Commit Graph

247 Commits

Author SHA1 Message Date
Vivek Goyal
878eaddd05 cfq-iosched: Do not access cfqq after freeing it
Fix a crash during boot reported by Jeff Moyer. Fix the issue of accessing
cfqq after freeing it.

Reported-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Jens Axboe <axboe@carl.(none)>
2009-12-07 19:37:15 +01:00
Jens Axboe
bb729bc98c cfq-iosched: use call_rcu() instead of doing grace period stall on queue exit
After the merge of the IO controller patches, booting on my megaraid
box ran much slower. Vivek Goyal traced it down to megaraid discovery
creating tons of devices, each suffering a grace period when they later
kill that queue (if no device is found).

So lets use call_rcu() to batch these deferred frees, instead of taking
the grace period hit for each one.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-12-06 09:54:19 +01:00
Vivek Goyal
3e25206689 blkio: Implement dynamic io controlling policy registration
o One of the goals of block IO controller is that it should be able to
  support mulitple io control policies, some of which be operational at
  higher level in storage hierarchy.

o To begin with, we had one io controlling policy implemented by CFQ, and
  I hard coded the CFQ functions called by blkio. This created issues when
  CFQ is compiled as module.

o This patch implements a basic dynamic io controlling policy registration
  functionality in blkio. This is similar to elevator functionality where
  ioschedulers register the functions dynamically.

o Now in future, when more IO controlling policies are implemented, these
  can dynakically register with block IO controller.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-12-04 16:38:14 +01:00
Vivek Goyal
9d6a986c0b blkio: Export some symbols from blkio as its user CFQ can be a module
o blkio controller is inside the kernel and cfq makes use of interfaces
  exported by blkio. CFQ can be a module too, hence export symbols used
  by CFQ.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-12-04 16:38:14 +01:00
Shaohua Li
3c764b7a65 cfq-iosched: make nonrot check logic consistent
cfq_arm_slice_timer() has logic to disable idle window for SSD device. The same
thing should be done at cfq_select_queue() too, otherwise we will still see
idle window. This makes the nonrot check logic consistent in cfq.
Tests in a intel SSD with low_latency knob close, below patch can triple disk
thoughput for muti-thread sequential read.

Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-12-04 13:12:06 +01:00
Jens Axboe
f2eecb9152 cfq-iosched: move IO controller declerations to a header file
They should not be declared inside some other file that's not related
to CFQ.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-12-04 10:06:35 +01:00
Vivek Goyal
c04645e592 blkio: Wait on sync-noidle queue even if rq_noidle = 1
o rq_noidle() is supposed to tell cfq that do not expect a request after this
  one, hence don't idle. But this does not seem to work very well. For example
  for direct random readers, rq_noidle = 1 but there is next request coming
  after this. Not idling, leads to a group not getting its share even if
  group_isolation=1.

o The right solution for this issue is to scan the higher layers and set
  right flag (WRITE_SYNC or WRITE_ODIRECT). For the time being, this single
  line fix helps. This should not have any significant impact when we are
  not using cgroups. I will later figure out IO paths in higher layer and
  fix it.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-12-03 19:28:53 +01:00
Vivek Goyal
ae30c28655 blkio: Implement group_isolation tunable
o If a group is running only a random reader, then it will not have enough
  traffic to keep disk busy and we will reduce overall throughput. This
  should result in better latencies for random reader though. If we don't
  idle on random reader service tree, then this random reader will experience
  large latencies if there are other groups present in system with sequential
  readers running in these.

o One solution suggested by corrado is that by default keep the random readers
  or sync-noidle workload in root group so that during one dispatch round
  we idle only once on sync-noidle tree. This means that all the sync-idle
  workload queues will be in their respective group and we will see service
  differentiation in those but not on sync-noidle workload.

o Provide a tunable group_isolation. If set, this will make sure that even
  sync-noidle queues go in their respective group and we wait on these. This
  provides stronger isolation between groups but at the expense of throughput
  if group does not have enough traffic to keep the disk busy.

