[Devel] Re: [RFC] IO scheduler based IO controller V8
Vivek Goyal
vgoyal at redhat.com
Sun Aug 16 12:53:02 PDT 2009
On Sun, Aug 16, 2009 at 03:30:22PM -0400, Vivek Goyal wrote:
>
> Hi All,
>
> Here is the V8 of the IO controller patches generated on top of 2.6.31-rc6.
>
Forgot to mention that for ease of patching a consolidated patch is here.
http://people.redhat.com/~vgoyal/io-controller/io-scheduler-based-io-controller-v8.patch
Thanks
Vivek
> Previous versions of the patches was posted here.
>
> (V1) http://lkml.org/lkml/2009/3/11/486
> (V2) http://lkml.org/lkml/2009/5/5/275
> (V3) http://lkml.org/lkml/2009/5/26/472
> (V4) http://lkml.org/lkml/2009/6/8/580
> (V5) http://lkml.org/lkml/2009/6/19/279
> (V6) http://lkml.org/lkml/2009/7/2/369
> (V7) http://lkml.org/lkml/2009/7/24/253
>
> Changes from V7
> ===============
> - Replaced BFQ with CFS+CFQ like hierarchical scheduler.
>
> Moving to time domain as service parameter had broken BFQ's assumptions
> about how long a queue runs (queue can run more than budget) and that in
> turn has potential to break the O(1) gurantees of BFQ.
>
> In addition, BFQ was relatively complex and not sure if benefits were
> proportionate in time domain setup. Hence for the time being trying to
> replace BFQ with a simpler scheduler and see how well does it perform.
>
> This scheduler borrows the ideas from CFS and CFQ. Time slices to queues are
> allocated based on their priority (like CFQ). These disk times are converted
> to virtual disk time and we keep track of each queue's vdisktime and each
> service tree's min_vdisktime to determine who has consumed how much disk
> time and who should run next (like CFS).
>
> - Fixed few issues reported by Jerome Marchand.
>
> Apart from this there are miscellaneous cleaups like getting rid of not so
> necessary comments, function renames, debug code re-organization etc.
>
> Limitations
> ===========
>
> - This IO controller provides the bandwidth control at the IO scheduler
> level (leaf node in stacked hiearchy of logical devices). So there can
> be cases (depending on configuration) where application does not see
> proportional BW division at higher logical level device.
>
> LWN has written an article about the issue here.
>
> http://lwn.net/Articles/332839/
>
> How to solve the issue of fairness at higher level logical devices
> ==================================================================
> (Do we really need it? That's not where the contention for resources is.)
>
> Couple of suggestions have come forward.
>
> - Implement IO control at IO scheduler layer and then with the help of
> some daemon, adjust the weight on underlying devices dynamiclly, depending
> on what kind of BW gurantees are to be achieved at higher level logical
> block devices.
>
> - Also implement a higher level IO controller along with IO scheduler
> based controller and let user choose one depending on his needs.
>
> A higher level controller does not know about the assumptions/policies
> of unerldying IO scheduler, hence it has the potential to break down
> the IO scheduler's policy with-in cgroup. A lower level controller
> can work with IO scheduler much more closely and efficiently.
>
> Other active IO controller developments
> =======================================
>
> IO throttling
> -------------
>
> This is a max bandwidth controller and not the proportional one. Secondly
> it is a second level controller which can break the IO scheduler's
> policy/assumtions with-in cgroup.
>
> dm-ioband
> ---------
>
> This is a proportional bandwidth controller implemented as device mapper
> driver. It is also a second level controller which can break the
> IO scheduler's policy/assumptions with-in cgroup.
>
> TODO
> ====
> - code cleanups, testing, bug fixing, optimizations, benchmarking etc...
>
> Testing
> =======
>
> I have been able to do some testing as follows. All my testing is with ext3
> file system with a SATA drive which supports queue depth of 31.
