o Documentation for io-controller. Signed-off-by: Vivek Goyal <vgoyal@xxxxxxxxxx> --- Documentation/block/00-INDEX | 2 + Documentation/block/io-controller.txt | 452 +++++++++++++++++++++++++++++++++ 2 files changed, 454 insertions(+), 0 deletions(-) create mode 100644 Documentation/block/io-controller.txt diff --git a/Documentation/block/00-INDEX b/Documentation/block/00-INDEX index 961a051..dc8bf95 100644 --- a/Documentation/block/00-INDEX +++ b/Documentation/block/00-INDEX @@ -10,6 +10,8 @@ capability.txt - Generic Block Device Capability (/sys/block/<disk>/capability) deadline-iosched.txt - Deadline IO scheduler tunables +io-controller.txt + - IO controller for provding hierarchical IO scheduling ioprio.txt - Block io priorities (in CFQ scheduler) request.txt diff --git a/Documentation/block/io-controller.txt b/Documentation/block/io-controller.txt new file mode 100644 index 0000000..82804ad --- /dev/null +++ b/Documentation/block/io-controller.txt @@ -0,0 +1,452 @@ + IO Controller + ============= + +Overview +======== + +This patchset implements a proportional weight IO controller. That is one +can create cgroups and assign prio/weights to those cgroups and task group +will get access to disk proportionate to the weight of the group. + +These patches modify elevator layer and individual IO schedulers to do +IO control hence this io controller works only on block devices which use +one of the standard io schedulers can not be used with any xyz logical block +device. + +The assumption/thought behind modifying IO scheduler is that resource control +is primarily needed on leaf nodes where the actual contention for resources is +present and not on intertermediate logical block devices. + +Consider following hypothetical scenario. Lets say there are three physical +disks, namely sda, sdb and sdc. Two logical volumes (lv0 and lv1) have been +created on top of these. Some part of sdb is in lv0 and some part is in lv1. + + lv0 lv1 + / \ / \ + sda sdb sdc + +Also consider following cgroup hierarchy + + root + / \ + A B + / \ / \ + T1 T2 T3 T4 + +A and B are two cgroups and T1, T2, T3 and T4 are tasks with-in those cgroups. +Assuming T1, T2, T3 and T4 are doing IO on lv0 and lv1. These tasks should +get their fair share of bandwidth on disks sda, sdb and sdc. There is no +IO control on intermediate logical block nodes (lv0, lv1). + +So if tasks T1 and T2 are doing IO on lv0 and T3 and T4 are doing IO on lv1 +only, there will not be any contetion for resources between group A and B if +IO is going to sda or sdc. But if actual IO gets translated to disk sdb, then +IO scheduler associated with the sdb will distribute disk bandwidth to +group A and B proportionate to their weight. + +CFQ already has the notion of fairness and it provides differential disk +access based on priority and class of the task. Just that it is flat and +with cgroup stuff, it needs to be made hierarchical to achive a good +hierarchical control on IO. + +Rest of the IO schedulers (noop, deadline and AS) don't have any notion +of fairness among various threads. They maintain only one queue where all +the IO gets queued (internally this queue is split in read and write queue +for deadline and AS). With this patchset, now we maintain one queue per +cgropu per device and then try to do fair queuing among those queues. + +One of the concerns raised with modifying IO schedulers was that we don't +want to replicate the code in all the IO schedulers. These patches share +the fair queuing code which has been moved to a common layer (elevator +layer). Hence we don't end up replicating code across IO schedulers. Following +diagram depicts the concept. + + -------------------------------- + | Elevator Layer + Fair Queuing | + -------------------------------- + | | | | + NOOP DEADLINE AS CFQ + +Design +====== +This patchset primarily uses BFQ (Budget Fair Queuing) code to provide +fairness among different IO queues. Fabio and Paolo implemented BFQ which uses +B-WF2Q+ algorithm for fair queuing. + +Why BFQ? + +- Not sure if weighted round robin logic of CFQ can be easily extended for + hierarchical mode. One of the things is that we can not keep dividing + the time slice of parent group among childrens. Deeper we go in hierarchy + time slice will get smaller. + + One of the ways to implement hierarchical support could be to keep track + of virtual time and service provided to queue/group and select a queue/group + for service based on any of the various available algoriths. + + BFQ already had support for hierarchical scheduling, taking those patches + was easier. + +- BFQ was designed to provide tighter bounds/delay w.r.t service provided + to a queue. Delay/Jitter with BFQ is O(1). + + Note: BFQ originally used amount of IO done (number of sectors) as notion + of service provided. IOW, it tried to provide fairness in terms of + actual IO done and not in terms of actual time disk access was + given