o By default group_isolation = 0

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-12-03 19:28:53 +01:00
Vivek Goyal
f26bd1f0a3 blkio: Determine async workload length based on total number of queues
o Async queues are not per group. Instead these are system wide and maintained
  in root group. Hence their workload slice length should be calculated
  based on total number of queues in the system and not just queues in the
  root group.

o As root group's default weight is 1000, make sure to charge async queue
  more in terms of vtime so that it does not get more time on disk because
  root group has higher weight.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-12-03 19:28:53 +01:00
Vivek Goyal
f75edf2dc8 blkio: Wait for cfq queue to get backlogged if group is empty
o If a queue consumes its slice and then gets deleted from service tree, its
  associated group will also get deleted from service tree if this was the
  only queue in the group. That will make group loose its share.

o For the queues on which we have idling on and if these have used their
  slice, wait a bit for these queues to get backlogged again and then
  expire these queues so that group does not loose its share.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-12-03 19:28:53 +01:00
Vivek Goyal
f8d461d692 blkio: Propagate cgroup weight updation to cfq groups
o Propagate blkio cgroup weight updation to associated cfq groups.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-12-03 19:28:53 +01:00
Vivek Goyal
24610333d5 blkio: Drop the reference to queue once the task changes cgroup
o If a task changes cgroup, drop reference to the cfqq associated with io
  context and set cfqq pointer stored in ioc to NULL so that upon next request
  arrival we will allocate a  new queue in new group.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-12-03 19:28:52 +01:00
Vivek Goyal
8682e1f15f blkio: Provide some isolation between groups
o Do not allow following three operations across groups for isolation.
	- selection of co-operating queues
	- preemtpions across groups
	- request merging across groups.

o Async queues are currently global and not per group. Allow preemption of
  an async queue if a sync queue in other group gets backlogged.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-12-03 19:28:52 +01:00
Vivek Goyal
220841906f blkio: Export disk time and sectors used by a group to user space
o Export disk time and sector used by a group to user space through cgroup
  interface.

o Also export a "dequeue" interface to cgroup which keeps track of how many
  a times a group was deleted from service tree. Helps in debugging.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-12-03 19:28:52 +01:00
Vivek Goyal
2868ef7b39 blkio: Some debugging aids for CFQ
o Some debugging aids for CFQ.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-12-03 19:28:52 +01:00
Vivek Goyal
b1c3576961 blkio: Take care of cgroup deletion and cfq group reference counting
o One can choose to change elevator or delete a cgroup. Implement group
  reference counting so that both elevator exit and cgroup deletion can
  take place gracefully.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Nauman Rafique <nauman@google.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-12-03 19:28:52 +01:00
Vivek Goyal
25fb5169d4 blkio: Dynamic cfq group creation based on cgroup tasks belongs to
o Determine the cgroup IO submitting task belongs to and create the cfq
  group if it does not exist already.

o Also link cfqq and associated cfq group.

o Currently all async IO is mapped to root group.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-12-03 19:28:52 +01:00
Vivek Goyal
dae739ebc4 blkio: Group time used accounting and workload context save restore
o This patch introduces the functionality to do the accounting of group time
  when a queue expires. This time used decides which is the group to go
  next.

