154 lines
7.3 KiB
ReStructuredText
154 lines
7.3 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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================================================
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Multi-Queue Block IO Queueing Mechanism (blk-mq)
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================================================
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The Multi-Queue Block IO Queueing Mechanism is an API to enable fast storage
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devices to achieve a huge number of input/output operations per second (IOPS)
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through queueing and submitting IO requests to block devices simultaneously,
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benefiting from the parallelism offered by modern storage devices.
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Introduction
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============
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Background
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----------
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Magnetic hard disks have been the de facto standard from the beginning of the
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development of the kernel. The Block IO subsystem aimed to achieve the best
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performance possible for those devices with a high penalty when doing random
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access, and the bottleneck was the mechanical moving parts, a lot slower than
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any layer on the storage stack. One example of such optimization technique
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involves ordering read/write requests according to the current position of the
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hard disk head.
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However, with the development of Solid State Drives and Non-Volatile Memories
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without mechanical parts nor random access penalty and capable of performing
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high parallel access, the bottleneck of the stack had moved from the storage
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device to the operating system. In order to take advantage of the parallelism
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in those devices' design, the multi-queue mechanism was introduced.
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The former design had a single queue to store block IO requests with a single
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lock. That did not scale well in SMP systems due to dirty data in cache and the
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bottleneck of having a single lock for multiple processors. This setup also
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suffered with congestion when different processes (or the same process, moving
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to different CPUs) wanted to perform block IO. Instead of this, the blk-mq API
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spawns multiple queues with individual entry points local to the CPU, removing
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the need for a lock. A deeper explanation on how this works is covered in the
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following section (`Operation`_).
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Operation
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---------
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When the userspace performs IO to a block device (reading or writing a file,
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for instance), blk-mq takes action: it will store and manage IO requests to
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the block device, acting as middleware between the userspace (and a file
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system, if present) and the block device driver.
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blk-mq has two group of queues: software staging queues and hardware dispatch
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queues. When the request arrives at the block layer, it will try the shortest
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path possible: send it directly to the hardware queue. However, there are two
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cases that it might not do that: if there's an IO scheduler attached at the
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layer or if we want to try to merge requests. In both cases, requests will be
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sent to the software queue.
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Then, after the requests are processed by software queues, they will be placed
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at the hardware queue, a second stage queue where the hardware has direct access
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to process those requests. However, if the hardware does not have enough
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resources to accept more requests, blk-mq will places requests on a temporary
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queue, to be sent in the future, when the hardware is able.
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Software staging queues
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~~~~~~~~~~~~~~~~~~~~~~~
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The block IO subsystem adds requests in the software staging queues
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(represented by struct blk_mq_ctx) in case that they weren't sent
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directly to the driver. A request is one or more BIOs. They arrived at the
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block layer through the data structure struct bio. The block layer
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will then build a new structure from it, the struct request that will
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be used to communicate with the device driver. Each queue has its own lock and
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the number of queues is defined by a per-CPU or per-node basis.
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The staging queue can be used to merge requests for adjacent sectors. For
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instance, requests for sector 3-6, 6-7, 7-9 can become one request for 3-9.
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Even if random access to SSDs and NVMs have the same time of response compared
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to sequential access, grouped requests for sequential access decreases the
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number of individual requests. This technique of merging requests is called
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plugging.
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Along with that, the requests can be reordered to ensure fairness of system
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resources (e.g. to ensure that no application suffers from starvation) and/or to
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improve IO performance, by an IO scheduler.
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IO Schedulers
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^^^^^^^^^^^^^
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There are several schedulers implemented by the block layer, each one following
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a heuristic to improve the IO performance. They are "pluggable" (as in plug
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and play), in the sense of they can be selected at run time using sysfs. You
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can read more about Linux's IO schedulers `here
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<https://www.kernel.org/doc/html/latest/block/index.html>`_. The scheduling
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happens only between requests in the same queue, so it is not possible to merge
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requests from different queues, otherwise there would be cache trashing and a
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need to have a lock for each queue. After the scheduling, the requests are
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eligible to be sent to the hardware. One of the possible schedulers to be
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selected is the NONE scheduler, the most straightforward one. It will just
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place requests on whatever software queue the process is running on, without
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any reordering. When the device starts processing requests in the hardware
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queue (a.k.a. run the hardware queue), the software queues mapped to that
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hardware queue will be drained in sequence according to their mapping.
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Hardware dispatch queues
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~~~~~~~~~~~~~~~~~~~~~~~~
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The hardware queue (represented by struct blk_mq_hw_ctx) is a struct
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used by device drivers to map the device submission queues (or device DMA ring
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buffer), and are the last step of the block layer submission code before the
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low level device driver taking ownership of the request. To run this queue, the
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block layer removes requests from the associated software queues and tries to
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dispatch to the hardware.
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If it's not possible to send the requests directly to hardware, they will be
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added to a linked list (``hctx->dispatch``) of requests. Then,
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next time the block layer runs a queue, it will send the requests laying at the
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``dispatch`` list first, to ensure a fairness dispatch with those
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requests that were ready to be sent first. The number of hardware queues
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depends on the number of hardware contexts supported by the hardware and its
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device driver, but it will not be more than the number of cores of the system.
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There is no reordering at this stage, and each software queue has a set of
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hardware queues to send requests for.
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.. note::
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Neither the block layer nor the device protocols guarantee
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the order of completion of requests. This must be handled by
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higher layers, like the filesystem.
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Tag-based completion
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~~~~~~~~~~~~~~~~~~~~
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In order to indicate which request has been completed, every request is
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identified by an integer, ranging from 0 to the dispatch queue size. This tag
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is generated by the block layer and later reused by the device driver, removing
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the need to create a redundant identifier. When a request is completed in the
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driver, the tag is sent back to the block layer to notify it of the finalization.
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This removes the need to do a linear search to find out which IO has been
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completed.
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Further reading
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---------------
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- `Linux Block IO: Introducing Multi-queue SSD Access on Multi-core Systems <http://kernel.dk/blk-mq.pdf>`_
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- `NOOP scheduler <https://en.wikipedia.org/wiki/Noop_scheduler>`_
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- `Null block device driver <https://www.kernel.org/doc/html/latest/block/null_blk.html>`_
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Source code documentation
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=========================
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.. kernel-doc:: include/linux/blk-mq.h
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.. kernel-doc:: block/blk-mq.c
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