Re: [PATCH 0/4] iopoll support for io_uring/nvme passthrough

[Kanchan Joshi <joshi.k@samsung.com>]

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On Fri, Aug 05, 2022 at 12:15:24PM -0600, Jens Axboe wrote:
>On 8/5/22 12:11 PM, Keith Busch wrote:
>> On Fri, Aug 05, 2022 at 11:18:38AM -0600, Jens Axboe wrote:
>>> On 8/5/22 11:04 AM, Jens Axboe wrote:
>>>> On 8/5/22 9:42 AM, Kanchan Joshi wrote:
>>>>> Hi,
>>>>>
>>>>> Series enables async polling on io_uring command, and nvme passthrough
>>>>> (for io-commands) is wired up to leverage that.
>>>>>
>>>>> 512b randread performance (KIOP) below:
>>>>>
>>>>> QD_batch    block    passthru    passthru-poll   block-poll
>>>>> 1_1          80        81          158            157
>>>>> 8_2         406       470          680            700
>>>>> 16_4        620       656          931            920
>>>>> 128_32      879       1056        1120            1132
>>>>
>>>> Curious on why passthru is slower than block-poll? Are we missing
>>>> something here?
>>>
>>> I took a quick peek, running it here. List of items making it slower:
>>>
>>> - No fixedbufs support for passthru, each each request will go through
>>>   get_user_pages() and put_pages() on completion. This is about a 10%
>>>   change for me, by itself.
>>
>> Enabling fixed buffer support through here looks like it will take a
>> little bit of work. The driver needs an opcode or flag to tell it the
>> user address is a fixed buffer, and io_uring needs to export its
>> registered buffer for a driver like nvme to get to.
>
>Yeah, it's not a straight forward thing. But if this will be used with
>recycled buffers, then it'll definitely be worthwhile to look into.

Had posted bio-cache and fixedbufs in the initial round but retracted
to get the foundation settled first.
https://lore.kernel.org/linux-nvme/20220308152105.309618-1-joshi.k@samsung.com/

I see that you brought back bio-cache already. I can refresh fixedbufs.
Completion-batching seems too tightly coupled to block-path.

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