>
> Test1 (Isolation between two KVM virtual machines)
> ==================================================
> Created two KVM virtual machines. Partitioned a disk on host in two partitions
> and gave one partition to each virtual machine. Put both the virtual machines
> in two different cgroup of weight 1000 and 500 each. Virtual machines created
> ext3 file system on the partitions exported from host and did buffered writes.
> Host seems writes as synchronous and virtual machine with higher weight gets
> double the disk time of virtual machine of lower weight. Used deadline
> scheduler in this test case.
>
> Some more details about configuration are in documentation patch.
>
> Test2 (Fairness for synchronous reads)
> ======================================
> - Two dd in two cgroups with cgrop weights 1000 and 500. Ran two "dd" in those
> cgroups (With CFQ scheduler and /sys/block/<device>/queue/fairness = 1)
>
> Higher weight dd finishes first and at that point of time my script takes
> care of reading cgroup files io.disk_time and io.disk_sectors for both the
> groups and display the results.
>
> dd if=/mnt/$BLOCKDEV/zerofile1 of=/dev/null &
> dd if=/mnt/$BLOCKDEV/zerofile2 of=/dev/null &
>
> group1 time=8:16 2452 group1 sectors=8:16 457856
> group2 time=8:16 1317 group2 sectors=8:16 247008
>
> 234179072 bytes (234 MB) copied, 3.90912 s, 59.9 MB/s
> 234179072 bytes (234 MB) copied, 5.15548 s, 45.4 MB/s
>
> First two fields in time and sectors statistics represent major and minor
> number of the device. Third field represents disk time in milliseconds and
> number of sectors transferred respectively.
>
> This patchset tries to provide fairness in terms of disk time received. group1
> got almost double of group2 disk time (At the time of first dd finish). These
> time and sectors statistics can be read using io.disk_time and io.disk_sector
> files in cgroup. More about it in documentation file.
>
> Test3 (Reader Vs Buffered Writes)
> ================================
> Buffered writes can be problematic and can overwhelm readers, especially with
> noop and deadline. IO controller can provide isolation between readers and
> buffered (async) writers.
>
> First I ran the test without io controller to see the severity of the issue.
> Ran a hostile writer and then after 10 seconds started a reader and then
> monitored the completion time of reader. Reader reads a 256 MB file. Tested
> this with noop scheduler.
>
> sample script
> ------------
> sync
> echo 3 > /proc/sys/vm/drop_caches
> time dd if=/dev/zero of=/mnt/sdb/reader-writer-zerofile bs=4K count=2097152
> conv=fdatasync &
> sleep 10
> time dd if=/mnt/sdb/256M-file of=/dev/null &
>
> Results
> -------
> 8589934592 bytes (8.6 GB) copied, 106.045 s, 81.0 MB/s (Writer)
> 268435456 bytes (268 MB) copied, 96.5237 s, 2.8 MB/s (Reader)
>
> Now it was time to test io controller whether it can provide isolation between
> readers and writers with noop. I created two cgroups of weight 1000 each and
> put reader in group1 and writer in group 2 and ran the test again. Upon
> comletion of reader, my scripts read io.disk_time and io.disk_sectors cgroup
> files to get an estimate how much disk time each group got and how many
> sectors each group did IO for.
>
> For more accurate accounting of disk time for buffered writes with queuing
> hardware I had to set /sys/block/<disk>/queue/iosched/fairness to "1".
>
> sample script
> -------------
> echo $$ > /cgroup/bfqio/test2/tasks
> dd if=/dev/zero of=/mnt/$BLOCKDEV/testzerofile bs=4K count=2097152 &
> sleep 10
> echo noop > /sys/block/$BLOCKDEV/queue/scheduler
> echo 1 > /sys/block/$BLOCKDEV/queue/iosched/fairness
> echo $$ > /cgroup/bfqio/test1/tasks
> dd if=/mnt/$BLOCKDEV/256M-file of=/dev/null &
> wait $!
> # Some code for reading cgroup files upon completion of reader.