to a queue. + + This patcheset modified BFQ to provide fairness in time domain because + that's what CFQ does. So idea was try not to deviate too much from + the CFQ behavior initially. + + Providing fairness in time domain makes accounting trciky because + due to command queueing, at one time there might be multiple requests + from different queues and there is no easy way to find out how much + disk time actually was consumed by the requests of a particular + queue. More about this in comments in source code. + +We have taken BFQ code as starting point for providing fairness among groups +because it already contained lots of features which we required to implement +hierarhical IO scheduling. With this patch set, I am not trying to ensure O(1) +delay here as my goal is to provide fairness among groups. Most likely that +will mean that latencies are not worse than what cfq currently provides (if +not improved ones). Once fairness is ensured, one can look into more in +ensuring O(1) latencies. + +From data structure point of view, one can think of a tree per device, where +io groups and io queues are hanging and are being scheduled using B-WF2Q+ +algorithm. io_queue, is end queue where requests are actually stored and +dispatched from (like cfqq). + +These io queues are primarily created by and managed by end io schedulers +depending on its semantics. For example, noop, deadline and AS ioschedulers +keep one io queues per cgroup and cfqq keeps one io queue per io_context in +a cgroup (apart from async queues). + +A request is mapped to an io group by elevator layer and which io queue it +is mapped to with in group depends on ioscheduler. Currently "current" task +is used to determine the cgroup (hence io group) of the request. Down the +line we need to make use of bio-cgroup patches to map delayed writes to +right group. + +Going back to old behavior +========================== +In new scheme of things essentially we are creating hierarchical fair +queuing logic in elevator layer and chaning IO schedulers to make use of +that logic so that end IO schedulers start supporting hierarchical scheduling. + +Elevator layer continues to support the old interfaces. So even if fair queuing +is enabled at elevator layer, one can have both new hierchical scheduler as +well as old non-hierarchical scheduler operating. + +Also noop, deadline and AS have option of enabling hierarchical scheduling. +If it is selected, fair queuing is done in hierarchical manner. If hierarchical +scheduling is disabled, noop, deadline and AS should retain their existing +behavior. + +CFQ is the only exception where one can not disable fair queuing as it is +needed for provding fairness among various threads even in non-hierarchical +mode. + +Various user visible config options +=================================== +CONFIG_IOSCHED_NOOP_HIER + - Enables hierchical fair queuing in noop. Not selecting this option + leads to old behavior of noop. + +CONFIG_IOSCHED_DEADLINE_HIER + - Enables hierchical fair queuing in deadline. Not selecting this + option leads to old behavior of deadline. + +CONFIG_IOSCHED_AS_HIER + - Enables hierchical fair queuing in AS. Not selecting this option + leads to old behavior of AS. + +CONFIG_IOSCHED_CFQ_HIER + - Enables hierarchical fair queuing in CFQ. Not selecting this option + still does fair queuing among various queus but it is flat and not + hierarchical. + +CGROUP_BLKIO + - This option enables blkio-cgroup controller for IO tracking + purposes. That means, by this controller one can attribute a write + to the original cgroup and not assume that it belongs to submitting + thread. + +CONFIG_TRACK_ASYNC_CONTEXT + - Currently CFQ attributes the writes to the submitting thread and + caches the async queue pointer in the io context of the process. + If this option is set, it tells cfq and elevator fair queuing logic + that for async writes make use of IO tracking patches and attribute + writes to original cgroup and not to write submitting thread. + + This should be primarily useful when lots of asynchronous writes + are being submitted by pdflush threads and we need to assign the + writes to right group. + +CONFIG_DEBUG_GROUP_IOSCHED + - Throws extra debug messages in blktrace output helpful in doing + doing debugging in hierarchical setup. + + - Also allows for export of extra debug statistics like group queue + and dequeue statistics on device through cgroup interface. + +Config options selected automatically +===================================== +These config options are not user visible and are selected/deselected +automatically based on IO scheduler configurations. + +CONFIG_ELV_FAIR_QUEUING + - Enables/Disables the fair queuing logic at elevator layer. + +CONFIG_GROUP_IOSCHED + - Enables/Disables hierarchical queuing and associated cgroup bits. + +HOWTO +===== +You can do a very simple testing of running two dd threads in two different +cgroups. Here