Messages in this thread Patch in this message | | | Date | Thu, 22 Jul 2010 14:36:59 -0700 | From | Randy Dunlap <> | Subject | Re: [PATCH 5/5] cfq-iosched: Documentation update |
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On Thu, 22 Jul 2010 17:29:32 -0400 Vivek Goyal wrote:
> o Documentation update for group_idle tunable and Group IOPS mode. > --- > Documentation/block/cfq-iosched.txt | 44 ++++++++++++++++++++++++++++ > Documentation/cgroups/blkio-controller.txt | 28 +++++++++++++++++ > 2 files changed, 72 insertions(+), 0 deletions(-) > create mode 100644 Documentation/block/cfq-iosched.txt > > diff --git a/Documentation/block/cfq-iosched.txt b/Documentation/block/cfq-iosched.txt > new file mode 100644 > index 0000000..6cc2151 > --- /dev/null > +++ b/Documentation/block/cfq-iosched.txt > @@ -0,0 +1,44 @@ > +CFQ ioscheduler tunables > +======================== > + > +slice_idle > +---------- > +This specifies how long CFQ should idle for next request on certain cfq queues > +(for sequential workloads) and service trees (for random workloads) before > +queue is expired and CFQ selects next queue to dispatch from. > + > +By default slice_idle is a non zero value. That means by default we idle on
non-zero
> +queues/service trees. This can be very helpful on highly seeky media like > +single spindle SATA/SAS disks where we can cut down on overall number of > +seeks and see improved throughput. > + > +Setting slice_idle to 0 will remove all the idling on queues/service tree > +level and one should see an overall improved throughput on faster storage > +devices like multiple SATA/SAS disks in hardware RAID configuration. The down > +side is that isolation provided from WRITES also goes down and notion of > +ioprio becomes weaker. > + > +So depending on storage and workload, it might be a useful to set slice_idle=0.
might be useful
> +In general I think for SATA/SAS disks and software RAID of SATA/SAS disks > +keeping slice_idle enabled should be useful. For any configurations where > +there are multiple spindles behind single LUN (Host based hardware RAID > +controller or for storage arrays), setting slice_idle=0 might end up in better > +throughput and acceptable latencies. > + > +CFQ IOPS Mode for group scheduling > +================================== > +Basic CFQ design is to provide prio based time slices. Higher prio process > +gets bigger time slice and lower prio process gets smaller time slice.
s/prio/priority/ multiple places.
> +Measuring time becomes harder if storage is fast and supports NCQ and it would > +be better to dispatch multiple requests from multiple cfq queues in request > +queue at a time. In such scenario, it is not possible to measure time consumed > +by single queue accurately. > + > +What is possible though to measure number of requests dispatched from a single
though is to measure (?)
> +queue and also allow dispatch from multiple cfqq at the same time. This
what is cfqq? ^^^^
> +effectively becomes the fairness in terms of IOPS (IO operations per second). > + > +If one sets slice_idle=0 and if storage supports NCQ, CFQ internally switches > +to IOPS mode and starts providing fairness in terms of number of requests > +dispatched. Note that this mode switching takes effect only for group > +scheduling. For non cgroup users nothing should change.
non-cgroup
> diff --git a/Documentation/cgroups/blkio-controller.txt b/Documentation/cgroups/blkio-controller.txt > index 48e0b21..6919d62 100644 > --- a/Documentation/cgroups/blkio-controller.txt > +++ b/Documentation/cgroups/blkio-controller.txt > @@ -217,6 +217,7 @@ Details of cgroup files > CFQ sysfs tunable > ================= > /sys/block/<disk>/queue/iosched/group_isolation > +----------------------------------------------- > > If group_isolation=1, it provides stronger isolation between groups at the > expense of throughput. By default group_isolation is 0. In general that > @@ -243,6 +244,33 @@ By default one should run with group_isolation=0. If that is not sufficient > and one wants stronger isolation between groups, then set group_isolation=1 > but this will come at cost of reduced throughput. > > +/sys/block/<disk>/queue/iosched/slice_idle > +------------------------------------------ > +On a faster hardware CFQ can be slow, especially with sequential workload. > +This happens because CFQ idles on a single queue and single queue might not > +drive deeper request queue depths to keep the storage busy. In such scenarios > +one can try setting slice_idle=0 and that would switch CFQ to IOPS > +(IO operations per second) mode on NCQ supporting hardware. > + > +That means CFQ will not idle between cfq queues of a cfq group and hence be > +able to driver higher queue depth and achieve better throughput. That also > +means that cfq provides fairness among groups in terms of IOPS and not in > +terms of disk time. > + > +/sys/block/<disk>/queue/iosched/group_idle > +------------------------------------------ > +If one disables idling on individual cfq queues and cfq service trees by > +setting slice_idle=0, group_idle kicks in. That means CFQ will still idle > +on the group in an attempt to provide fairness among groups. > + > +By default group_idle is same as slice_idle and does not do anything if > +slice_idle is enabled. > + > +One can experience an overall throughput drop if you have created multiple > +groups and put applications in that group which are not driving enough > +IO to keep disk busy. In that case set group_idle=0, and CFQ will not idle > +on individual groups and throughput should improve. > + > What works > ========== > - Currently only sync IO queues are support. All the buffered writes are > --
--- ~Randy *** Remember to use Documentation/SubmitChecklist when testing your code ***
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