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SubjectRe: [PATCH] docs: block: Create blk-mq documentation
On 5/27/20 1:09 PM, André Almeida wrote:
> Create a documentation providing a background and explanation around the
> operation of the Multi-Queue Block IO Queueing Mechanism (blk-mq).
> The reference for writing this documentation was the source code and
> "Linux Block IO: Introducing Multi-queue SSD Access on Multi-core
> Systems", by Axboe et al.
> Signed-off-by: André Almeida <>
> ---
> Hello,
> This commit was tested using "make htmldocs" and the HTML output has
> been verified.
> Thanks,
> André
> ---
> Documentation/block/blk-mq.rst | 154 +++++++++++++++++++++++++++++++++
> Documentation/block/index.rst | 1 +
> 2 files changed, 155 insertions(+)
> create mode 100644 Documentation/block/blk-mq.rst
> diff --git a/Documentation/block/blk-mq.rst b/Documentation/block/blk-mq.rst
> new file mode 100644
> index 000000000000..4c37b37df50e
> --- /dev/null
> +++ b/Documentation/block/blk-mq.rst
> @@ -0,0 +1,154 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +================================================
> +Multi-Queue Block IO Queueing Mechanism (blk-mq)
> +================================================
> +
> +The Multi-Queue Block IO Queueing Mechanism is an API to enable fast storage
> +devices to achieve a huge number of input/output operations per second (IOPS)
> +through queueing and submitting IO requests to block devices simultaneously,
> +benefiting from the parallelism offered by modern storage devices.
> +
> +Introduction
> +============
> +
> +Background
> +----------
> +
> +Magnetic hard disks have been the de facto standard from the beginning of the
> +development of the kernel. The Block IO subsystem aimed to achieve the best
> +performance possible for those devices with a high penalty when doing random
> +access, and the bottleneck was the mechanical moving parts, a lot more slower
> +than any layer on the storage stack. One example of such optimization technique
> +involves ordering read/write requests accordingly to the current position of
> +the hard disk head.
> +
> +However, with the development of Solid State Drivers and Non-Volatile Memories

Drives ??
> +without mechanical parts nor random access penalty and capable of performing
> +high parallel access, the bottleneck of the stack had moved from the storage
> +device to the operating system. In order to take advantage of the parallelism
> +in those devices design, the multi-queue mechanism was introduced.
> +
> +The former design had a single queue to store block IO requests with a single
> +lock, that did not scale well in SMP systems due to dirty data in cache and the

lock. That did not

> +bottleneck of having a single lock for multiple processors. This setup also
> +suffered with congestion when different processes (or the same process, moving
> +to different CPUs) wanted to perform block IO. Instead of this, this API spawns

Instead of this, the blk-mq API spawns

> +multiple queues with individual entry points local to the CPU, removing the
> +need for a lock. A deeper explanation on how this works is covered in the
> +following section (`Operation`_).
> +
> +Operation
> +---------
> +
> +When the userspace performs IO to a block device (reading or writing a file,
> +for instance), the blk-mq takes action: it will store and manage IO requests to

blk-mq takes action:

> +the block device, acting as a middleware between the userspace (and a file

acting as middleware

> +system, if present) and the block device driver.
> +
> +The blk-mq has two group of queues: software staging queues and hardware

The blk-mq mechanism has two groups
or just
blk-mq has two groups

> +dispatch queues. When the request arrives the block layer, it will try the

arrives at the block layer,

> +shortest path possible: send it directly to the hardware queue. However, there
> +are two cases that it might not to do that: if there's an IO scheduler attached

might not do that:

> +at the layer or if we want to try to merge requests. In both cases, requests
> +will be sent to the software queue.
> +
> +Then, after the requests being processed at software queues, they will be

after the requests are processed at [or by] software queues,

> +placed at the hardware queue, a second stage queue were the hardware has direct
> +access to process those requests. However, if the hardware has not enough

hardware does not have enough

> +resources to accept more requests, it will place requests at temporary queue,

What does "it" refer to? ^^^^
places requests on a temporary queue,

> +to be sent in the future, when the hardware is able.
> +
> +Software staging queues
> +~~~~~~~~~~~~~~~~~~~~~~~
> +
> +The block IO subsystem adds requests (represented by struct
> +:c:type:`blk_mq_ctx`) in the software staging queues in case that they weren't
> +sent directly to the driver. A request is a collection of BIOs. They arrived at
> +the block layer through the data structures struct :c:type:`bio`. The block

data structure

> +layer will then build a new structure from it, the struct :c:type:`request`
> +that will be used to communicate with the device driver. Each queue has its
> +owns lock and the number of queues is defined by a per-CPU or per-node basis.


