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[dm-devel] Re: [RFC] IO scheduler based IO controller V7



Hi Vivek,

Here are some test results for normal reads and write for IO Controller V7 by fio.
Tested with "fairness == 0". It seems performance gets better comparing with V6.

Mode         Normal read   |   Random read   |   Normal write   |   Random write  |  Direct read  |  Direct Write

2.6.31-rc1   71,613KiB/s       3,606KiB/s        66,250KiB/s        9,420KiB/s       51,535KiB/s     55,752KiB/s

V7           70,540KiB/s       3,551KiB/s        64,548KiB/s        9,677KiB/s       53,530KiB/s     54,145KiB/s

Performance  -1.5%             -1.5%             -2.6%              +2.7%            +3.9%           -2.9%


Vivek Goyal wrote:
> Hi All,
> 
> Here is the V7 of the IO controller patches generated on top of 2.6.31-rc4.
> 
> For ease of patching, a consolidated patch is available here.
> 
> http://people.redhat.com/~vgoyal/io-controller/io-scheduler-based-io-controller-v7.patch
> 
> 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
> 
> Changes from V6
> ===============
> - Introduced the notion of group_idling where we idle for next request to
>   come from the same group before we expire it. It is along the lines of
>   cfq's slice_idle thing to provide fairness. Switching to group idling
>   now helps in the sense that we don't have to rely whether queue idling
>   was turned on or not by CFQ. It becomes too much of debugging pain with
>   different work loads and different kind of storage media. Introduction
>   of group_idle should help.
> 
> - Moved some of the code like dynamic queue idling update, arming queue
>   idling timer, keeping track of average think time etc back to CFQ. With
>   group idling we don't need it now. Reduce the amount of change.
> 
> - Enabled cfq's close cooperator functionality in groups. So far this worked
>   only in root group. Now it should work in non-root groups also.
> 
> - Got rid of the patch where we calculated disk time based on average disk
>   rate in some circumstances. It was giving bad numbers in early queue
>   deletion cases. Also did not think that it was helping a lot. Remvoed it
>   for the time being.
>  
> - Added an experimental patch to map sync requests using bio tracking info and
>   not task context. This is only for noop, deadline and AS.
> 
> - Got rid of experimental patch of idling for async queues. Don't think it
>   was helping.
> 
> - Got rid of wait_busy and wait_busy_done logic from queue. Instead
>   implemented it for groups.
> 
> - Introduced oom_ioq to accomodate oom_cfqq change recently.
> 
> - Broke-up elv_init_ioq() fn into smaller functions. It had 7 arguments and
>   looked complicated.
> 
> - Fixed a bug in blk_queue_io_group_congested(). Thanks to Munehiro Ikeda.
> 
> - Merged gui's patch to fix the cgroup file format issue.
> 
> - Merged gui's patch to update per group congestion limit when
>   q->nr_group_requests is updated.
> 
> - Fixed a bug where close cooperation will not work if we wait for all the
>   requests to finish from previous queue.
> 
> - Fixed group deletion accouting where deletion from idle tree were also
>   appearing in the log.
> 
> - Got rid of busy_rt_queues infrastructure.
> 
> - Got rid of elv_ioq_request_dispatched(). An helper function just to
>   increment a variable.
>   
> 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 &
> 
>   234179072 bytes (234 MB) copied, 3.9065 s, 59.9 MB/s
>   234179072 bytes (234 MB) copied, 5.19232 s, 45.1 MB/s
> 
>   group1 time=8 16 2471 group1 sectors=8 16 457840
>   group2 time=8 16 1220 group2 sectors=8 16 225736
> 
> 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.dis_time and io.disk_group 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 $!
> # Some code for reading cgroup files upon completion of reader.
> -------------------------
> 
> Results
> =======
> 268435456 bytes (268 MB) copied, 6.65819 s, 40.3 MB/s (Reader) 
> 
> group1 time=8 16 3063	group1 sectors=8 16 524808
> group2 time=8 16 3071	group2 sectors=8 16 441752
> 
> Note, reader finishes now much lesser time and both group1 and group2
> got almost 3 seconds of disk time. Hence io-controller provides isolation
> from buffered writes.
> 
> Test4 (AIO)
> ===========
> 
> AIO reads
> -----------
> Set up two fio, AIO read jobs in two cgroup with weight 1000 and 500
> respectively. I am using cfq scheduler. Following are some lines from my test
> script.
> 
> ---------------------------------------------------------------
> echo 1000 > /cgroup/bfqio/test1/io.weight
> echo 500 > /cgroup/bfqio/test2/io.weight
> 
> fio_args="--ioengine=libaio --rw=read --size=512M --direct=1"
> echo 1 > /sys/block/$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/$BLOCKDEV/fio2/test2.log &
> ----------------------------------------------------------------
> 
> test1 and test2 are two groups with weight 1000 and 500 respectively.
> "read-and-display-group-stats.sh" is one small script which reads the
> test1 and test2 cgroup files to determine how much disk time each group
> got till first fio job finished.
> 
> Results
> ------
> test1 statistics: time=8 16 22403   sectors=8 16 1049640
> test2 statistics: time=8 16 11400   sectors=8 16 552864
> 
> Above shows that by the time first fio (higher weight), finished, group
> test1 got 22403 ms of disk time and group test2 got 11400 ms of disk time.
> similarly the statistics for number of sectors transferred are also shown.
> 
> Note that disk time given to group test1 is almost double of group2 disk
> time.
> 
> AIO writes
> ----------
> Set up two fio, AIO direct write jobs in two cgroup with weight 1000 and 500
> respectively. I am using cfq scheduler. Following are some lines from my test
> script.
> 
> ------------------------------------------------
> echo 1000 > /cgroup/bfqio/test1/io.weight
> echo 500 > /cgroup/bfqio/test2/io.weight
> fio_args="--ioengine=libaio --rw=write --size=512M --direct=1"
> 
> echo 1 > /sys/block/$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 29085   sectors=8 16 1049656
> test2 statistics: time=8 16 14652   sectors=8 16 516728
> 
> Above shows that by the time first fio (higher weight), finished, group
> test1 got 28085 ms of disk time and group test2 got 14652 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.
> 
> 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 48 1315   sectors=8 48 55776 dq=8 48 1
> test2 statistics: time=8 48 633   sectors=8 48 14720 dq=8 48 2
> 
> test1 statistics: time=8 48 5586   sectors=8 48 339064 dq=8 48 2
> test2 statistics: time=8 48 2985   sectors=8 48 146656 dq=8 48 3
> 
> test1 statistics: time=8 48 9935   sectors=8 48 628728 dq=8 48 3
> test2 statistics: time=8 48 5265   sectors=8 48 278688 dq=8 48 4
> 
> test1 statistics: time=8 48 14156   sectors=8 48 932488 dq=8 48 6
> test2 statistics: time=8 48 7646   sectors=8 48 412704 dq=8 48 7
> 
> test1 statistics: time=8 48 18141   sectors=8 48 1231488 dq=8 48 10
> test2 statistics: time=8 48 9820   sectors=8 48 548400 dq=8 48 8
> 
> test1 statistics: time=8 48 21953   sectors=8 48 1485632 dq=8 48 13
> test2 statistics: time=8 48 12394   sectors=8 48 698288 dq=8 48 10
> 
> test1 statistics: time=8 48 25167   sectors=8 48 1705264 dq=8 48 13
> test2 statistics: time=8 48 14042   sectors=8 48 817808 dq=8 48 10
> 
> 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.
> 
> Your feedback is welcome.
> 
> Thanks
> Vivek
> 
> 
> 

-- 
Regards
Gui Jianfeng


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