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block: replace REQ_NOIDLE with REQ_IDLE

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Noidle should be the default for writes as seen by all the compounds
definitions in fs.h using it.  In fact only direct I/O really should
be using NODILE, so turn the whole flag around to get the defaults
right, which will make our life much easier especially onces the
WRITE_* defines go away.

This assumes all the existing "raw" users of REQ_SYNC for writes
want noidle behavior, which seems to be spot on from a quick audit.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>
This commit is contained in:
Christoph Hellwig
2016-11-01 07:40:09 -06:00
committed by Jens Axboe
parent b685d3d65a
commit a2b809672e
6 changed files with 33 additions and 28 deletions
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32
Documentation/block/cfq-iosched.txt
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@@ -240,11 +240,11 @@ 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.
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.
@@ -257,17 +257,17 @@ tree idling provides isolation with buffered write queues on async tree.
FAQ
===
Q1. Why to idle at all on queues marked with REQ_NOIDLE.
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 marked
with REQ_NOIDLE. This helps in providing isolation with all the sync-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_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
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.
@@ -276,16 +276,16 @@ A1. We only do tree idle (all queues on sync-noidle tree) on queues marked
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
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_NOIDLE flag on requests
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_NOIDLE
A2. I would think whenever one is doing synchronous write and not expecting
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_NOIDLE on writes and that probably should work well for
to specify REQ_IDLE on writes and that probably should work well for
most of the cases.