path: root/Documentation
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authorLinus Torvalds <torvalds@linux-foundation.org>2011-08-19 10:47:07 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2011-08-19 10:47:07 -0700
commit5ccc38740a283aba81a00e92941310d0c1aeb2ee (patch)
treeba7d725947975a9391e085bd1d5958b004bfdc3e /Documentation
parent0c3bef612881ee6216a36952ffaabfc35b83545c (diff)
parentb53d1ed734a2b9af8da115b836b658daa7d47a48 (diff)
Merge branch 'for-linus' of git://git.kernel.dk/linux-block
* 'for-linus' of git://git.kernel.dk/linux-block: (23 commits) Revert "cfq: Remove special treatment for metadata rqs." block: fix flush machinery for stacking drivers with differring flush flags block: improve rq_affinity placement blktrace: add FLUSH/FUA support Move some REQ flags to the common bio/request area allow blk_flush_policy to return REQ_FSEQ_DATA independent of *FLUSH xen/blkback: Make description more obvious. cfq-iosched: Add documentation about idling block: Make rq_affinity = 1 work as expected block: swim3: fix unterminated of_device_id table block/genhd.c: remove useless cast in diskstats_show() drivers/cdrom/cdrom.c: relax check on dvd manufacturer value drivers/block/drbd/drbd_nl.c: use bitmap_parse instead of __bitmap_parse bsg-lib: add module.h include cfq-iosched: Reduce linked group count upon group destruction blk-throttle: correctly determine sync bio loop: fix deadlock when sysfs and LOOP_CLR_FD race against each other loop: add BLK_DEV_LOOP_MIN_COUNT=%i to allow distros 0 pre-allocated loop devices loop: add management interface for on-demand device allocation loop: replace linked list of allocated devices with an idr index ...
Diffstat (limited to 'Documentation')
2 files changed, 77 insertions, 3 deletions
diff --git a/Documentation/block/cfq-iosched.txt b/Documentation/block/cfq-iosched.txt
index e578feed6d81..6d670f570451 100644
--- a/Documentation/block/cfq-iosched.txt
+++ b/Documentation/block/cfq-iosched.txt
@@ -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
+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.
+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.
diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 6ca1f5cb71e0..614d0382e2cb 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -1350,9 +1350,12 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
it is equivalent to "nosmp", which also disables
the IO APIC.
- max_loop= [LOOP] Maximum number of loopback devices that can
- be mounted
- Format: <1-256>
+ max_loop= [LOOP] The number of loop block devices that get
+ (loop.max_loop) unconditionally pre-created at init time. The default
+ number is configured by BLK_DEV_LOOP_MIN_COUNT. Instead
+ of statically allocating a predefined number, loop
+ devices can be requested on-demand with the
+ /dev/loop-control interface.
mcatest= [IA-64]

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