is what you can do. + +- Enable hierarchical scheduling in io scheuduler of your choice (say cfq). + CONFIG_IOSCHED_CFQ_HIER=y + +- Enable IO tracking for async writes. + CONFIG_TRACK_ASYNC_CONTEXT=y + + (This will automatically select CGROUP_BLKIO) + +- Compile and boot into kernel and mount IO controller and blkio io tracking + controller. + + mount -t cgroup -o io,blkio none /cgroup + +- Create two cgroups + mkdir -p /cgroup/test1/ /cgroup/test2 + +- Set weights of group test1 and test2 + echo 1000 > /cgroup/test1/io.weight + echo 500 > /cgroup/test2/io.weight + +- Set "fairness" parameter to 1 at the disk you are testing. + + echo 1 > /sys/block/<disk>/queue/iosched/fairness + +- Create two same size files (say 512MB each) on same disk (file1, file2) and + launch two dd threads in different cgroup to read those files. Make sure + right io scheduler is being used for the block device where files are + present (the one you compiled in hierarchical mode). + + sync + echo 3 > /proc/sys/vm/drop_caches + + dd if=/mnt/sdb/zerofile1 of=/dev/null & + echo $! > /cgroup/test1/tasks + cat /cgroup/test1/tasks + + dd if=/mnt/sdb/zerofile2 of=/dev/null & + echo $! > /cgroup/test2/tasks + cat /cgroup/test2/tasks + +- At macro level, first dd should finish first. To get more precise data, keep + on looking at (with the help of script), at io.disk_time and io.disk_sectors + files of both test1 and test2 groups. This will tell how much disk time + (in milli seconds), each group got and how many secotors each group + dispatched to the disk. We provide fairness in terms of disk time, so + ideally io.disk_time of cgroups should be in proportion to the weight. + +Some High Level Test setups +=========================== +One of the use cases of IO controller is to provide some kind of IO isolation +between multiple virtual machines on the same host. Following is one +example setup which worked for me. + + + KVM KVM + Guest1 Guest2 + --------- ---------- + | ----- | | ------ | + | | vdb | | | | vdb | | + | ----- | | ------ | + --------- ---------- + + --------------------------- + | Host | + | ------------- | + | | sdb1 | sdb2 | | + | ------------- | + --------------------------- + +On host machine, I had a spare SATA disk. I created two partitions sdb1 +and sdb2 and gave this partitions as additional storage to kvm guests. sdb1 +to KVM guest1 and sdb2 KVM guest2. These storage appeared as /dev/vdb in +both the guests. Formatted the /dev/vdb and created ext3 file system and +started a 1G file writeout in both the guests. Before writeout I had created +two cgroups of weight 1000 and 500 and put virtual machines in two different +groups. + +Following is write I started in both the guests. + +dd if=/dev/zero of=/mnt/vdb/zerofile1 bs=4K count=262144 conv=fdatasync + +Following are the results on host with "deadline" scheduler. + +group1 time=8:16 17755 group1 sectors=8:16 2104608 +group2 time=8:16 9649 group2 sectors=8:16 1180480 + +Virtual machine with cgroup weight 1000 got almost double the time of virtual +machine with weight 500. + +What Works and What Does not +============================ +Service differentiation at application level can be noticed only if completely +parallel IO paths are created from application to IO scheduler and there +are no serializations introduced by any intermediate layer. For example, +in some cases file system and page cache layer introduce serialization and +we don't see service difference between higher weight and lower weight +process groups. + +For example, when I start an O_SYNC write out on an ext3 file system (file +is being created newly), I see lots of activity from kjournald. I have not +gone into details yet, but my understanding is that there are lot more +journal commits and kjournald kind of introduces serialization between two +processes. So even if you put these two processes in two different cgroups +with different weights, higher weight process will not see more IO done. + +It does work very well when we bypass filesystem layer and IO is raw. For +example in above virtual machine case, host sees raw synchronous writes +coming from two guest machines and filesystem layer at host is not introducing +any kind of serialization hence we can see the service difference. + +It also works very well for reads even on the same file system as for reads +file system journalling activity does not kick in and we can create parallel +IO paths from application to all the way down to IO scheduler and get more +IO done on the IO path with higher weight. + +Regarding "fairness" parameter +============================== +IO controller has introduced a "fairness" tunable for every io scheduler. +Currently this tunable can assume values 0, 1. + +If fairness is set to 1, then IO controller waits for requests to finish from +previous queue before requests from new queue are dispatched. This helps in +doing better accouting of disk time consumed