Commit 4c9f7836 authored by Tejun Heo's avatar Tejun Heo Committed by Jens Axboe

[PATCH] 05/05 update biodoc to match new generic dispatch api

Updates biodoc to reflect changes in elevator API
Signed-off-by: default avatarTejun Heo <>
Signed-off-by: default avatarJens Axboe <>
parent 98b11471
......@@ -906,9 +906,20 @@ Aside:
4. The I/O scheduler
I/O schedulers are now per queue. They should be runtime switchable and modular
but aren't yet. Jens has most bits to do this, but the sysfs implementation is
I/O scheduler, a.k.a. elevator, is implemented in two layers. Generic dispatch
queue and specific I/O schedulers. Unless stated otherwise, elevator is used
to refer to both parts and I/O scheduler to specific I/O schedulers.
Block layer implements generic dispatch queue in ll_rw_blk.c and elevator.c.
The generic dispatch queue is responsible for properly ordering barrier
requests, requeueing, handling non-fs requests and all other subtleties.
Specific I/O schedulers are responsible for ordering normal filesystem
requests. They can also choose to delay certain requests to improve
throughput or whatever purpose. As the plural form indicates, there are
multiple I/O schedulers. They can be built as modules but at least one should
be built inside the kernel. Each queue can choose different one and can also
change to another one dynamically.
A block layer call to the i/o scheduler follows the convention elv_xxx(). This
calls elevator_xxx_fn in the elevator switch (drivers/block/elevator.c). Oh,
......@@ -921,44 +932,36 @@ keeping work.
The functions an elevator may implement are: (* are mandatory)
elevator_merge_fn called to query requests for merge with a bio
elevator_merge_req_fn " " " with another request
elevator_merge_req_fn called when two requests get merged. the one
which gets merged into the other one will be
never seen by I/O scheduler again. IOW, after
being merged, the request is gone.
elevator_merged_fn called when a request in the scheduler has been
involved in a merge. It is used in the deadline
scheduler for example, to reposition the request
if its sorting order has changed.
*elevator_next_req_fn returns the next scheduled request, or NULL
if there are none (or none are ready).
elevator_dispatch_fn fills the dispatch queue with ready requests.
I/O schedulers are free to postpone requests by
not filling the dispatch queue unless @force
is non-zero. Once dispatched, I/O schedulers
are not allowed to manipulate the requests -
they belong to generic dispatch queue.
*elevator_add_req_fn called to add a new request into the scheduler
elevator_add_req_fn called to add a new request into the scheduler
elevator_queue_empty_fn returns true if the merge queue is empty.
Drivers shouldn't use this, but rather check
if elv_next_request is NULL (without losing the
request if one exists!)
elevator_remove_req_fn This is called when a driver claims ownership of
the target request - it now belongs to the
driver. It must not be modified or merged.
Drivers must not lose the request! A subsequent
call of elevator_next_req_fn must return the
_next_ request.
elevator_requeue_req_fn called to add a request to the scheduler. This
is used when the request has alrnadebeen
returned by elv_next_request, but hasn't
completed. If this is not implemented then
elevator_add_req_fn is called instead.
elevator_latter_req_fn These return the request before or after the
one specified in disk sort order. Used by the
block layer to find merge possibilities.
elevator_completed_req_fn called when a request is completed. This might
come about due to being merged with another or
when the device completes the request.
elevator_completed_req_fn called when a request is completed.
elevator_may_queue_fn returns true if the scheduler wants to allow the
current context to queue a new request even if
......@@ -967,13 +970,33 @@ elevator_may_queue_fn returns true if the scheduler wants to allow the
elevator_put_req_fn Must be used to allocate and free any elevator
specific storate for a request.
specific storage for a request.
elevator_activate_req_fn Called when device driver first sees a request.
I/O schedulers can use this callback to
determine when actual execution of a request
elevator_deactivate_req_fn Called when device driver decides to delay
a request by requeueing it.
elevator_exit_fn Allocate and free any elevator specific storage
for a queue.
4.2 I/O scheduler implementation
4.2 Request flows seen by I/O schedulers
All requests seens by I/O schedulers strictly follow one of the following three
set_req_fn ->
i. add_req_fn -> (merged_fn ->)* -> dispatch_fn -> activate_req_fn ->
(deactivate_req_fn -> activate_req_fn ->)* -> completed_req_fn
ii. add_req_fn -> (merged_fn ->)* -> merge_req_fn
iii. [none]
-> put_req_fn
4.3 I/O scheduler implementation
The generic i/o scheduler algorithm attempts to sort/merge/batch requests for
optimal disk scan and request servicing performance (based on generic
principles and device capabilities), optimized for:
......@@ -993,18 +1016,7 @@ request in sort order to prevent binary tree lookups.
This arrangement is not a generic block layer characteristic however, so
elevators may implement queues as they please.
ii. Last merge hint
The last merge hint is part of the generic queue layer. I/O schedulers must do
some management on it. For the most part, the most important thing is to make
sure q->last_merge is cleared (set to NULL) when the request on it is no longer
a candidate for merging (for example if it has been sent to the driver).
The last merge performed is cached as a hint for the subsequent request. If
sequential data is being submitted, the hint is used to perform merges without
any scanning. This is not sufficient when there are multiple processes doing
I/O though, so a "merge hash" is used by some schedulers.
iii. Merge hash
ii. Merge hash
AS and deadline use a hash table indexed by the last sector of a request. This
enables merging code to quickly look up "back merge" candidates, even when
multiple I/O streams are being performed at once on one disk.
......@@ -1013,29 +1025,8 @@ multiple I/O streams are being performed at once on one disk.
are far less common than "back merges" due to the nature of most I/O patterns.
Front merges are handled by the binary trees in AS and deadline schedulers.
iv. Handling barrier cases
A request with flags REQ_HARDBARRIER or REQ_SOFTBARRIER must not be ordered
around. That is, they must be processed after all older requests, and before
any newer ones. This includes merges!
In AS and deadline schedulers, barriers have the effect of flushing the reorder
queue. The performance cost of this will vary from nothing to a lot depending
on i/o patterns and device characteristics. Obviously they won't improve
performance, so their use should be kept to a minimum.
v. Handling insertion position directives
A request may be inserted with a position directive. The directives are one of
ELEVATOR_INSERT_SORT is a general directive for non-barrier requests.
ELEVATOR_INSERT_BACK is used to insert a barrier to the back of the queue.
ELEVATOR_INSERT_FRONT is used to insert a barrier to the front of the queue, and
overrides the ordering requested by any previous barriers. In practice this is
harmless and required, because it is used for SCSI requeueing. This does not
require flushing the reorder queue, so does not impose a performance penalty.
vi. Plugging the queue to batch requests in anticipation of opportunities for
merge/sort optimizations
iii. Plugging the queue to batch requests in anticipation of opportunities for
merge/sort optimizations
This is just the same as in 2.4 so far, though per-device unplugging
support is anticipated for 2.5. Also with a priority-based i/o scheduler,
......@@ -1069,7 +1060,7 @@ Aside:
blk_kick_queue() to unplug a specific queue (right away ?)
or optionally, all queues, is in the plan.
4.3 I/O contexts
4.4 I/O contexts
I/O contexts provide a dynamically allocated per process data area. They may
be used in I/O schedulers, and in the block layer (could be used for IO statis,
priorities for example). See *io_context in drivers/block/ll_rw_blk.c, and
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