linux-ext4 - Re: bio splits unnecessarily due to BH

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Date:	Mon, 01 Apr 2013 17:25:56 +0900
From:	Kazuya Mio <k-mio@...jp.nec.com>
To:	Jan Kara <jack@...e.cz>
CC:	akpm@...ux-foundation.org, adilger.kernel@...ger.ca,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: bio splits unnecessarily due to BH_Boundary in ext3 direct I/O

2013/03/30 2:15, Jan Kara wrote:
>   Sorry for not getting to you earlier. So we agree in our analysis. Do you
> agree with the attached patch? Does it help your workload?

According to your two patches, direct I/O works as the following steps:
1. add sdio->cur_page to sdio->bio by dio_new_bio() or dio_bio_add_page()
2. submit sdio->bio if sdio->boudary is set
3. set the curret page to sdio->cur_page in submit_page_section()

Only one page is submitted separately because of submitting bio before step 3.

For example, we write 52KB data with O_DIRECT, it is ideal that filesystem
submits a bio twice (48KB and 4KB). However, after applying your two patches,
52KB data is split into three bios (44KB, 4KB and 4KB).

Regards,
Kazuya Mio
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