle
----------
This specifies how long CFQ should idle for next request on certain cfq queues
(for sequential workloads) and service trees (for random workloads) before
queue is expired and CFQ selects next queue to dispatch from.

By default slice_idle is a non-zero value. That means by default we idle on
queues/service trees. This 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/service tree
level and one should see an overall improved throughput on faster storage
devices like multiple SATA/SAS disks in hardware RAID configuration. The down
side is that isolation provided from WRITES also goes down and notion of
IO priority becomes weaker.

So depending on storage and workload, it might be useful to set slice_idle=0.
In general I think 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.

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.

group_idle
-----------
This parameter forces idling at the CFQ group level instead of CFQ
queue level. This was introduced after a bottleneck was observed
in higher end storage due to idle on sequential queue and allow dispatch
from a single queue. The idea with this parameter is that it can be run with
slice_idle=0 and group_idle=8, so that idling does not happen on individual
queues in the group but happens overall on the group and thus still keeps the
IO controller working.
Not idling on individual queues in the group will dispatch requests from
multiple queues in the group at the same time and achieve higher throughput
on higher end storage.

Default value for this parameter is 8ms.

low_latency
-----------
This parameter is used to enable/disable the low latency mode of the CFQ
scheduler. If enabled, CFQ tries to recompute the slice time for each process
based on the target_latency set for the system. This favors fairness over
throughput. Disabling low latency (setting it to 0) ignores target latency,
allowing each process in the system to get a full time slice.

By default low latency mode is enabled.

target_latency
--------------
This parameter is used to calculate the time slice for a process if cfq's
latency mode is enabled. It will ensure that sync requests have an estimated
latency. But if sequential workload is higher(e.g. sequential read),
then to meet the latency constraints, throughput may decrease because of less
time for each process to issue I/O request before the cfq queue is switched.

Though this can be overcome by disabling the latency_mode, it may increase
the read latency for some applications. This parameter allows for changing
target_latency through the sysfs interface which can provide the balanced
throughput and read latency.

Default value for target_latency is 300ms.

slice_async
-----------
This parameter is same as of slice_sync but for asynchronous queue. The
default value is 40ms.

slice_async_rq
--------------
This parameter is used to limit the dispatching of asynchronous request to
device request queue in queue's slice time. The maximum number of request that
are allowed to be dispatched also depends upon the io priority. Default value
for this is 2.

slice_sync
----------
When a queue is selected for execution, the queues IO requests are only
executed for a certain amount of time(time_slice) before switching to another
queue. This parameter is used to calculate the time slice of synchronous
queue.

time_slice is computed using the below equation:-
time_slice = slice_sync + (slice_sync/5 * (4 - prio)). To increase the
time_slice of synchronous queue, increase the value of slice_sync. Default
value is 100ms.

quantum
-------
This specifies the number of request dispatched to the device queue. In a
queue's time slice, a request will not be dispatched if the number of request
in the device exceeds this parameter. This parameter is used for synchronous
request.

In case of storage with several disk, this setting can limit the parallel
processing of request. Therefore, increasing the value can improve the
performance although this can cause the latency of some I/O to increase due
to more number of requests.

CFQ Group scheduling
====================

CFQ supports blkio cgroup and has "blkio." prefixed files in each
blkio cgroup directory. It is weight-based and there are four knobs
for configuration - weight[_device] and leaf_weight[_device].
Internal cgroup nodes (the ones with children) can also have tasks in
them, so the former two configure how much proportion the cgroup as a
whole is entitled to at its parent's level while the latter two
configure how much proportion the tasks in the cgroup have compared to
its direct children.

Another way to think about it is assuming that each internal node has
an implicit leaf child node which hosts all the tasks whose weight is
configured by leaf_weight[_device]. Let's assume a blkio hierarchy
composed of five cgroups - root, A, B, AA and AB - with the following
weights where the names represent the hierarchy.

        weight leaf_weight
 root :  125    125
 A    :  500    750
 B    :  250    500
 AA   :  500    500
 AB   : 1000    500

root never has a parent making its weight is meaningless. For backward
compatibility, weight is always kept in sync with leaf_weight. B, AA
and AB have no child and thus its tasks have no children cgroup to
compete with. They always get 100% of what the cgroup won at the
parent level. Considering only the weights which matter, the hierarchy
looks like the following.

          root
       /    |   \
      A     B    leaf
     500   250   125
   /  |  \
  AA  AB  leaf
 500 1000 750

If all cgroups have active IOs and competing with each other, disk
time will be distributed like the following.