by a queue. If this is not done +then on a queuing hardware, there can be requests from multiple queues and +we will not have any idea which queue consumed how much of disk time. + +Details of cgroup files +======================= +- io.ioprio_class + - Specifies class of the cgroup (RT, BE, IDLE). This is default io + class of the group on all the devices until and unless overridden by + per device rule. (See io.policy). + + 1 = RT; 2 = BE, 3 = IDLE + +- io.weight + - Specifies per cgroup weight. This is default weight of the group + on all the devices until and unless overridden by per device rule. + (See io.policy). + + Currently allowed range of weights is from 1 to 1000. + +- io.disk_time + - disk time allocated to cgroup per device in milliseconds. First + two fields specify the major and minor number of the device and + third field specifies the disk time allocated to group in + milliseconds. + +- io.disk_sectors + - number of sectors transferred to/from disk by the group. First + two fields specify the major and minor number of the device and + third field specifies the number of sectors transferred by the + group to/from the device. + +- io.disk_queue + - Debugging aid only enabled if CONFIG_DEBUG_GROUP_IOSCHED=y. This + gives the statistics about how many a times a group was queued + on service tree of the device. First two fields specify the major + and minor number of the device and third field specifies the number + of times a group was queued on a particular device. + +- io.disk_queue + - Debugging aid only enabled if CONFIG_DEBUG_GROUP_IOSCHED=y. This + gives the statistics about how many a times a group was de-queued + or removed from the service tree of the device. This basically gives + and idea if we can generate enough IO to create continuously + backlogged groups. First two fields specify the major and minor + number of the device and third field specifies the number + of times a group was de-queued on a particular device. + +- io.policy + - One can specify per cgroup per device rules using this interface. + These rules override the default value of group weight and class as + specified by io.weight and io.ioprio_class. + + Following is the format. + + #echo dev_maj:dev_minor weight ioprio_class > /patch/to/cgroup/io.policy + + weight=0 means removing a policy. + + Examples: + + Configure weight=300 ioprio_class=2 on /dev/hdb (8:16) in this cgroup + # echo 8:16 300 2 > io.policy + # cat io.policy + dev weight class + 8:16 300 2 + + Configure weight=500 ioprio_class=1 on /dev/hda (8:0) in this cgroup + # echo 8:0 500 1 > io.policy + # cat io.policy + dev weight class + 8:0 500 1 + 8:16 300 2 + + Remove the policy for /dev/hda in this cgroup + # echo 8:0 0 1 > io.policy + # cat io.policy + dev weight class + 8:16 300 2 + +About configuring request desriptors +==================================== +Traditionally there are 128 request desriptors allocated per request queue +where io scheduler is operating (/sys/block/<disk>/queue/nr_requests). If these +request descriptors are exhausted, processes will put to sleep and woken +up once request descriptors are available. + +With io controller and cgroup stuff, one can not afford to allocate requests +from single pool as one group might allocate lots of requests and then tasks +from other groups might be put to sleep and this other group might be a +higher weight group. Hence to make sure that a group always can get the +request descriptors it is entitled to, one needs to make request descriptor +limit per group on every queue. + +A new parameter /sys/block/<disk>/queue/nr_group_requests has been introduced +and this parameter controlls the maximum number of requests per group. +nr_requests still continues to control total number of request descriptors +on the queue. + +Ideally one should set nr_requests to be following. + +nr_requests = number_of_cgroups * nr_group_requests + +This will make sure that at any point of time nr_group_requests number of +request descriptors will be available for any of the cgroups. + +Currently default nr_requests=512 and nr_group_requests=128. This will make +sure that apart from root group one can create 3 more group without running +into any issues. If one decides to create more cgorus, nr_requests and +nr_group_requests should be adjusted accordingly. + +Issues +====== +- How to do more accurate disk time accounting, especially with CFQ. We + don't start disk time accounting till first request from the queue has + completed. But there are cases like above virtual machine setup, where + closely cooperating threads issue 1-2 request per thread/io context and + expire. That means many a times, queue does not get serviced for a long + period and gets expired immediately after dispatching one request. First + request is free so we are left with queue disk time as zero and that's + problematic. -- 1.6.0.6 -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
