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 <andrealmeid@xxxxxxxxxxxxx> --- 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 +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 +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 +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 +the block device, acting as a middleware between the userspace (and a file +system, if present) and the block device driver. + +The blk-mq has two group of queues: software staging queues and hardware +dispatch queues. When the request arrives the block layer, it will try the +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 +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 +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 +resources to accept more requests, it will place requests at 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 +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 +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 +<https://www.kernel.org/doc/html/latest/block/index.html>`_. 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. + +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 <http://kernel.dk/blk-mq.pdf>`_ + +- `NOOP scheduler <https://en.wikipedia.org/wiki/Noop_scheduler>`_ + +- `Null block device driver <https://www.kernel.org/doc/html/latest/block/null_blk.html>`_ + +Source code documentation +========================= + +.. kernel-doc:: include/linux/blk-mq.h + +.. kernel-doc:: block/blk-mq.c diff --git a/Documentation/block/index.rst b/Documentation/block/index.rst index 3fa7a52fafa4..3a3f38322185 100644 --- a/Documentation/block/index.rst +++ b/Documentation/block/index.rst @@ -10,6 +10,7 @@ Block bfq-iosched biodoc biovecs + blk-mq capability cmdline-partition data-integrity -- 2.26.2
