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From:   Changheun Lee <nanich.lee@samsung.com>
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Subject: Re: [RESEND,v5,1/2] bio: limit bio max size
Date:   Tue, 13 Apr 2021 13:37:33 +0900
Message-Id: <20210413043733.28880-1-nanich.lee@samsung.com>
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> 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?

almost one time called.

> 
> 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.

Thank you for your advice. I'll analyze more about your point later. :)
But I think it's different from finding main time spend point in
do_direct_IO(). I think excessive loop should be controlled.
8,192 loops in do_direct_IO() - for 32MB - to submit one bio is too much
on 4KB page system. I want to apply a optional solution to avoid
excessive loop casued by multipage bvec.

Thanks,

Changheun Lee