Re: [PATCH V4] block: optimize for small block size IO

[Jens Axboe <axboe@kernel.dk>]

On 11/4/19 11:17 AM, Kent Overstreet wrote:
> On Mon, Nov 04, 2019 at 10:15:41AM -0800, Christoph Hellwig wrote:
>> On Mon, Nov 04, 2019 at 01:14:03PM -0500, Kent Overstreet wrote:
>>> On Sat, Nov 02, 2019 at 03:29:11PM +0800, Ming Lei wrote:
>>>> __blk_queue_split() may be a bit heavy for small block size(such as
>>>> 512B, or 4KB) IO, so introduce one flag to decide if this bio includes
>>>> multiple page. And only consider to try splitting this bio in case
>>>> that the multiple page flag is set.
>>>
>>> So, back in the day I had an alternative approach in mind: get rid of
>>> blk_queue_split entirely, by pushing splitting down to the request layer - when
>>> we map the bio/request to sgl, just have it map as much as will fit in the sgl
>>> and if it doesn't entirely fit bump bi_remaining and leave it on the request
>>> queue.
>>>
>>> This would mean there'd be no need for counting segments at all, and would cut a
>>> fair amount of code out of the io path.
>>
>> I thought about that to, but it will take a lot more effort.  Mostly
>> because md/dm heavily rely on splitting as well.  I still think it is
>> worthwhile, it will just take a significant amount of time and we
>> should have the quick improvement now.
> 
> We can do it one driver at a time - driver sets a flag to disable
> blk_queue_split(). Obvious one to do first would be nvme since that's where it
> shows up the most.
> 
> And md/md do splitting internally, but I'm not so sure they need
> blk_queue_split().

I'm a big proponent of doing something like that instead, but it is a
lot of work. I absolutely hate the splitting we're doing now, even
though the original "let's work as hard as we add add page time to get
things right" was pretty abysmal as well.

-- 
Jens Axboe