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

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Message-ID: <BL0PR04MB65148B0629FF23847341BAE8E7719@BL0PR04MB6514.namprd04.prod.outlook.com>
Date:   Sun, 11 Apr 2021 22:13:01 +0000
From:   Damien Le Moal <Damien.LeMoal@....com>
To:     Bart Van Assche <bvanassche@....org>,
        Changheun Lee <nanich.lee@...sung.com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
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Subject: Re: [RESEND,v5,1/2] bio: limit bio max size

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.

> 
> Thanks,
> 
> Bart.
> 


-- 
Damien Le Moal
Western Digital Research