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SubjectRe: [PATCH v8 0/5] Add MMC software queue support
On Wed, 5 Feb 2020 at 13:51, Baolin Wang <baolin.wang7@gmail.com> wrote:
>
> Hi All,
>
> Now the MMC read/write stack will always wait for previous request is
> completed by mmc_blk_rw_wait(), before sending a new request to hardware,
> or queue a work to complete request, that will bring context switching
> overhead, especially for high I/O per second rates, to affect the IO
> performance.

In the regular request path (non CQE), we call mmc_blk_card_busy() to
complete a request. For write I/O, this leads to calling
card_busy_detect(), which starts to poll the card by sending a CMD13.

At least one CMD13 will be sent to the card, before we exit the
polling loop and a new I/O request can get submitted. However, in many
cases, depending on the controller/host/card/request-size, my best
guess is that *one* CMD13 might not be sufficient. At least, that is
what I have observed on those platforms I recently have been working
on.

That said, I am wondering if you have done some measurement/profiling
on this particular behaviour for your controller/driver? For example,
how many CMD13 gets sent for random small writes during polling?

Why am I asking this? Because, unless I am mistaken, when using the
new hsq path that you introduce in $subject series, based on the cqe
ops, then mmc_blk_card_busy() is not being called at all. In other
words, you rely on HW busy detection from the controller/driver,
rather than polling with CMD13. Is that correct?

This seems like an additional reason to why you achieve significant
improvements for the random write case. Don't you think?

>
> Thus this patch set will introduce the MMC software command queue support
> based on command queue engine's interfaces, and set the queue depth as 64
> to allow more requests can be be prepared, merged and inserted into IO
> scheduler, but we only allow 2 requests in flight, that is enough to let
> the irq handler always trigger the next request without a context switch,
> as well as avoiding a long latency.
>
> Moreover we can expand the MMC software queue interface to support
> MMC packed request or packed command instead of adding new interfaces,
> according to previosus discussion.
>
> Below are some comparison data with fio tool. The fio command I used
> is like below with changing the '--rw' parameter and enabling the direct
> IO flag to measure the actual hardware transfer speed in 4K block size.
>
> ./fio --filename=/dev/mmcblk0p30 --direct=1 --iodepth=20 --rw=read --bs=4K --size=1G --group_reporting --numjobs=20 --name=test_read
>
> My eMMC card working at HS400 Enhanced strobe mode:
> [ 2.229856] mmc0: new HS400 Enhanced strobe MMC card at address 0001
> [ 2.237566] mmcblk0: mmc0:0001 HBG4a2 29.1 GiB
> [ 2.242621] mmcblk0boot0: mmc0:0001 HBG4a2 partition 1 4.00 MiB
> [ 2.249110] mmcblk0boot1: mmc0:0001 HBG4a2 partition 2 4.00 MiB
> [ 2.255307] mmcblk0rpmb: mmc0:0001 HBG4a2 partition 3 4.00 MiB, chardev (248:0)
>
> 1. Without MMC software queue
> I tested 5 times for each case and output a average speed.
>
> 1) Sequential read:
> Speed: 59.4MiB/s, 63.4MiB/s, 57.5MiB/s, 57.2MiB/s, 60.8MiB/s
> Average speed: 59.66MiB/s
>
> 2) Random read:
> Speed: 26.9MiB/s, 26.9MiB/s, 27.1MiB/s, 27.1MiB/s, 27.2MiB/s
> Average speed: 27.04MiB/s
>
> 3) Sequential write:
> Speed: 71.6MiB/s, 72.5MiB/s, 72.2MiB/s, 64.6MiB/s, 67.5MiB/s
> Average speed: 69.68MiB/s
>
> 4) Random write:
> Speed: 36.3MiB/s, 35.4MiB/s, 38.6MiB/s, 34MiB/s, 35.5MiB/s
> Average speed: 35.96MiB/s
>
> 2. With MMC software queue
> I tested 5 times for each case and output a average speed.
>
> 1) Sequential read:
> Speed: 59.2MiB/s, 60.4MiB/s, 63.6MiB/s, 60.3MiB/s, 59.9MiB/s
> Average speed: 60.68MiB/s
>
> 2) Random read:
> Speed: 31.3MiB/s, 31.4MiB/s, 31.5MiB/s, 31.3MiB/s, 31.3MiB/s
> Average speed: 31.36MiB/s
>
> 3) Sequential write:
> Speed: 71MiB/s, 71.8MiB/s, 72.3MiB/s, 72.2MiB/s, 71MiB/s
> Average speed: 71.66MiB/s
>
> 4) Random write:
> Speed: 68.9MiB/s, 68.7MiB/s, 68.8MiB/s, 68.6MiB/s, 68.8MiB/s
> Average speed: 68.76MiB/s
>
> Form above data, we can see the MMC software queue can help to improve some
> performance obviously for random read and write, though no obvious improvement
> for sequential read and write.
>
> Any comments are welcome. Thanks a lot.
>
> Changes from v7:
> - Add reviewed tag from Arnd.
> - Use the 'hsq' acronym for varibles and functions in the core layer.
> - Check the 'card->ext_csd.cmdq_en' in cqhci.c to make sure the CQE
> can work normally.
> - Add a new patch to enable the host software queue for the SD card.
> - Use the default MMC queue depth for host software queue.

It would be nice to also have some measurements for an SD card, now
that the series supports this. Is that possible for you test as well?

[...]

Kind regards
Uffe

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