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Subject: Re: [PATCH v9] bio: limit bio max size
To:     Changheun Lee <nanich.lee@samsung.com>, bvanassche@acm.org,
        yi.zhang@redhat.com, ming.lei@redhat.com, bgoncalv@redhat.com,
        hch@lst.de, jaegeuk@kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, patchwork-bot@kernel.org
References: <CGME20210503101008epcas1p25d6b727dafcf9ff24db6aa41a3f611fa@epcas1p2.samsung.com>
 <20210503095203.29076-1-nanich.lee@samsung.com>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <c2951dc5-18bc-6c99-7cdc-21e6a8a12f9c@kernel.dk>
Date:   Mon, 3 May 2021 11:42:06 -0600
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On 5/3/21 3:52 AM, Changheun Lee wrote:
> bio size can grow up to 4GB when muli-page bvec is enabled.
> but sometimes it would lead to inefficient behaviors.
> in case of large chunk direct I/O, - 32MB chunk read in user space -
> all pages for 32MB would be merged to a bio structure if the pages
> physical addresses are contiguous. it makes some delay to submit
> until merge complete. bio max size should be limited to a proper size.
> 
> When 32MB chunk read with direct I/O option is coming from userspace,
> kernel behavior is below now in do_direct_IO() loop. it's timeline.
> 
>  | bio merge for 32MB. total 8,192 pages are merged.
>  | total elapsed time is over 2ms.
>  |------------------ ... ----------------------->|
>                                                  | 8,192 pages merged a bio.
>                                                  | at this time, first bio submit is done.
>                                                  | 1 bio is split to 32 read request and issue.
>                                                  |--------------->
>                                                   |--------------->
>                                                    |--------------->
>                                                               ......
>                                                                    |--------------->
>                                                                     |--------------->|
>                           total 19ms elapsed to complete 32MB read done from device. |
> 
> If bio max size is limited with 1MB, behavior is changed below.
> 
>  | bio merge for 1MB. 256 pages are merged for each bio.
>  | total 32 bio will be made.
>  | total elapsed time is over 2ms. it's same.
>  | but, first bio submit timing is fast. about 100us.
>  |--->|--->|--->|---> ... -->|--->|--->|--->|--->|
>       | 256 pages merged a bio.
>       | at this time, first bio submit is done.
>       | and 1 read request is issued for 1 bio.
>       |--------------->
>            |--------------->
>                 |--------------->
>                                       ......
>                                                  |--------------->
>                                                   |--------------->|
>         total 17ms elapsed to complete 32MB read done from device. |
> 
> As a result, read request issue timing is faster if bio max size is limited.
> Current kernel behavior with multipage bvec, super large bio can be created.
> And it lead to delay first I/O request issue.

Applied, thanks.

-- 
Jens Axboe