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cfq-iosched: Add documentation about idling

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There are always questions about why CFQ is idling on various conditions.
Recent ones is Christoph asking again why to idle on REQ_NOIDLE. His
assertion is that XFS is relying more and more on workqueues and is
concerned that CFQ idling on IO from every workqueue will impact
XFS badly.

So he suggested that I add some more documentation about CFQ idling
and that can provide more clarity on the topic and also gives an
opprotunity to poke a hole in theory and lead to improvements.

So here is my attempt at that. Any comments are welcome.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
This commit is contained in:
Vivek Goyal 2011-08-05 09:42:20 +02:00 committed by Jens Axboe
parent 35ae66e0a0
commit 4931402a9d
1 changed files with 71 additions and 0 deletions
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Documentation/block/cfq-iosched.txt
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@ -43,3 +43,74 @@ 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.