> -------------------------
>
> Results
> =======
> 68435456 bytes (268 MB) copied, 6.87668 s, 39.0 MB/s
>
> group1 time=8:16 3719 group1 sectors=8:16 524816
> group2 time=8:16 3659 group2 sectors=8:16 638712
>
> Note, reader finishes now much lesser time and both group1 and group2
> got almost 3 seconds of disk time. Hence io-controller provides isolation
> from buffered writes.
>
> Test4 (AIO)
> ===========
>
> AIO reads
> -----------
> Set up two fio, AIO read jobs in two cgroup with weight 1000 and 500
> respectively. I am using cfq scheduler. Following are some lines from my test
> script.
>
> ---------------------------------------------------------------
> echo 1000 > /cgroup/bfqio/test1/io.weight
> echo 500 > /cgroup/bfqio/test2/io.weight
>
> fio_args="--ioengine=libaio --rw=read --size=512M --direct=1"
> echo 1 > /sys/block/$BLOCKDEV/queue/iosched/fairness
>
> echo $$ > /cgroup/bfqio/test1/tasks
> fio $fio_args --name=test1 --directory=/mnt/$BLOCKDEV/fio1/
> --output=/mnt/$BLOCKDEV/fio1/test1.log
> --exec_postrun="../read-and-display-group-stats.sh $maj_dev $minor_dev" &
>
> echo $$ > /cgroup/bfqio/test2/tasks
> fio $fio_args --name=test2 --directory=/mnt/$BLOCKDEV/fio2/
> --output=/mnt/$BLOCKDEV/fio2/test2.log &
> ----------------------------------------------------------------
>
> test1 and test2 are two groups with weight 1000 and 500 respectively.
> "read-and-display-group-stats.sh" is one small script which reads the
> test1 and test2 cgroup files to determine how much disk time each group
> got till first fio job finished.
>
> Results
> ------
> test1 statistics: time=8:16 17686 sectors=8:16 1049664
> test2 statistics: time=8:16 9036 sectors=8:16 585152
>
> Above shows that by the time first fio (higher weight), finished, group
> test1 got 17686 ms of disk time and group test2 got 9036 ms of disk time.
> similarly the statistics for number of sectors transferred are also shown.
>
> Note that disk time given to group test1 is almost double of group2 disk
> time.
>
> AIO writes
> ----------
> Set up two fio, AIO direct write jobs in two cgroup with weight 1000 and 500
> respectively. I am using cfq scheduler. Following are some lines from my test
> script.
>
> ------------------------------------------------
> echo 1000 > /cgroup/bfqio/test1/io.weight
> echo 500 > /cgroup/bfqio/test2/io.weight
> fio_args="--ioengine=libaio --rw=write --size=512M --direct=1"
>
> echo 1 > /sys/block/$BLOCKDEV/queue/iosched/fairness
>
> echo $$ > /cgroup/bfqio/test1/tasks
> fio $fio_args --name=test1 --directory=/mnt/$BLOCKDEV/fio1/
> --output=/mnt/$BLOCKDEV/fio1/test1.log
> --exec_postrun="../read-and-display-group-stats.sh $maj_dev $minor_dev" &
>
> echo $$ > /cgroup/bfqio/test2/tasks
> fio $fio_args --name=test2 --directory=/mnt/$BLOCKDEV/fio2/
> --output=/mnt/$BLOCKDEV/fio2/test2.log &
> -------------------------------------------------
>
> test1 and test2 are two groups with weight 1000 and 500 respectively.
> "read-and-display-group-stats.sh" is one small script which reads the
> test1 and test2 cgroup files to determine how much disk time each group
> got till first fio job finished.
>
> Following are the results.
>
> test1 statistics: time=8:16 25509 sectors=8:16 1049688
> test2 statistics: time=8:16 12863 sectors=8:16 527104
>
> Above shows that by the time first fio (higher weight), finished, group
> test1 got almost double the disk time of group test2.
>
> Test5 (Fairness for async writes, Buffered Write Vs Buffered Write)
> ===================================================================
> Fairness for async writes is tricky and biggest reason is that async writes
> are cached in higher layers (page cahe) as well as possibly in file system
> layer also (btrfs, xfs etc), and are dispatched to lower layers not necessarily
> in proportional manner.
