linux/block
Jens Axboe 320ae51fee blk-mq: new multi-queue block IO queueing mechanism
Linux currently has two models for block devices:

- The classic request_fn based approach, where drivers use struct
  request units for IO. The block layer provides various helper
  functionalities to let drivers share code, things like tag
  management, timeout handling, queueing, etc.

- The "stacked" approach, where a driver squeezes in between the
  block layer and IO submitter. Since this bypasses the IO stack,
  driver generally have to manage everything themselves.

With drivers being written for new high IOPS devices, the classic
request_fn based driver doesn't work well enough. The design dates
back to when both SMP and high IOPS was rare. It has problems with
scaling to bigger machines, and runs into scaling issues even on
smaller machines when you have IOPS in the hundreds of thousands
per device.

The stacked approach is then most often selected as the model
for the driver. But this means that everybody has to re-invent
everything, and along with that we get all the problems again
that the shared approach solved.

This commit introduces blk-mq, block multi queue support. The
design is centered around per-cpu queues for queueing IO, which
then funnel down into x number of hardware submission queues.
We might have a 1:1 mapping between the two, or it might be
an N:M mapping. That all depends on what the hardware supports.

blk-mq provides various helper functions, which include:

- Scalable support for request tagging. Most devices need to
  be able to uniquely identify a request both in the driver and
  to the hardware. The tagging uses per-cpu caches for freed
  tags, to enable cache hot reuse.

- Timeout handling without tracking request on a per-device
  basis. Basically the driver should be able to get a notification,
  if a request happens to fail.

- Optional support for non 1:1 mappings between issue and
  submission queues. blk-mq can redirect IO completions to the
  desired location.

- Support for per-request payloads. Drivers almost always need
  to associate a request structure with some driver private
  command structure. Drivers can tell blk-mq this at init time,
  and then any request handed to the driver will have the
  required size of memory associated with it.

- Support for merging of IO, and plugging. The stacked model
  gets neither of these. Even for high IOPS devices, merging
  sequential IO reduces per-command overhead and thus
  increases bandwidth.

For now, this is provided as a potential 3rd queueing model, with
the hope being that, as it matures, it can replace both the classic
and stacked model. That would get us back to having just 1 real
model for block devices, leaving the stacked approach to dm/md
devices (as it was originally intended).

Contributions in this patch from the following people:

Shaohua Li <shli@fusionio.com>
Alexander Gordeev <agordeev@redhat.com>
Christoph Hellwig <hch@infradead.org>
Mike Christie <michaelc@cs.wisc.edu>
Matias Bjorling <m@bjorling.me>
Jeff Moyer <jmoyer@redhat.com>

