Re: [RESEND,v5,1/2] bio: limit bio max size

[Ming Lei <ming.lei@redhat.com>]

On Sun, Apr 11, 2021 at 10:13:01PM +0000, Damien Le Moal wrote:
> On 2021/04/09 23:47, Bart Van Assche wrote:
> > On 4/7/21 3:27 AM, Damien Le Moal wrote:
> >> On 2021/04/07 18:46, Changheun Lee wrote:
> >>> I'll prepare new patch as you recommand. It will be added setting of
> >>> limit_bio_size automatically when queue max sectors is determined.
> >>
> >> Please do that in the driver for the HW that benefits from it. Do not do this
> >> for all block devices.
> > 
> > Hmm ... is it ever useful to build a bio with a size that exceeds 
> > max_hw_sectors when submitting a bio directly to a block device, or in 
> > other words, if no stacked block driver sits between the submitter and 
> > the block device? Am I perhaps missing something?
> 
> Device performance wise, the benefits are certainly not obvious to me either.
> But for very fast block devices, I think the CPU overhead of building more
> smaller BIOs may be significant compared to splitting a large BIO into multiple
> requests. Though it may be good to revisit this with some benchmark numbers.

This patch tries to address issue[1] in do_direct_IO() in which
Changheun observed that other operations takes time between adding page
to bio.

However, do_direct_IO() just does following except for adding bio and
submitting bio:

- retrieves pages at batch(pin 64 pages each time from VM) and 

- retrieve block mapping(get_more_blocks), which is still done usually
very less times for 32MB; for new mapping, clean_bdev_aliases() may
take a bit time.

If there isn't system memory pressure, pin 64 pages won't be slow, but
get_more_blocks() may take a bit time.

Changheun, can you check if multiple get_more_blocks() is called for submitting
32MB in your test?

In my 32MB sync dio f2fs test on x86_64 VM, one buffer_head mapping can
hold 32MB, but it is one freshly new f2fs.

I'd suggest to understand the issue completely before figuring out one
solution.


[1] https://lore.kernel.org/linux-block/20210202041204.28995-1-nanich.lee@samsung.com/


Thanks,
Ming