block: remove legacy IO schedulers

Retain the deadline documentation, as that carries over to mq-deadline as well. Tested-by: Ming Lei <ming.lei@redhat.com> Reviewed-by: Omar Sandoval <osandov@fb.com> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2026-08-13 14:19:53 -07:00 · 2018-11-07 13:42:32 -07:00
parent 404b8f5a03
commit f382fb0bce
7 changed files with 0 additions and 6025 deletions
@@ -1,291 +0,0 @@
 CFQ (Complete Fairness Queueing)
 ===============================
 The main aim of CFQ scheduler is to provide a fair allocation of the disk
 I/O bandwidth for all the processes which requests an I/O operation.
 CFQ maintains the per process queue for the processes which request I/O
 operation(synchronous requests). In case of asynchronous requests, all the
 requests from all the processes are batched together according to their
 process's I/O priority.
 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
 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
 IO priority becomes weaker.
 So depending on storage and workload, it might be useful to set slice_idle=0.
 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.
 back_seek_max
 -------------
 This specifies, given in Kbytes, the maximum "distance" for backward seeking.
 The distance is the amount of space from the current head location to the
 sectors that are backward in terms of distance.
 This parameter allows the scheduler to anticipate requests in the "backward"
 direction and consider them as being the "next" if they are within this
 distance from the current head location.
 back_seek_penalty
 -----------------
 This parameter is used to compute the cost of backward seeking. If the
 backward distance of request is just 1/back_seek_penalty from a "front"
 request, then the seeking cost of two requests is considered equivalent.
 So scheduler will not bias toward one or the other request (otherwise scheduler
 will bias toward front request). Default value of back_seek_penalty is 2.
 fifo_expire_async
 -----------------
 This parameter is used to set the timeout of asynchronous requests. Default
 value of this is 248ms.
 fifo_expire_sync
 ----------------
 This parameter is used to set the timeout of synchronous requests. Default
 value of this is 124ms. In case to favor synchronous requests over asynchronous
 one, this value should be decreased relative to fifo_expire_async.
 group_idle
 -----------
 This parameter forces idling at the CFQ group level instead of CFQ
 queue level. This was introduced after a bottleneck was observed
 in higher end storage due to idle on sequential queue and allow dispatch
 from a single queue. The idea with this parameter is that it can be run with
 slice_idle=0 and group_idle=8, so that idling does not happen on individual
 queues in the group but happens overall on the group and thus still keeps the
 IO controller working.
 Not idling on individual queues in the group will dispatch requests from
 multiple queues in the group at the same time and achieve higher throughput
 on higher end storage.
 Default value for this parameter is 8ms.
 low_latency
 -----------
 This parameter is used to enable/disable the low latency mode of the CFQ
 scheduler. If enabled, CFQ tries to recompute the slice time for each process
 based on the target_latency set for the system. This favors fairness over
 throughput. Disabling low latency (setting it to 0) ignores target latency,
 allowing each process in the system to get a full time slice.
 By default low latency mode is enabled.
 target_latency
 --------------
 This parameter is used to calculate the time slice for a process if cfq's
 latency mode is enabled. It will ensure that sync requests have an estimated
 latency. But if sequential workload is higher(e.g. sequential read),
 then to meet the latency constraints, throughput may decrease because of less
 time for each process to issue I/O request before the cfq queue is switched.
 Though this can be overcome by disabling the latency_mode, it may increase
 the read latency for some applications. This parameter allows for changing
 target_latency through the sysfs interface which can provide the balanced
 throughput and read latency.
