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

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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.

Many drivers don't need sgl via blk_rq_map_sg(), but still need to split bio.

> 
> 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.

No counting segments involved in this small block size case, the handling
in blk_bio_segment_split() should be simple enough, still not understand
why IOPS drop is observable.

Thanks,
Ming





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