lkml.org 
[lkml]   [2009]   [Aug]   [16]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    From
    Subject[RFC] IO scheduler based IO controller V8
    Date

    Hi All,

    Here is the V8 of the IO controller patches generated on top of 2.6.31-rc6.

    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


    \
     
     \ /
      Last update: 2009-08-16 21:57    [W:4.235 / U:0.240 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site