 Default value for target_latency is 300ms.
 slice_async
 -----------
 This parameter is same as of slice_sync but for asynchronous queue. The
 default value is 40ms.
 slice_async_rq
 --------------
 This parameter is used to limit the dispatching of asynchronous request to
 device request queue in queue's slice time. The maximum number of request that
 are allowed to be dispatched also depends upon the io priority. Default value
 for this is 2.
 slice_sync
 ----------
 When a queue is selected for execution, the queues IO requests are only
 executed for a certain amount of time(time_slice) before switching to another
 queue. This parameter is used to calculate the time slice of synchronous
 queue.
 time_slice is computed using the below equation:-
 time_slice = slice_sync + (slice_sync/5 * (4 - prio)). To increase the
 time_slice of synchronous queue, increase the value of slice_sync. Default
 value is 100ms.
 quantum
 -------
 This specifies the number of request dispatched to the device queue. In a
 queue's time slice, a request will not be dispatched if the number of request
 in the device exceeds this parameter. This parameter is used for synchronous
 request.
 In case of storage with several disk, this setting can limit the parallel
 processing of request. Therefore, increasing the value can improve the
 performance although this can cause the latency of some I/O to increase due
 to more number of requests.
 CFQ Group scheduling
 ====================
 CFQ supports blkio cgroup and has "blkio." prefixed files in each
 blkio cgroup directory. It is weight-based and there are four knobs
 for configuration - weight[_device] and leaf_weight[_device].
 Internal cgroup nodes (the ones with children) can also have tasks in
 them, so the former two configure how much proportion the cgroup as a
 whole is entitled to at its parent's level while the latter two
 configure how much proportion the tasks in the cgroup have compared to
 its direct children.
 Another way to think about it is assuming that each internal node has
 an implicit leaf child node which hosts all the tasks whose weight is
 configured by leaf_weight[_device]. Let's assume a blkio hierarchy
 composed of five cgroups - root, A, B, AA and AB - with the following
 weights where the names represent the hierarchy.
        weight leaf_weight
 root :  125    125
 A    :  500    750
 B    :  250    500
 AA   :  500    500
 AB   : 1000    500
 root never has a parent making its weight is meaningless. For backward
 compatibility, weight is always kept in sync with leaf_weight. B, AA
 and AB have no child and thus its tasks have no children cgroup to
 compete with. They always get 100% of what the cgroup won at the
 parent level. Considering only the weights which matter, the hierarchy
 looks like the following.
          root
       /    |   \
      A     B    leaf
     500   250   125
   /  |  \
  AA  AB  leaf
 500 1000 750
 If all cgroups have active IOs and competing with each other, disk
 time will be distributed like the following.
 Distribution below root. The total active weight at this level is
 A:500 + B:250 + C:125 = 875.
 root-leaf :   125 /  875      =~ 14%
 A         :   500 /  875      =~ 57%
 B(-leaf)  :   250 /  875      =~ 28%
 A has children and further distributes its 57% among the children and
 the implicit leaf node. The total active weight at this level is
 AA:500 + AB:1000 + A-leaf:750 = 2250.
 A-leaf    : ( 750 / 2250) * A =~ 19%
 AA(-leaf) : ( 500 / 2250) * A =~ 12%
 AB(-leaf) : (1000 / 2250) * A =~ 25%
 CFQ IOPS Mode for group scheduling
 ===================================
 Basic CFQ design is to provide priority based time slices. Higher priority
 process gets bigger time slice and lower priority process gets smaller time
 slice. 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 is to measure number of requests dispatched from a
 single queue and also allow dispatch from multiple cfq queue at the same time.
 This 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.
 CFQ IO scheduler Idling Theory
 ===============================
 Idling on a queue is primarily about waiting for the next request to come
 on same queue after completion of a request. In this process CFQ will not
 dispatch requests from other cfq queues even if requests are pending there.
 The rationale behind idling is that it can cut down on number of seeks
 on rotational media. For example, if a process is doing dependent
 sequential reads (next read will come on only after completion of previous
 one), then not dispatching request from other queue should help as we
 did not move the disk head and kept on dispatching sequential IO from
 one queue.
 CFQ has following service trees and various queues are put on these trees.
 	sync-idle	sync-noidle	async
 All cfq queues doing synchronous sequential IO go on to sync-idle tree.
 On this tree we idle on each queue individually.
