Re: [PATCH v1 2/6] fs: use on-stack-bio if backing device has BDI_CAP_SYNC capability

[Jens Axboe]

On 08/11/2017 04:46 AM, Christoph Hellwig wrote:
> On Wed, Aug 09, 2017 at 08:06:24PM -0700, Dan Williams wrote:
>> I like it, but do you think we should switch to sbvec[<constant>] to
>> preclude pathological cases where nr_pages is large?
> 
> Yes, please.
> 
> Then I'd like to see that the on-stack bio even matters for
> mpage_readpage / mpage_writepage.  Compared to all the buffer head
> overhead the bio allocation should not actually matter in practice.

I'm skeptical for that path, too. I also wonder how far we could go
with just doing a per-cpu bio recycling facility, to reduce the cost
of having to allocate a bio. The on-stack bio parts are fine for
simple use case, where simple means that the patch just special
cases the allocation, and doesn't have to change much else.

I had a patch for bio recycling and batched freeing a year or two
ago, I'll see if I can find and resurrect it.

-- 
Jens Axboe