Re: [PATCH v10 0/1] bio: limit bio max size

[Christoph Hellwig]

On Fri, May 14, 2021 at 03:32:41PM +0900, Changheun Lee wrote:
> I tested 512MB file read with direct I/O. and chunk size is 64MB.
>  - on SCSI disk, with no limit of bio max size(4GB) : avg. 630 MB/s
>  - on SCSI disk, with limit bio max size to 1MB     : avg. 645 MB/s
>  - on ramdisk, with no limit of bio max size(4GB)   : avg. 2749 MB/s
>  - on ramdisk, with limit bio max size to 1MB       : avg. 3068 MB/s
> 
> I set ramdisk environment as below.
>  - dd if=/dev/zero of=/mnt/ramdisk.img bs=$((1024*1024)) count=1024
>  - mkfs.ext4 /mnt/ramdisk.img
>  - mkdir /mnt/ext4ramdisk
>  - mount -o loop /mnt/ramdisk.img /mnt/ext4ramdisk
> 
> With low performance disk, bio submit delay caused by large bio size is
> not big protion. So it can't be feel easily. But it will be shown in high
> performance disk.

So let's attack the problem properly:

 1) switch f2fs to a direct I/O implementation that does not suck
 2) look into optimizing the iomap code to e.g. submit the bio once
    it is larger than queue_io_opt() without failing to add to a bio
    which would be annoying for things like huge pages.