o Also introduce the functionlity to save and restore the workload type
  context with-in group. It might happen that once we expire the cfq queue
  and group, a different group will schedule in and we will lose the context
  of the workload type. Hence save and restore it upon queue expiry.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-12-03 19:28:52 +01:00
Vivek Goyal
58ff82f34c blkio: Implement per cfq group latency target and busy queue avg
o So far we had 300ms soft target latency system wide. Now with the
  introduction of cfq groups, divide that latency by number of groups so
  that one can come up with group target latency which will be helpful
  in determining the workload slice with-in group and also the dynamic
  slice length of the cfq queue.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-12-03 19:28:52 +01:00
Vivek Goyal
25bc6b0776 blkio: Introduce per cfq group weights and vdisktime calculations
o Bring in the per cfq group weight and how vdisktime is calculated for the
  group. Also bring in the functionality of updating the min_vdisktime of
  the group service tree.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-12-03 19:28:52 +01:00
Vivek Goyal
1fa8f6d68b blkio: Introduce the root service tree for cfq groups
o So far we just had one cfq_group in cfq_data. To create space for more than
  one cfq_group, we need to have a service tree of groups where all the groups
  can be queued if they have active cfq queues backlogged in these.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-12-03 19:28:51 +01:00
Vivek Goyal
f04a642463 blkio: Keep queue on service tree until we expire it
o Currently cfqq deletes a queue from service tree if it is empty (even if
  we might idle on the queue). This patch keeps the queue on service tree
  hence associated group remains on the service tree until we decide that
  we are not going to idle on the queue and expire it.

o This just helps in time accounting for queue/group and in implementation
  of rest of the patches.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-12-03 19:28:51 +01:00
Vivek Goyal
615f0259e6 blkio: Implement macro to traverse each service tree in group
o Implement a macro to traverse each service tree in the group. This avoids
  usage of double for loop and special condition for idle tree 4 times.

o Macro is little twisted because of special handling of idle class service
  tree.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-12-03 19:28:51 +01:00
Vivek Goyal
cdb16e8f73 blkio: Introduce the notion of cfq groups
o This patch introduce the notion of cfq groups. Soon we will can have multiple
  groups of different weights in the system.

o Various service trees (prioclass and workload type trees), will become per
  cfq group. So hierarchy looks as follows.

			cfq_groups
			   |
			workload type
			   |
		        cfq queue

o When an scheduling decision has to be taken, first we select the cfq group
  then workload with-in the group and then cfq queue with-in the workload
  type.

o This patch just makes various workload service tree per cfq group and
  introduce the function to be able to choose a group for scheduling.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-12-03 19:28:51 +01:00
Vivek Goyal
bf79193710 blkio: Set must_dispatch only if we decided to not dispatch the request
o must_dispatch flag should be set only if we decided not to run the queue
  and dispatch the request.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-12-03 19:28:51 +01:00
Shaohua Li
474b18ccc2 cfq-iosched: no dispatch limit for single queue
Since commit 2f5cb7381b, each queue can send
up to 4 * 4 requests if only one queue exists. I wonder why we have such limit.
Device supports tag can send more requests. For example, AHCI can send 31
requests. Test (direct aio randread) shows the limits reduce about 4% disk
thoughput.
On the other hand, since we send one request one time, if other queue
pop when current is sending more than cfq_quantum requests, current queue will
stop send requests soon after one request, so sounds there is no big latency.

Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-12-03 12:58:05 +01:00
Jens Axboe
464191c65b Revert "cfq: Make use of service count to estimate the rb_key offset"
This reverts commit 3586e917f2.

Corrado Zoccolo <czoccolo@gmail.com> correctly points out, that we need
consistency of rb_key offset across groups. This means we cannot properly
use the per-service_tree service count. Revert this change.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-11-30 09:38:13 +01:00
Corrado Zoccolo
8e550632cc cfq-iosched: fix corner cases in idling logic
Idling logic was disabled in some corner cases, leading to unfair share
 for noidle queues.
 * the idle timer was not armed if there were other requests in the
   driver. unfortunately, those requests could come from other workloads,
   or queues for which we don't enable idling. So we will check only
   pending requests from the active queue
 * rq_noidle check on no-idle queue could disable the end of tree idle if
   the last completed request was rq_noidle. Now, we will disable that
   idle only if all the queues served in the no-idle tree had rq_noidle
   requests.

Reported-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Corrado Zoccolo <czoccolo@gmail.com>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-11-26 10:39:31 +01:00
Corrado Zoccolo
76280aff1c cfq-iosched: idling on deep seeky sync queues
Seeky sync queues with large depth can gain unfairly big share of disk
 time, at the expense of other seeky queues. This patch ensures that
 idling will be enabled for queues with I/O depth at least 4, and small
 think time. The decision to enable idling is sticky, until an idle
 window times out without seeing a new request.