Acked-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
2013-10-25 11:56:00 +01:00
..
partitions block: change config option name for cmdline partition parsing 2013-09-30 14:31:02 -07:00
blk-cgroup.c Merge branch 'for-3.12/core' of git://git.kernel.dk/linux-block 2013-09-22 15:00:11 -07:00
blk-cgroup.h cgroup: make css_for_each_descendant() and friends include the origin css in the iteration 2013-08-08 20:11:27 -04:00
blk-core.c blk-mq: new multi-queue block IO queueing mechanism 2013-10-25 11:56:00 +01:00
blk-exec.c blk-mq: new multi-queue block IO queueing mechanism 2013-10-25 11:56:00 +01:00
blk-flush.c blk-mq: new multi-queue block IO queueing mechanism 2013-10-25 11:56:00 +01:00
blk-integrity.c scatterlist: introduce sg_unmark_end 2013-03-20 15:43:04 +10:30
blk-ioc.c lib/radix-tree.c: make radix_tree_node_alloc() work correctly within interrupt 2013-09-11 15:59:36 -07:00
blk-iopoll.c block: delete __cpuinit usage from all block files 2013-07-14 19:36:59 -04:00
blk-lib.c block: account iowait time when waiting for completion of IO request 2013-02-15 16:45:07 +01:00
blk-map.c block: re-use existing 'reading' variable instead of checking direction again 2011-12-21 15:27:24 +01:00
blk-merge.c scatterlist: introduce sg_unmark_end 2013-03-20 15:43:04 +10:30
blk-mq-cpu.c blk-mq: new multi-queue block IO queueing mechanism 2013-10-25 11:56:00 +01:00
blk-mq-cpumap.c blk-mq: new multi-queue block IO queueing mechanism 2013-10-25 11:56:00 +01:00
blk-mq-sysfs.c blk-mq: new multi-queue block IO queueing mechanism 2013-10-25 11:56:00 +01:00
blk-mq-tag.c blk-mq: new multi-queue block IO queueing mechanism 2013-10-25 11:56:00 +01:00
blk-mq-tag.h blk-mq: new multi-queue block IO queueing mechanism 2013-10-25 11:56:00 +01:00
blk-mq.c blk-mq: new multi-queue block IO queueing mechanism 2013-10-25 11:56:00 +01:00
blk-mq.h blk-mq: new multi-queue block IO queueing mechanism 2013-10-25 11:56:00 +01:00
blk-settings.c block: discard granularity might not be power of 2 2012-12-14 20:46:04 +01:00
blk-softirq.c block: delete __cpuinit usage from all block files 2013-07-14 19:36:59 -04:00
blk-sysfs.c blk-mq: new multi-queue block IO queueing mechanism 2013-10-25 11:56:00 +01:00
blk-tag.c block: Reserve only one queue tag for sync IO if only 3 tags are available 2013-06-28 21:32:27 +02:00
blk-throttle.c cgroup: make css_for_each_descendant() and friends include the origin css in the iteration 2013-08-08 20:11:27 -04:00
blk-timeout.c blk-mq: new multi-queue block IO queueing mechanism 2013-10-25 11:56:00 +01:00
blk.h blk-mq: new multi-queue block IO queueing mechanism 2013-10-25 11:56:00 +01:00
bsg-lib.c bsg: Remove unused function bsg_goose_queue() 2012-12-06 14:33:02 +01:00
bsg.c hlist: drop the node parameter from iterators 2013-02-27 19:10:24 -08:00
cfq-iosched.c Merge branch 'for-3.12/core' of git://git.kernel.dk/linux-block 2013-09-22 15:00:11 -07:00
cmdline-parser.c block: support embedded device command line partition 2013-09-11 15:56:57 -07:00
compat_ioctl.c kernel-wide: fix missing validations on __get/__put/__copy_to/__copy_from_user() 2013-09-11 15:58:18 -07:00
deadline-iosched.c block: Convert kmalloc_node(...GFP_ZERO...) to kzalloc_node(...) 2013-09-11 13:22:03 -06:00
elevator.c block: Convert kmalloc_node(...GFP_ZERO...) to kzalloc_node(...) 2013-09-11 13:22:03 -06:00
genhd.c block: Convert kmalloc_node(...GFP_ZERO...) to kzalloc_node(...) 2013-09-11 13:22:03 -06:00
ioctl.c Merge branch 'for-3.7/core' of git://git.kernel.dk/linux-block 2012-10-11 09:04:23 +09:00
Kconfig block: change config option name for cmdline partition parsing 2013-09-30 14:31:02 -07:00
Kconfig.iosched blkcg: make CONFIG_BLK_CGROUP bool 2012-03-06 21:27:21 +01:00
Makefile blk-mq: new multi-queue block IO queueing mechanism 2013-10-25 11:56:00 +01:00
noop-iosched.c elevator: Fix a race in elevator switching 2013-07-03 13:25:24 +02:00
partition-generic.c Revert "loop: cleanup partitions when detaching loop device" 2013-04-08 10:12:11 +02:00
scsi_ioctl.c aio: don't include aio.h in sched.h 2013-05-07 20:16:25 -07:00