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From:   Damien Le Moal <Damien.LeMoal@wdc.com>
To:     Bart Van Assche <bvanassche@acm.org>,
        Changheun Lee <nanich.lee@samsung.com>,
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Subject: Re: [RESEND,v5,1/2] bio: limit bio max size
Thread-Topic: [RESEND,v5,1/2] bio: limit bio max size
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Date:   Sun, 11 Apr 2021 22:13:01 +0000
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On 2021/04/09 23:47, Bart Van Assche wrote:=0A=
> On 4/7/21 3:27 AM, Damien Le Moal wrote:=0A=
>> On 2021/04/07 18:46, Changheun Lee wrote:=0A=
>>> I'll prepare new patch as you recommand. It will be added setting of=0A=
>>> limit_bio_size automatically when queue max sectors is determined.=0A=
>>=0A=
>> Please do that in the driver for the HW that benefits from it. Do not do=
 this=0A=
>> for all block devices.=0A=
> =0A=
> Hmm ... is it ever useful to build a bio with a size that exceeds =0A=
> max_hw_sectors when submitting a bio directly to a block device, or in =
=0A=
> other words, if no stacked block driver sits between the submitter and =
=0A=
> the block device? Am I perhaps missing something?=0A=
=0A=
Device performance wise, the benefits are certainly not obvious to me eithe=
r.=0A=
But for very fast block devices, I think the CPU overhead of building more=
=0A=
smaller BIOs may be significant compared to splitting a large BIO into mult=
iple=0A=
requests. Though it may be good to revisit this with some benchmark numbers=
.=0A=
=0A=
> =0A=
> Thanks,=0A=
> =0A=
> Bart.=0A=
> =0A=
=0A=
=0A=
-- =0A=
Damien Le Moal=0A=
Western Digital Research=0A=