The reasoning behind the decision is that, if an application is using
large I/O depth, it is already optimized to make full utilization of
the hardware, and therefore we reserve a slice of exclusive use for it.

Reported-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Corrado Zoccolo <czoccolo@gmail.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-11-26 10:39:31 +01:00
Corrado Zoccolo
e4a229196a cfq-iosched: fix no-idle preemption logic
An incoming no-idle queue should preempt the active no-idle queue
 only if the active queue is idling due to service tree empty.
 Previous code was buggy in two ways:
 * it relied on service_tree field to be set on the active queue, while
   it is not set when the code is idling for a new request
 * it didn't check for the service tree empty condition, so could lead to
   LIFO behaviour if multiple queues with depth > 1 were preempting each
   other on an non-NCQ device.

Reported-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Corrado Zoccolo <czoccolo@gmail.com>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-11-26 10:39:31 +01:00
Corrado Zoccolo
e459dd08f4 cfq-iosched: fix ncq detection code
CFQ's detection of queueing devices initially assumes a queuing device
and detects if the queue depth reaches a certain threshold.
However, it will reconsider this choice periodically.

Unfortunately, if device is considered not queuing, CFQ will force a
unit queue depth for some workloads, thus defeating the detection logic.
This leads to poor performance on queuing hardware,
since the idle window remains enabled.

Given this premise, switching to hw_tag = 0 after we have proved at
least once that the device is NCQ capable is not a good choice.

The new detection code starts in an indeterminate state, in which CFQ behaves
as if hw_tag = 1, and then, if for a long observation period we never saw
large depth, we switch to hw_tag = 0, otherwise we stick to hw_tag = 1,
without reconsidering it again.

Signed-off-by: Corrado Zoccolo <czoccolo@gmail.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-11-26 10:02:57 +01:00
Corrado Zoccolo
c16632bab1 cfq-iosched: cleanup unreachable code
cfq_should_idle returns false for no-idle queues that are not the last,
so the control flow will never reach the removed code in a state that
satisfies the if condition.
The unreachable code was added to emulate previous cfq behaviour for
non-NCQ rotational devices. My tests show that even without it, the
performances and fairness are comparable with previous cfq, thanks to
the fact that all seeky queues are grouped together, and that we idle at
the end of the tree.

Signed-off-by: Corrado Zoccolo <czoccolo@gmail.com>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-11-26 09:46:46 +01:00
Gui Jianfeng
3586e917f2 cfq: Make use of service count to estimate the rb_key offset
For the moment, different workload cfq queues are put into different
service trees. But CFQ still uses "busy_queues" to estimate rb_key
offset when inserting a cfq queue into a service tree. I think this
isn't appropriate, and it should make use of service tree count to do
this estimation. This patch is for for-2.6.33 branch.

Signed-off-by: Gui Jianfeng <guijianfeng@cn.fujitsu.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-11-26 09:14:11 +01:00
Randy Dunlap
ad5ebd2fa2 block: jiffies fixes
Use HZ-independent calculation of milliseconds.
Add jiffies.h where it was missing since functions or macros
from it are used.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-11-11 13:47:45 +01:00
Corrado Zoccolo
cf7c25cf91 cfq-iosched: fix next_rq computation
Cfq has a bug in computation of next_rq, that affects transition
between multiple sequential request streams in a single queue
(e.g.: two sequential buffered writers of the same priority),
causing the alternation between the two streams for a transient period.

  8,0    1    18737     0.260400660  5312  D   W 141653311 + 256
  8,0    1    20839     0.273239461  5400  D   W 141653567 + 256
  8,0    1    20841     0.276343885  5394  D   W 142803919 + 256
  8,0    1    20843     0.279490878  5394  D   W 141668927 + 256
  8,0    1    20845     0.292459993  5400  D   W 142804175 + 256
  8,0    1    20847     0.295537247  5400  D   W 141668671 + 256
  8,0    1    20849     0.298656337  5400  D   W 142804431 + 256
  8,0    1    20851     0.311481148  5394  D   W 141668415 + 256
  8,0    1    20853     0.314421305  5394  D   W 142804687 + 256
  8,0    1    20855     0.318960112  5400  D   W 142804943 + 256

The fix makes sure that the next_rq is computed from the last
dispatched request, and not affected by merging.

