Hi All, Here is the V5 of the IO controller patches generated on top of 2.6.30. 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 This patchset is still work in progress but I want to keep on getting the snapshot of my tree out at regular intervals to get the feedback hence V5. Changes from V4 =============== - Implemented bdi_*_congested_group() functions to also determine if a particular io group on a bdi is congested or not. So far we only used determine whether bdi is congested or not. But now there is one request list per group and one also needs to check whether the particular io group io is going into is congested or not. - Fixed preemption logic in hiearchical mode. In hiearchical mode, one needs to traverse up the hiearchy so that current queue and new queue are at same level to make a decision whether preeption should be done or not. Took the idea and code from CFS cpu scheduler. - There were some tunables which were appearing under /sys/block/<device>/queue dir but these tunables actually belonged to ioschedulers in hierarhical moded. Fixed it. - Fixed another preemption issue where if any RT queue was pending (busy_rt_queues), current queue was being expired. Now this preemption is done only if there are busy_rt_queues in the same group. (Though I think that busy_rt_queues is redundant code as the moment RT request comes, we preempt the BE queue so we should never run into the issue of RT reuqest pending while BE is running. Keeping the code for the time being). - Applied the patch from Gui where he got rid of only_root_group code and now used cgroups children list to determine if root group is only group or there are childrens too. - Applied few cleanup patches from Gui. - We store the device id (major, minor) in io group. Previously I was retrieving that info from bio. Switched to gettting that info from backing device. 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 ================================================================== 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. Testing ======= I have been able to do only very basic testing of reads and writes. Test1 (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) 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. Test2 (Fairness for async writes) ================================= 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. To get fairness for async writes in all cases, higher layer needs to be fixed. That probably is a lot of work. Do we really care that much for fairness among two writer cgroups? One can choose to do direct IO if fairness for buffered writes really matters for him. I think we care more for fairness in following cases and with this patch we should be able to achive that. - Read Vs Read - Read Vs Writes (Buffered writes or direct IO writes) - Making sure that isolation is achieved between reader and writer cgroup. - All form of direct IO. Following is the only case where it is hard to ensure fairness between cgroups because of higher layer design. - Buffered writes Vs Buffered Writes. So to test async writes I 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. TODO ==== - Lots of code cleanups, testing, bug fixing, optimizations, benchmarking etc... - Work on a better interface (possibly cgroup based) for configuring per group request descriptor limits. - Debug and fix some of the areas like page cache where higher weight cgroup async writes are stuck behind lower weight cgroup async writes. Thanks Vivek -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
