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1) group_wait_time - This is the amount of time the cgroup had to wait to get a timeslice for one of its queues from when it became busy, i.e., went from 0 to 1 request queued. This is different from the io_wait_time which is the cumulative total of the amount of time spent by each IO in that cgroup waiting in the scheduler queue. This stat is a great way to find out any jobs in the fleet that are being starved or waiting for longer than what is expected (due to an IO controller bug or any other issue). 2) empty_time - This is the amount of time a cgroup spends w/o any pending requests. This stat is useful when a job does not seem to be able to use its assigned disk share by helping check if that is happening due to an IO controller bug or because the job is not submitting enough IOs. 3) idle_time - This is the amount of time spent by the IO scheduler idling for a given cgroup in anticipation of a better request than the exising ones from other queues/cgroups. All these stats are recorded using start and stop events. When reading these stats, we do not add the delta between the current time and the last start time if we're between the start and stop events. We avoid doing this to make sure that these numbers are always monotonically increasing when read. Since we're using sched_clock() which may use the tsc as its source, it may induce some inconsistency (due to tsc resync across cpus) if we included the current delta. Signed-off-by: Divyesh Shah<dpshah@google.com> Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
227 lines
9.9 KiB
Plaintext
227 lines
9.9 KiB
Plaintext
Block IO Controller
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===================
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Overview
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========
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cgroup subsys "blkio" implements the block io controller. There seems to be
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a need of various kinds of IO control policies (like proportional BW, max BW)
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both at leaf nodes as well as at intermediate nodes in a storage hierarchy.
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Plan is to use the same cgroup based management interface for blkio controller
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and based on user options switch IO policies in the background.
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In the first phase, this patchset implements proportional weight time based
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division of disk policy. It is implemented in CFQ. Hence this policy takes
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effect only on leaf nodes when CFQ is being used.
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HOWTO
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=====
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You can do a very simple testing of running two dd threads in two different
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cgroups. Here is what you can do.
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- Enable group scheduling in CFQ
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CONFIG_CFQ_GROUP_IOSCHED=y
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- Compile and boot into kernel and mount IO controller (blkio).
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mount -t cgroup -o blkio none /cgroup
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- Create two cgroups
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mkdir -p /cgroup/test1/ /cgroup/test2
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- Set weights of group test1 and test2
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echo 1000 > /cgroup/test1/blkio.weight
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echo 500 > /cgroup/test2/blkio.weight
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- Create two same size files (say 512MB each) on same disk (file1, file2) and
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launch two dd threads in different cgroup to read those files.
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sync
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echo 3 > /proc/sys/vm/drop_caches
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dd if=/mnt/sdb/zerofile1 of=/dev/null &
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echo $! > /cgroup/test1/tasks
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cat /cgroup/test1/tasks
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dd if=/mnt/sdb/zerofile2 of=/dev/null &
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echo $! > /cgroup/test2/tasks
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cat /cgroup/test2/tasks
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- At macro level, first dd should finish first. To get more precise data, keep
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on looking at (with the help of script), at blkio.disk_time and
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blkio.disk_sectors files of both test1 and test2 groups. This will tell how
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much disk time (in milli seconds), each group got and how many secotors each
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group dispatched to the disk. We provide fairness in terms of disk time, so
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ideally io.disk_time of cgroups should be in proportion to the weight.
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Various user visible config options
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===================================
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CONFIG_CFQ_GROUP_IOSCHED
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- Enables group scheduling in CFQ. Currently only 1 level of group
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creation is allowed.
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CONFIG_DEBUG_CFQ_IOSCHED
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- Enables some debugging messages in blktrace. Also creates extra
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cgroup file blkio.dequeue.
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Config options selected automatically
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=====================================
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These config options are not user visible and are selected/deselected
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automatically based on IO scheduler configuration.
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CONFIG_BLK_CGROUP
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- Block IO controller. Selected by CONFIG_CFQ_GROUP_IOSCHED.
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CONFIG_DEBUG_BLK_CGROUP
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- Debug help. Selected by CONFIG_DEBUG_CFQ_IOSCHED.
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Details of cgroup files
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=======================
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- blkio.weight
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- Specifies per cgroup weight.
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Currently allowed range of weights is from 100 to 1000.
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- blkio.time
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- disk time allocated to cgroup per device in milliseconds. First
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two fields specify the major and minor number of the device and
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third field specifies the disk time allocated to group in
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milliseconds.
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- blkio.sectors
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- number of sectors transferred to/from disk by the group. First
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two fields specify the major and minor number of the device and
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third field specifies the number of sectors transferred by the
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group to/from the device.
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- blkio.io_service_bytes
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- Number of bytes transferred to/from the disk by the group. These
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are further divided by the type of operation - read or write, sync
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or async. First two fields specify the major and minor number of the
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device, third field specifies the operation type and the fourth field
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specifies the number of bytes.
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- blkio.io_serviced
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- Number of IOs completed to/from the disk by the group. These
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are further divided by the type of operation - read or write, sync
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or async. First two fields specify the major and minor number of the
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device, third field specifies the operation type and the fourth field
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specifies the number of IOs.