 All synchronous non-sequential queues go on sync-noidle tree. Also any
 synchronous write request which is not marked with REQ_IDLE goes on this
 service tree. On this tree we do not idle on individual queues instead idle
 on the whole group of queues or the tree. So if there are 4 queues waiting
 for IO to dispatch we will idle only once last queue has dispatched the IO
 and there is no more IO on this service tree.
 All async writes go on async service tree. There is no idling on async
 queues.
 CFQ has some optimizations for SSDs and if it detects a non-rotational
 media which can support higher queue depth (multiple requests at in
 flight at a time), then it cuts down on idling of individual queues and
 all the queues move to sync-noidle tree and only tree idle remains. This
 tree idling provides isolation with buffered write queues on async tree.
 FAQ
 ===
 Q1. Why to idle at all on queues not marked with REQ_IDLE.
 A1. We only do tree idle (all queues on sync-noidle tree) on queues not marked
    with REQ_IDLE. This helps in providing isolation with all the sync-idle
    queues. Otherwise in presence of many sequential readers, other
    synchronous IO might not get fair share of disk.
    For example, if there are 10 sequential readers doing IO and they get
    100ms each. If a !REQ_IDLE request comes in, it will be scheduled
    roughly after 1 second. If after completion of !REQ_IDLE request we
    do not idle, and after a couple of milli seconds a another !REQ_IDLE
    request comes in, again it will be scheduled after 1second. Repeat it
    and notice how a workload can lose its disk share and suffer due to
    multiple sequential readers.
    fsync can generate dependent IO where bunch of data is written in the
    context of fsync, and later some journaling data is written. Journaling
    data comes in only after fsync has finished its IO (atleast for ext4
    that seemed to be the case). Now if one decides not to idle on fsync
    thread due to !REQ_IDLE, then next journaling write will not get
    scheduled for another second. A process doing small fsync, will suffer
    badly in presence of multiple sequential readers.
    Hence doing tree idling on threads using !REQ_IDLE flag on requests
    provides isolation from multiple sequential readers and at the same
    time we do not idle on individual threads.
 Q2. When to specify REQ_IDLE
 A2. I would think whenever one is doing synchronous write and expecting
    more writes to be dispatched from same context soon, should be able
    to specify REQ_IDLE on writes and that probably should work well for
    most of the cases.
@@ -3,67 +3,6 @@ if BLOCK
 menu "IO Schedulers"
 config IOSCHED_NOOP
 	bool
 	default y
 	---help---
 	  The no-op I/O scheduler is a minimal scheduler that does basic merging
 	  and sorting. Its main uses include non-disk based block devices like
 	  memory devices, and specialised software or hardware environments
 	  that do their own scheduling and require only minimal assistance from
 	  the kernel.
 config IOSCHED_DEADLINE
 	tristate "Deadline I/O scheduler"
 	default y
 	---help---
 	  The deadline I/O scheduler is simple and compact. It will provide
 	  CSCAN service with FIFO expiration of requests, switching to
 	  a new point in the service tree and doing a batch of IO from there
 	  in case of expiry.
 config IOSCHED_CFQ
 	tristate "CFQ I/O scheduler"
 	default y
 	---help---
 	  The CFQ I/O scheduler tries to distribute bandwidth equally
 	  among all processes in the system. It should provide a fair
 	  and low latency working environment, suitable for both desktop
 	  and server systems.
 	  This is the default I/O scheduler.
 config CFQ_GROUP_IOSCHED
 	bool "CFQ Group Scheduling support"
 	depends on IOSCHED_CFQ && BLK_CGROUP
 	---help---
 	  Enable group IO scheduling in CFQ.
 choice
 	prompt "Default I/O scheduler"
 	default DEFAULT_CFQ
 	help
 	  Select the I/O scheduler which will be used by default for all
 	  block devices.