Distribution below root. The total active weight at this level is
A:500 + B:250 + C:125 = 875.

 root-leaf :   125 /  875      =~ 14%
 A         :   500 /  875      =~ 57%
 B(-leaf)  :   250 /  875      =~ 28%

A has children and further distributes its 57% among the children and
the implicit leaf node. The total active weight at this level is
AA:500 + AB:1000 + A-leaf:750 = 2250.

 A-leaf    : ( 750 / 2250) * A =~ 19%
 AA(-leaf) : ( 500 / 2250) * A =~ 12%
 AB(-leaf) : (1000 / 2250) * A =~ 25%

CFQ IOPS Mode for group scheduling
===================================
Basic CFQ design is to provide priority based time slices. Higher priority
process gets bigger time slice and lower priority process gets smaller time
slice. Measuring time becomes harder if storage is fast and supports NCQ and
it would be better to dispatch multiple requests from multiple cfq queues in
request queue at a time. In such scenario, it is not possible to measure time
consumed by single queue accurately.

What is possible though is to measure number of requests dispatched from a
single queue and also allow dispatch from multiple cfq queue at the same time.
This effectively becomes the fairness in terms of IOPS (IO operations per
second).

If one sets slice_idle=0 and if storage supports NCQ, CFQ internally switches
to IOPS mode and starts providing fairness in terms of number of requests
dispatched. Note that this mode switching takes effect only for group
scheduling. For non-cgroup users nothing should change.

CFQ IO scheduler Idling Theory
===============================
Idling on a queue is primarily about waiting for the next request to come
on same queue after completion of a request. In this process CFQ will not
dispatch requests from other cfq queues even if requests are pending there.

The rationale behind idling is that it can cut down on number of seeks
on rotational media. For example, if a process is doing dependent
sequential reads (next read will come on only after completion of previous
one), then not dispatching request from other queue should help as we
did not move the disk head and kept on dispatching sequential IO from
one queue.

CFQ has following service trees and various queues are put on these trees.

	sync-idle	sync-noidle	async

All cfq queues doing synchronous sequential IO go on to sync-idle tree.
On this tree we idle on each queue individually.

All synchronous non-sequential queues go on sync-noidle tree. Also any
synchronous write request which is not marked with REQ_IDLE goes on this
service tree. On this tree we do not idle on individual queues instead idle
on the whole group of queues or the tree. So if there are 4 queues waiting
for IO to dispatch we will idle only once last queue has dispatched the IO
and there is no more IO on this service tree.

All async writes go on async service tree. There is no idling on async
queues.

CFQ has some optimizations for SSDs and if it detects a non-rotational
media which can support higher queue depth (multiple requests at in
flight at a time), then it cuts down on idling of individual queues and
all the queues move to sync-noidle tree and only tree idle remains. This
tree idling provides isolation with buffered write queues on async tree.

FAQ
===
Q1. Why to idle at all on queues not marked with REQ_IDLE.

A1. We only do tree idle (all queues on sync-noidle tree) on queues not marked
    with REQ_IDLE. This helps in providing isolation with all the sync-idle
    queues. Otherwise in presence of many sequential readers, other
    synchronous IO might not get fair share of disk.

    For example, if there are 10 sequential readers doing IO and they get
    100ms each. If a !REQ_IDLE request comes in, it will be scheduled
    roughly after 1 second. If after completion of !REQ_IDLE request we
    do not idle, and after a couple of milli seconds a another !REQ_IDLE
    request comes in, again it will be scheduled after 1second. Repeat it
    and notice how a workload can lose its disk share and suffer due to
    multiple sequential readers.

    fsync can generate dependent IO where bunch of data is written in the
    context of fsync, and later some journaling data is written. Journaling
    data comes in only after fsync has finished its IO (atleast for ext4
    that seemed to be the case). Now if one decides not to idle on fsync
    thread due to !REQ_IDLE, then next journaling write will not get
    scheduled for another second. A process doing small fsync, will suffer
    badly in presence of multiple sequential readers.

    Hence doing tree idling on threads using !REQ_IDLE flag on requests
    provides isolation from multiple sequential readers and at the same
    time we do not idle on individual threads.

Q2. When to specify REQ_IDLE
A2. I would think whenever one is doing synchronous write and expecting
    more writes to be dispatched from same context soon, should be able
    to specify REQ_IDLE on writes and that probably should work well for
    most of the cases.