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- blkio.io_service_time
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- Total amount of time between request dispatch and request completion
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for the IOs done by this cgroup. This is in nanoseconds to make it
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meaningful for flash devices too. For devices with queue depth of 1,
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this time represents the actual service time. When queue_depth > 1,
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that is no longer true as requests may be served out of order. This
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may cause the service time for a given IO to include the service time
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of multiple IOs when served out of order which may result in total
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io_service_time > actual time elapsed. This time is further divided by
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the type of operation - read or write, sync or async. First two fields
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specify the major and minor number of the device, third field
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specifies the operation type and the fourth field specifies the
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io_service_time in ns.
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- blkio.io_wait_time
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- Total amount of time the IOs for this cgroup spent waiting in the
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scheduler queues for service. This can be greater than the total time
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elapsed since it is cumulative io_wait_time for all IOs. It is not a
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measure of total time the cgroup spent waiting but rather a measure of
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the wait_time for its individual IOs. For devices with queue_depth > 1
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this metric does not include the time spent waiting for service once
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the IO is dispatched to the device but till it actually gets serviced
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(there might be a time lag here due to re-ordering of requests by the
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device). This is in nanoseconds to make it meaningful for flash
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devices too. This time is further divided by the type of operation -
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read or write, sync or async. First two fields specify the major and
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minor number of the device, third field specifies the operation type
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and the fourth field specifies the io_wait_time in ns.
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- blkio.io_merged
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- Total number of bios/requests merged into requests belonging to this
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cgroup. This is further divided by the type of operation - read or
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write, sync or async.
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- blkio.io_queued
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- Total number of requests queued up at any given instant for this
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cgroup. This is further divided by the type of operation - read or
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write, sync or async.
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- blkio.avg_queue_size
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- Debugging aid only enabled if CONFIG_DEBUG_CFQ_IOSCHED=y.
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The average queue size for this cgroup over the entire time of this
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cgroup's existence. Queue size samples are taken each time one of the
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queues of this cgroup gets a timeslice.
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- blkio.group_wait_time
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- Debugging aid only enabled if CONFIG_DEBUG_CFQ_IOSCHED=y.
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This is the amount of time the cgroup had to wait since it became busy
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(i.e., went from 0 to 1 request queued) to get a timeslice for one of
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its queues. This is different from the io_wait_time which is the
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cumulative total of the amount of time spent by each IO in that cgroup
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waiting in the scheduler queue. This is in nanoseconds. If this is
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read when the cgroup is in a waiting (for timeslice) state, the stat
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will only report the group_wait_time accumulated till the last time it
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got a timeslice and will not include the current delta.
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- blkio.empty_time
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- Debugging aid only enabled if CONFIG_DEBUG_CFQ_IOSCHED=y.
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This is the amount of time a cgroup spends without any pending
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requests when not being served, i.e., it does not include any time
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spent idling for one of the queues of the cgroup. This is in
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nanoseconds. If this is read when the cgroup is in an empty state,
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the stat will only report the empty_time accumulated till the last
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time it had a pending request and will not include the current delta.
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- blkio.idle_time
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- Debugging aid only enabled if CONFIG_DEBUG_CFQ_IOSCHED=y.
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This is the amount of time spent by the IO scheduler idling for a
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given cgroup in anticipation of a better request than the exising ones
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from other queues/cgroups. This is in nanoseconds. If this is read
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when the cgroup is in an idling state, the stat will only report the
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idle_time accumulated till the last idle period and will not include
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the current delta.
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- blkio.dequeue
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- Debugging aid only enabled if CONFIG_DEBUG_CFQ_IOSCHED=y. This
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gives the statistics about how many a times a group was dequeued
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from service tree of the device. First two fields specify the major
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and minor number of the device and third field specifies the number
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of times a group was dequeued from a particular device.
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- blkio.reset_stats
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- Writing an int to this file will result in resetting all the stats
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for that cgroup.
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CFQ sysfs tunable
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=================
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/sys/block/<disk>/queue/iosched/group_isolation
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If group_isolation=1, it provides stronger isolation between groups at the
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expense of throughput. By default group_isolation is 0. In general that
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means that if group_isolation=0, expect fairness for sequential workload
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only. Set group_isolation=1 to see fairness for random IO workload also.
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Generally CFQ will put random seeky workload in sync-noidle category. CFQ
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will disable idling on these queues and it does a collective idling on group
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of such queues. Generally these are slow moving queues and if there is a
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sync-noidle service tree in each group, that group gets exclusive access to
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disk for certain period. That means it will bring the throughput down if
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group does not have enough IO to drive deeper queue depths and utilize disk
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capacity to the fullest in the slice allocated to it. But the flip side is
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that even a random reader should get better latencies and overall throughput
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if there are lots of sequential readers/sync-idle workload running in the
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system.
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If group_isolation=0, then CFQ automatically moves all the random seeky queues
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in the root group. That means there will be no service differentiation for
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that kind of workload. This leads to better throughput as we do collective
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idling on root sync-noidle tree.
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By default one should run with group_isolation=0. If that is not sufficient
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and one wants stronger isolation between groups, then set group_isolation=1
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but this will come at cost of reduced throughput.
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What works
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==========
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- Currently only sync IO queues are support. All the buffered writes are
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still system wide and not per group. Hence we will not see service
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differentiation between buffered writes between groups.
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