Re: [PATCH] Revert "block: simplify set_init_blocksize" to regain lost performance

[Bart Van Assche <bvanassche@acm.org>]

On 1/26/21 11:59 AM, Maxim Mikityanskiy wrote:
> The cited commit introduced a serious regression with SATA write speed,
> as found by bisecting. This patch reverts this commit, which restores
> write speed back to the values observed before this commit.
> 
> The performance tests were done on a Helios4 NAS (2nd batch) with 4 HDDs
> (WD8003FFBX) using dd (bs=1M count=2000). "Direct" is a test with a
> single HDD, the rest are different RAID levels built over the first
> partitions of 4 HDDs. Test results are in MB/s, R is read, W is write.
> 
>                 | Direct | RAID0 | RAID10 f2 | RAID10 n2 | RAID6
> ----------------+--------+-------+-----------+-----------+--------
> 9011495c9466    | R:256  | R:313 | R:276     | R:313     | R:323
> (before faulty) | W:254  | W:253 | W:195     | W:204     | W:117
> ----------------+--------+-------+-----------+-----------+--------
> 5ff9f19231a0    | R:257  | R:398 | R:312     | R:344     | R:391
> (faulty commit) | W:154  | W:122 | W:67.7    | W:66.6    | W:67.2
> ----------------+--------+-------+-----------+-----------+--------
> 5.10.10         | R:256  | R:401 | R:312     | R:356     | R:375
> unpatched       | W:149  | W:123 | W:64      | W:64.1    | W:61.5
> ----------------+--------+-------+-----------+-----------+--------
> 5.10.10         | R:255  | R:396 | R:312     | R:340     | R:393
> patched         | W:247  | W:274 | W:220     | W:225     | W:121
> 
> Applying this patch doesn't hurt read performance, while improves the
> write speed by 1.5x - 3.5x (more impact on RAID tests). The write speed
> is restored back to the state before the faulty commit, and even a bit
> higher in RAID tests (which aren't HDD-bound on this device) - that is
> likely related to other optimizations done between the faulty commit and
> 5.10.10 which also improved the read speed.
> 
> Signed-off-by: Maxim Mikityanskiy <maxtram95@gmail.com>
> Fixes: 5ff9f19231a0 ("block: simplify set_init_blocksize")
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Jens Axboe <axboe@kernel.dk>
> ---
>  fs/block_dev.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/block_dev.c b/fs/block_dev.c
> index 3b8963e228a1..235b5042672e 100644
> --- a/fs/block_dev.c
> +++ b/fs/block_dev.c
> @@ -130,7 +130,15 @@ EXPORT_SYMBOL(truncate_bdev_range);
>  
>  static void set_init_blocksize(struct block_device *bdev)
>  {
> -	bdev->bd_inode->i_blkbits = blksize_bits(bdev_logical_block_size(bdev));
> +	unsigned int bsize = bdev_logical_block_size(bdev);
> +	loff_t size = i_size_read(bdev->bd_inode);
> +
> +	while (bsize < PAGE_SIZE) {
> +		if (size & bsize)
> +			break;
> +		bsize <<= 1;
> +	}
> +	bdev->bd_inode->i_blkbits = blksize_bits(bsize);
>  }
>  
>  int set_blocksize(struct block_device *bdev, int size)

How can this patch affect write speed? I haven't found any calls of
set_init_blocksize() in the I/O path. Did I perhaps overlook something?

Bart.