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

[Maxim Mikityanskiy]

On Wed, Jan 27, 2021 at 6:23 AM Bart Van Assche <bvanassche@xxxxxxx> wrote:
>
> 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@xxxxxxxxx>
> > Fixes: 5ff9f19231a0 ("block: simplify set_init_blocksize")
> > Cc: Christoph Hellwig <hch@xxxxxx>
> > Cc: Jens Axboe <axboe@xxxxxxxxx>
> > ---
> >  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?

I don't know the exact mechanism how this change affects the speed,
I'm not an expert in the block device subsystem (I'm a networking
guy). This commit was found by git bisect, and my performance test
confirmed that reverting it fixes the bug.

It looks to me as this function sets the block size as part of control
flow, and this size is used later in the fast path, and the commit
that removed the loop decreased this block size.

> Bart.
>
>