> +
> +The staging queue can be used to merge requests for adjacent sectors. For
> +instance, requests for sector 3-6, 6-7, 7-9 can become one request for 3-9.
> +Even if random access to SSDs and NVMs have the same time of response compared
> +to sequential access, grouped requests for sequential access decreases the
> +number of individual requests. This technique of merging requests is called
> +plugging.
> +
> +Along with that, the requests can be reordered to ensure fairness of system
> +resources (e.g. to ensure that no application suffer from starvation) and/or to


> +improve IO performance, by an IO scheduler.
> +
> +IO Schedulers
> +^^^^^^^^^^^^^
> +
> +There are several schedulers implemented by the block layer, each one following
> +a heuristics to improve the IO performance. They are "pluggable" (as in plug

a heuristic

> +and play), in the sense of they can be selected at run time using sysfs. You
> +can read more about Linux's IO schedulers `here
> +<>`_. The scheduling
> +happens only between requests in the same queue, so it is not possible to merge
> +requests from different queues, otherwise there would be cache trashing and a
> +need to have a lock for each queue. After the scheduling, the requests are
> +eligible to be sent to the hardware. One of the possibles schedulers to be


> +selected is the NOOP scheduler, the most straightforward one, that implements a
> +simple FIFO, without performing any reordering. This is useful in the following
> +scenarios: when scheduling will be performed in a next step somewhere in the
> +stack, like block devices controllers; the actual sector position of blocks are


> +transparent for the host, meaning it hasn't enough information to take a proper
> +decision; or the overhead of reordering is higher than the handicap of
> +non-sequential accesses.
> +
> +Hardware dispatch queues
> +~~~~~~~~~~~~~~~~~~~~~~~~
> +
> +The hardware queue is a memory space shared with the block device (e.g. DMA)
> +where the hardware can access and dispatch requests (represented by struct
> +:c:type:`blk_mq_hw_ctx`). To run this queue, the block layer removes
> +requests from the associated software queues and tries to dispatch to the
> +hardware.

This paragraph (above) says to me that these HW devices understand struct blk_mq_hw_ctx.
Is that correct? Is is some kind of standard? If so, where?

> +If it's not possible to send the requests directly to hardware, they will be
> +added to a linked list (:c:type:`hctx->dispatched`) of requests. Then,
> +next time the block layer runs a queue, it will send the requests laying at the
> +:c:type:`dispatched` list first, to ensure a fairness dispatch with those
> +requests that were ready to be sent first. The number of hardware queues
> +depends on the number of hardware context supported by the hardware and its


> +device driver, but it will not be more than the number of cores of the system.
> +There is no reordering at this stage, and each software queues has a set of


> +hardware queues to send requests for.
> +
> +.. note::
> +
> + Neither the block layer nor the device protocols guarantee
> + the order of completion of requests. This must be handled by
> + higher layers, like the filesystem.
> +
> +Tag-based completion
> +~~~~~~~~~~~~~~~~~~~~
> +
> +In order to indicate which request has been completed, every request is
> +identified by an integer, ranging from 0 to the dispatch queue size. This tag
> +is generated by the block layer and later reused by the device driver, removing
> +the need to create a redundant identifier. When a request is completed in the
> +drive, the tag is sent back to the block layer to notify it of the finalization.
> +This removes the need to do a linear search to find out which IO has been
> +completed.
> +
> +Further reading
> +---------------
> +
> +- `Linux Block IO: Introducing Multi-queue SSD Access on Multi-core Systems <>`_
> +
> +- `NOOP scheduler <>`_
> +
> +- `Null block device driver <>`_
> +
> +Source code documentation
> +=========================
> +
> +.. kernel-doc:: include/linux/blk-mq.h
> +
> +.. kernel-doc:: block/blk-mq.c

thanks for the documentation.

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