  8,0    1    37776     4.305161306     0  D   W 141738087 + 256
  8,0    1    37778     4.308298091     0  D   W 141738343 + 256
  8,0    1    37780     4.312885190     0  D   W 141738599 + 256
  8,0    1    37782     4.315933291     0  D   W 141738855 + 256
  8,0    1    37784     4.319064459     0  D   W 141739111 + 256
  8,0    1    37786     4.331918431  5672  D   W 142803007 + 256
  8,0    1    37788     4.334930332  5672  D   W 142803263 + 256
  8,0    1    37790     4.337902723  5672  D   W 142803519 + 256
  8,0    1    37792     4.342359774  5672  D   W 142803775 + 256
  8,0    1    37794     4.345318286     0  D   W 142804031 + 256

Signed-off-by: Corrado Zoccolo <czoccolo@gmail.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-11-08 17:16:46 +01:00
Jens Axboe
e00ef79971 cfq-iosched: get rid of the coop_preempt flag
We need to rework this logic post the cooperating cfq_queue merging,
for now just get rid of it and Jeff Moyer will fix the fall out.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-11-04 08:54:55 +01:00
Jens Axboe
125c4f221a cfq-iosched: fix merge error
We ended up with testing the same condition twice, pretty
pointless. Remove that first if.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-11-03 21:25:45 +01:00
Jens Axboe
2058297d2d Merge branch 'for-linus' into for-2.6.33
Conflicts:
	block/cfq-iosched.c

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-11-03 21:14:39 +01:00
Jens Axboe
150e6c67f4 Merge branch 'cfq-2.6.33' into for-2.6.33 2009-11-03 21:12:10 +01:00
Shaohua Li
4b27e1bb44 cfq-iosched: limit coop preemption
CFQ has an optimization for cooperated applications. if several
io-context have close requests, they will get boost. But the
optimization get abused. Considering thread a, b, which work on one
file. a reads sectors s, s+2, s+4, ...; b reads sectors s+1, s+3, s
+5, ... Both a and b are sequential read, so they can open idle window.
a reads a sector s and goes to idle window and wakeup b. b reads sector
s+1, since in current implementation, cfq_should_preempt() thinks a and
b are cooperators, b will preempt a. b then reads sector s+1 and goes to
idle window and wakeup a. for the same reason, a will preempt b and
reads s+2. a and b will continue the circle. The circle will be very
long, and a and b will occupy whole disk queue. Other applications will
nearly have no chance to run.

Fix this limiting coop preempt until a queue is scheduled normally
again.

Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Acked-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-11-03 20:25:02 +01:00
Jens Axboe
e6ec4fe245 cfq-iosched: fix bad return value cfq_should_preempt()
Commit a6151c3a5c inadvertently reversed
a preempt condition check, potentially causing a performance regression.
Make the meta check correct again.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-11-03 20:21:35 +01:00
Corrado Zoccolo
dddb74519a cfq-iosched: simplify prio-unboost code
Eliminate redundant checks.

Signed-off-by: Corrado Zoccolo <czoccolo@gmail.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-11-02 10:40:37 +01:00
Jens Axboe
5869619cb5 cfq-iosched: fix style issue in cfq_get_avg_queues()
Line breaks and bad brace placement.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-10-28 09:27:07 +01:00
Corrado Zoccolo
718eee0579 cfq-iosched: fairness for sync no-idle queues
Currently no-idle queues in cfq are not serviced fairly:
even if they can only dispatch a small number of requests at a time,
they have to compete with idling queues to be serviced, experiencing
large latencies.

