mirror of https://gitee.com/openkylin/linux.git
117 lines
5.7 KiB
Plaintext
117 lines
5.7 KiB
Plaintext
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.
|
|
|
|
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
|
|
request which are marked with REQ_NOIDLE go 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 marked with REQ_NOIDLE.
|
|
|
|
A1. We only do tree idle (all queues on sync-noidle tree) on queues marked
|
|
with REQ_NOIDLE. 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_NOIDLE request comes in, it will be scheduled
|
|
roughly after 1 second. If after completion of REQ_NOIDLE request we
|
|
do not idle, and after a couple of milli seconds a another REQ_NOIDLE
|
|
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_NOIDLE, 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_NOIDLE 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_NOIDLE
|
|
A2. I would think whenever one is doing synchronous write and not expecting
|
|
more writes to be dispatched from same context soon, should be able
|
|
to specify REQ_NOIDLE on writes and that probably should work well for
|
|
most of the cases.
|