>
> For example, consider two dd threads reading /dev/zero as input file and doing
> writes of huge files. Very soon we will cross vm_dirty_ratio and dd thread will
> be forced to write out some pages to disk before more pages can be dirtied. But
> not necessarily dirty pages of same thread are picked. It can very well pick
> the inode of lesser priority dd thread and do some writeout. So effectively
> higher weight dd is doing writeouts of lower weight dd pages and we don't see
> service differentation.
>
> IOW, the core problem with async write fairness is that higher weight thread
> does not throw enought IO traffic at IO controller to keep the queue
> continuously backlogged. In my testing, there are many .2 to .8 second
> intervals where higher weight queue is empty and in that duration lower weight
> queue get lots of job done giving the impression that there was no service
> differentiation.
>
> In summary, from IO controller point of view async writes support is there.
> Because page cache has not been designed in such a manner that higher
> prio/weight writer can do more write out as compared to lower prio/weight
> writer, gettting service differentiation is hard and it is visible in some
> cases and not visible in some cases.
>
> Do we really care that much for fairness among two writer cgroups? One can
> choose to do direct writes or sync writes if fairness for writes really
> matters for him.
>
> Following is the only case where it is hard to ensure fairness between cgroups.
>
> - Buffered writes Vs Buffered Writes.
>
> So to test async writes I created two partitions on a disk and created ext3
> file systems on both the partitions. Also created two cgroups and generated
> lots of write traffic in two cgroups (50 fio threads) and watched the disk
> time statistics in respective cgroups at the interval of 2 seconds. Thanks to
> ryo tsuruta for the test case.
>
> *****************************************************************
> sync
> echo 3 > /proc/sys/vm/drop_caches
>
> fio_args="--size=64m --rw=write --numjobs=50 --group_reporting"
>
> echo $$ > /cgroup/bfqio/test1/tasks
> fio $fio_args --name=test1 --directory=/mnt/sdd1/fio/ --output=/mnt/sdd1/fio/test1.log &
>
> echo $$ > /cgroup/bfqio/test2/tasks
> fio $fio_args --name=test2 --directory=/mnt/sdd2/fio/ --output=/mnt/sdd2/fio/test2.log &
> ***********************************************************************
>
> And watched the disk time and sector statistics for the both the cgroups
> every 2 seconds using a script. How is snippet from output.
>
> test1 statistics: time=8:16 1631 sectors=8:16 1680 dq=8:16 2
> test2 statistics: time=8:16 896 sectors=8:16 976 dq=8:16 1
>
> test1 statistics: time=8:16 6031 sectors=8:16 88536 dq=8:16 5
> test2 statistics: time=8:16 3192 sectors=8:16 4080 dq=8:16 1
>
> test1 statistics: time=8:16 10425 sectors=8:16 390496 dq=8:16 5
> test2 statistics: time=8:16 5272 sectors=8:16 77896 dq=8:16 4
>
> test1 statistics: time=8:16 15396 sectors=8:16 747256 dq=8:16 5
> test2 statistics: time=8:16 7852 sectors=8:16 235648 dq=8:16 4
>
> test1 statistics: time=8:16 20302 sectors=8:16 1180168 dq=8:16 5
> test2 statistics: time=8:16 10297 sectors=8:16 391208 dq=8:16 4
>
> test1 statistics: time=8:16 25244 sectors=8:16 1579928 dq=8:16 6
> test2 statistics: time=8:16 12748 sectors=8:16 613096 dq=8:16 4
>
> test1 statistics: time=8:16 30095 sectors=8:16 1927848 dq=8:16 6
> test2 statistics: time=8:16 15135 sectors=8:16 806112 dq=8:16 4
>
> First two fields in time and sectors statistics represent major and minor
> number of the device. Third field represents disk time in milliseconds and
> number of sectors transferred respectively.
>
> So disk time consumed by group1 is almost double of group2 in this case.
>
> Thanks
> Vivek
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