Messages in this thread | | | Subject | Re: [PATCH 00/14] introduce the BFQ-v0 I/O scheduler as an extra scheduler | From | Jens Axboe <> | Date | Fri, 28 Oct 2016 08:07:35 -0600 |
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On 10/27/2016 04:27 PM, Linus Walleij wrote: > On Thu, Oct 27, 2016 at 11:08 PM, Jens Axboe <axboe@kernel.dk> wrote: > >> blk-mq has evolved to support a variety of devices, there's nothing >> special about mmc that can't work well within that framework. > > There is. Read mmc_queue_thread() in drivers/mmc/card/queue.c > > This repeatedly calls req = blk_fetch_request(q);, starting one request > and then getting the next one off the queue, including reading > a few NULL requests off the end of the queue (to satisfy the > semantics of its state machine. > > It then preprocess each request by esstially calling .pre() and .post() > hooks all the way down to the driver, flushing its mapped > sglist from CPU to DMA device memory (not a problem on x86 and > other DMA-coherent archs, but a big win on the incoherent ones). > > In the attempt that was posted recently this is achieved by lying > and saying the HW queue is two items deep and eating requests > off that queue calling pre/post on them. > > But as there actually exist MMC cards with command queueing, this > would become hopeless to handle, the hw queue depth has to reflect > the real depth. What we need is for the block core to call pre/post > hooks on each request. > > The "only" thing that doesn't work well after that is that CFQ is no > longer in action, which will have interesting effects on MMC throughput > in any fio-like stress test as it is mostly single-hw-queue.
That will cause you pain with any IO scheduler that has more complex state, like CFQ and BFQ... I looked at the code but I don't quite get why it is handling requests like that. Care to expand? Is it a performance optimization? It looks fairly convoluted for some reason. I would imagine that latency would be one of the more important aspects for mmc, yet the driver has a context switch for each sync IO.
-- Jens Axboe
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