We should notice, instead, that no-idle queues are the ones that would
benefit most from having low latency, in fact they are any of:
* processes with large think times (e.g. interactive ones like file
  managers)
* seeky (e.g. programs faulting in their code at startup)
* or marked as no-idle from upper levels, to improve latencies of those
  requests.

This patch improves the fairness and latency for those queues, by:
* separating sync idle, sync no-idle and async queues in separate
  service_trees, for each priority
* service all no-idle queues together
* and idling when the last no-idle queue has been serviced, to
  anticipate for more no-idle work
* the timeslices allotted for idle and no-idle service_trees are
  computed proportionally to the number of processes in each set.

Servicing all no-idle queues together should have a performance boost
for NCQ-capable drives, without compromising fairness.

Signed-off-by: Corrado Zoccolo <czoccolo@gmail.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-10-28 09:23:26 +01:00
Corrado Zoccolo
a6d44e982d cfq-iosched: enable idling for last queue on priority class
cfq can disable idling for queues in various circumstances.
When workloads of different priorities are competing, if the higher
priority queue has idling disabled, lower priority queues may steal
its disk share. For example, in a scenario with an RT process
performing seeky reads vs a BE process performing sequential reads,
on an NCQ enabled hardware, with low_latency unset,
the RT process will dispatch only the few pending requests every full
slice of service for the BE process.

The patch solves this issue by always performing idle on the last
queue at a given priority class > idle. If the same process, or one
that can pre-empt it (so at the same priority or higher), submits a
new request within the idle window, the lower priority queue won't
dispatch, saving the disk bandwidth for higher priority ones.

Note: this doesn't touch the non_rotational + NCQ case (no hardware
to test if this is a benefit in that case).

Signed-off-by: Corrado Zoccolo <czoccolo@gmail.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-10-28 09:23:26 +01:00
Corrado Zoccolo
c0324a020e cfq-iosched: reimplement priorities using different service trees
We use different service trees for different priority classes.
This allows a simplification in the service tree insertion code, that no
longer has to consider priority while walking the tree.

Signed-off-by: Corrado Zoccolo <czoccolo@gmail.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-10-28 09:23:26 +01:00
Corrado Zoccolo
aa6f6a3de1 cfq-iosched: preparation to handle multiple service trees
We embed a pointer to the service tree in each queue, to handle multiple
service trees easily.
Service trees are enriched with a counter.
cfq_add_rq_rb is invoked after putting the rq in the fifo, to ensure
that all fields in rq are properly initialized.

Signed-off-by: Corrado Zoccolo <czoccolo@gmail.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-10-28 09:23:26 +01:00
Corrado Zoccolo
5db5d64277 cfq-iosched: adapt slice to number of processes doing I/O
When the number of processes performing I/O concurrently increases,
a fixed time slice per process will cause large latencies.

This patch, if low_latency mode is enabled,  will scale the time slice
assigned to each process according to a 300ms target latency.

In order to keep fairness among processes:
* The number of active processes is computed using a special form of
running average, that quickly follows sudden increases (to keep latency low),
and decrease slowly (to have fairness in spite of rapid decreases of this
value).

To safeguard sequential bandwidth, we impose a minimum time slice
(computed using 2*cfq_slice_idle as base, adjusted according to priority
and async-ness).

Signed-off-by: Corrado Zoccolo <czoccolo@gmail.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-10-28 09:23:26 +01:00
Shaohua Li
1a1238a7dd cfq-iosched: improve hw_tag detection
If active queue hasn't enough requests and idle window opens, cfq will not
dispatch sufficient requests to hardware. In such situation, current code
will zero hw_tag. But this is because cfq doesn't dispatch enough requests
instead of hardware queue doesn't work. Don't zero hw_tag in such case.

Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-10-27 08:46:23 +01:00
Jeff Moyer
e6c5bc737a cfq: break apart merged cfqqs if they stop cooperating
cfq_queues are merged if they are issuing requests within the mean seek
distance of one another.  This patch detects when the coopearting stops and
breaks the queues back up.

Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
2009-10-26 14:34:47 +01:00