mirror of
https://github.com/AuxXxilium/linux_dsm_epyc7002.git
synced 2024-11-24 16:30:52 +07:00
898bd37a92
Rename the block documentation files to ReST, add an index for them and adjust in order to produce a nice html output via the Sphinx build system. At its new index.rst, let's add a :orphan: while this is not linked to the main index.rst file, in order to avoid build warnings. Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
73 lines
2.9 KiB
ReStructuredText
73 lines
2.9 KiB
ReStructuredText
==============================
|
|
Deadline IO scheduler tunables
|
|
==============================
|
|
|
|
This little file attempts to document how the deadline io scheduler works.
|
|
In particular, it will clarify the meaning of the exposed tunables that may be
|
|
of interest to power users.
|
|
|
|
Selecting IO schedulers
|
|
-----------------------
|
|
Refer to Documentation/block/switching-sched.rst for information on
|
|
selecting an io scheduler on a per-device basis.
|
|
|
|
------------------------------------------------------------------------------
|
|
|
|
read_expire (in ms)
|
|
-----------------------
|
|
|
|
The goal of the deadline io scheduler is to attempt to guarantee a start
|
|
service time for a request. As we focus mainly on read latencies, this is
|
|
tunable. When a read request first enters the io scheduler, it is assigned
|
|
a deadline that is the current time + the read_expire value in units of
|
|
milliseconds.
|
|
|
|
|
|
write_expire (in ms)
|
|
-----------------------
|
|
|
|
Similar to read_expire mentioned above, but for writes.
|
|
|
|
|
|
fifo_batch (number of requests)
|
|
------------------------------------
|
|
|
|
Requests are grouped into ``batches`` of a particular data direction (read or
|
|
write) which are serviced in increasing sector order. To limit extra seeking,
|
|
deadline expiries are only checked between batches. fifo_batch controls the
|
|
maximum number of requests per batch.
|
|
|
|
This parameter tunes the balance between per-request latency and aggregate
|
|
throughput. When low latency is the primary concern, smaller is better (where
|
|
a value of 1 yields first-come first-served behaviour). Increasing fifo_batch
|
|
generally improves throughput, at the cost of latency variation.
|
|
|
|
|
|
writes_starved (number of dispatches)
|
|
--------------------------------------
|
|
|
|
When we have to move requests from the io scheduler queue to the block
|
|
device dispatch queue, we always give a preference to reads. However, we
|
|
don't want to starve writes indefinitely either. So writes_starved controls
|
|
how many times we give preference to reads over writes. When that has been
|
|
done writes_starved number of times, we dispatch some writes based on the
|
|
same criteria as reads.
|
|
|
|
|
|
front_merges (bool)
|
|
----------------------
|
|
|
|
Sometimes it happens that a request enters the io scheduler that is contiguous
|
|
with a request that is already on the queue. Either it fits in the back of that
|
|
request, or it fits at the front. That is called either a back merge candidate
|
|
or a front merge candidate. Due to the way files are typically laid out,
|
|
back merges are much more common than front merges. For some work loads, you
|
|
may even know that it is a waste of time to spend any time attempting to
|
|
front merge requests. Setting front_merges to 0 disables this functionality.
|
|
Front merges may still occur due to the cached last_merge hint, but since
|
|
that comes at basically 0 cost we leave that on. We simply disable the
|
|
rbtree front sector lookup when the io scheduler merge function is called.
|
|
|
|
|
|
Nov 11 2002, Jens Axboe <jens.axboe@oracle.com>
|