 	config DEFAULT_DEADLINE
 		bool "Deadline" if IOSCHED_DEADLINE=y
 	config DEFAULT_CFQ
 		bool "CFQ" if IOSCHED_CFQ=y
 	config DEFAULT_NOOP
 		bool "No-op"
 endchoice
 config DEFAULT_IOSCHED
 	string
 	default "deadline" if DEFAULT_DEADLINE
 	default "cfq" if DEFAULT_CFQ
 	default "noop" if DEFAULT_NOOP
 config MQ_IOSCHED_DEADLINE
 	tristate "MQ deadline I/O scheduler"
 	default y
@@ -18,9 +18,6 @@ obj-$(CONFIG_BLK_DEV_BSGLIB)	+= bsg-lib.o
 obj-$(CONFIG_BLK_CGROUP)	+= blk-cgroup.o
 obj-$(CONFIG_BLK_DEV_THROTTLING)	+= blk-throttle.o
 obj-$(CONFIG_BLK_CGROUP_IOLATENCY)	+= blk-iolatency.o
 obj-$(CONFIG_IOSCHED_NOOP)	+= noop-iosched.o
 obj-$(CONFIG_IOSCHED_DEADLINE)	+= deadline-iosched.o
 obj-$(CONFIG_IOSCHED_CFQ)	+= cfq-iosched.o
 obj-$(CONFIG_MQ_IOSCHED_DEADLINE)	+= mq-deadline.o
 obj-$(CONFIG_MQ_IOSCHED_KYBER)	+= kyber-iosched.o
 bfq-y				:= bfq-iosched.o bfq-wf2q.o bfq-cgroup.o
@@ -225,8 +225,6 @@ int elevator_init(struct request_queue *q)
 							chosen_elevator);
 	}
 	if (!e)
 		e = elevator_get(q, CONFIG_DEFAULT_IOSCHED, false);
 	if (!e) {
 		printk(KERN_ERR
 			"Default I/O scheduler not found. Using noop.\n");
@@ -356,68 +354,6 @@ struct request *elv_rb_find(struct rb_root *root, sector_t sector)
 }
 EXPORT_SYMBOL(elv_rb_find);
 /*
 * Insert rq into dispatch queue of q.  Queue lock must be held on
 * entry.  rq is sort instead into the dispatch queue. To be used by
 * specific elevators.
 */
 void elv_dispatch_sort(struct request_queue *q, struct request *rq)
 {
 	sector_t boundary;
 	struct list_head *entry;
 	if (q->last_merge == rq)
 		q->last_merge = NULL;
 	elv_rqhash_del(q, rq);
 	q->nr_sorted--;
 	boundary = q->end_sector;
 	list_for_each_prev(entry, &q->queue_head) {
 		struct request *pos = list_entry_rq(entry);
 		if (req_op(rq) != req_op(pos))
 			break;
 		if (rq_data_dir(rq) != rq_data_dir(pos))
 			break;
 		if (pos->rq_flags & (RQF_STARTED | RQF_SOFTBARRIER))
 			break;
 		if (blk_rq_pos(rq) >= boundary) {
 			if (blk_rq_pos(pos) < boundary)
 				continue;
 		} else {
 			if (blk_rq_pos(pos) >= boundary)
 				break;
 		}
 		if (blk_rq_pos(rq) >= blk_rq_pos(pos))
 			break;
 	}
 	list_add(&rq->queuelist, entry);
 }
 EXPORT_SYMBOL(elv_dispatch_sort);
 /*
 * Insert rq into dispatch queue of q.  Queue lock must be held on
 * entry.  rq is added to the back of the dispatch queue. To be used by
 * specific elevators.
