| .. 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 slower than |
| any layer on the storage stack. One example of such optimization technique |
| involves ordering read/write requests according to the current position of the |
| hard disk head. |
| |
| However, with the development of Solid State Drives 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, 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), blk-mq takes action: it will store and manage IO requests to |
| the block device, acting as middleware between the userspace (and a file |
| system, if present) and the block device driver. |
| |
| blk-mq has two group of queues: software staging queues and hardware dispatch |
| queues. When the request arrives at 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 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 are processed by 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 does not have enough |
| resources to accept more requests, blk-mq will 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 in the software staging queues |
| (represented by struct blk_mq_ctx) in case that they weren't sent |
| directly to the driver. A request is one or more BIOs. They arrived at the |
| block layer through the data structure struct bio. The block layer |
| will then build a new structure from it, the struct request that will |
| be used to communicate with the device driver. Each queue has its own 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 suffers 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 heuristic 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 possible schedulers to be |
| selected is the NONE scheduler, the most straightforward one. It will just |
| place requests on whatever software queue the process is running on, without |
| any reordering. When the device starts processing requests in the hardware |
| queue (a.k.a. run the hardware queue), the software queues mapped to that |
| hardware queue will be drained in sequence according to their mapping. |
| |
| Hardware dispatch queues |
| ~~~~~~~~~~~~~~~~~~~~~~~~ |
| |
| The hardware queue (represented by struct blk_mq_hw_ctx) is a struct |
| used by device drivers to map the device submission queues (or device DMA ring |
| buffer), and are the last step of the block layer submission code before the |
| low level device driver taking ownership of the request. 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 (``hctx->dispatch``) of requests. Then, |
| next time the block layer runs a queue, it will send the requests laying at the |
| ``dispatch`` 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 contexts 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 queue 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 |
| driver, 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 |