 */
 void elv_dispatch_add_tail(struct request_queue *q, struct request *rq)
 {
 	if (q->last_merge == rq)
 		q->last_merge = NULL;
 	elv_rqhash_del(q, rq);
 	q->nr_sorted--;
 	q->end_sector = rq_end_sector(rq);
 	q->boundary_rq = rq;
 	list_add_tail(&rq->queuelist, &q->queue_head);
 }
 EXPORT_SYMBOL(elv_dispatch_add_tail);
 enum elv_merge elv_merge(struct request_queue *q, struct request **req,
 		struct bio *bio)
 {
@@ -881,12 +817,6 @@ int elv_register(struct elevator_type *e)
 	list_add_tail(&e->list, &elv_list);
 	spin_unlock(&elv_list_lock);
 	/* print pretty message */
 	if (elevator_match(e, chosen_elevator) ||
 			(!*chosen_elevator &&
 			 elevator_match(e, CONFIG_DEFAULT_IOSCHED)))
 				def = " (default)";
 	printk(KERN_INFO "io scheduler %s registered%s\n", e->elevator_name,
 								def);
 	return 0;
@@ -1,124 +0,0 @@
 /*
 * elevator noop
 */
 #include <linux/blkdev.h>
 #include <linux/elevator.h>
 #include <linux/bio.h>
 #include <linux/module.h>
 #include <linux/slab.h>
 #include <linux/init.h>
 struct noop_data {
 	struct list_head queue;
 };
 static void noop_merged_requests(struct request_queue *q, struct request *rq,
 				 struct request *next)
 {
 	list_del_init(&next->queuelist);
 }
 static int noop_dispatch(struct request_queue *q, int force)
 {
 	struct noop_data *nd = q->elevator->elevator_data;
 	struct request *rq;
 	rq = list_first_entry_or_null(&nd->queue, struct request, queuelist);
 	if (rq) {
 		list_del_init(&rq->queuelist);
 		elv_dispatch_sort(q, rq);
 		return 1;
 	}
 	return 0;
 }
 static void noop_add_request(struct request_queue *q, struct request *rq)
 {
 	struct noop_data *nd = q->elevator->elevator_data;
 	list_add_tail(&rq->queuelist, &nd->queue);
 }
 static struct request *
 noop_former_request(struct request_queue *q, struct request *rq)
 {
 	struct noop_data *nd = q->elevator->elevator_data;
 	if (rq->queuelist.prev == &nd->queue)
 		return NULL;
 	return list_prev_entry(rq, queuelist);
 }
 static struct request *
 noop_latter_request(struct request_queue *q, struct request *rq)
 {
 	struct noop_data *nd = q->elevator->elevator_data;
 	if (rq->queuelist.next == &nd->queue)
 		return NULL;
 	return list_next_entry(rq, queuelist);
 }
 static int noop_init_queue(struct request_queue *q, struct elevator_type *e)
 {
 	struct noop_data *nd;
 	struct elevator_queue *eq;
 	eq = elevator_alloc(q, e);
 	if (!eq)
 		return -ENOMEM;
 	nd = kmalloc_node(sizeof(*nd), GFP_KERNEL, q->node);
 	if (!nd) {
 		kobject_put(&eq->kobj);
 		return -ENOMEM;
 	}
 	eq->elevator_data = nd;
 	INIT_LIST_HEAD(&nd->queue);
 	spin_lock_irq(q->queue_lock);
 	q->elevator = eq;
 	spin_unlock_irq(q->queue_lock);
 	return 0;
 }
 static void noop_exit_queue(struct elevator_queue *e)
 {
 	struct noop_data *nd = e->elevator_data;
 	BUG_ON(!list_empty(&nd->queue));
 	kfree(nd);
 }
 static struct elevator_type elevator_noop = {
 	.ops.sq = {
 		.elevator_merge_req_fn		= noop_merged_requests,
 		.elevator_dispatch_fn		= noop_dispatch,
 		.elevator_add_req_fn		= noop_add_request,
 		.elevator_former_req_fn		= noop_former_request,
 		.elevator_latter_req_fn		= noop_latter_request,
 		.elevator_init_fn		= noop_init_queue,
 		.elevator_exit_fn		= noop_exit_queue,
 	},
 	.elevator_name = "noop",
 	.elevator_owner = THIS_MODULE,
 };
 static int __init noop_init(void)
 {
 	return elv_register(&elevator_noop);
 }
 static void __exit noop_exit(void)
 {
 	elv_unregister(&elevator_noop);
 }
 module_init(noop_init);
 module_exit(noop_exit);
 MODULE_AUTHOR("Jens Axboe");
 MODULE_LICENSE("GPL");
 MODULE_DESCRIPTION("No-op IO scheduler");