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

Minchan Kim
2017-08-14T08:50:47Z

Hi Jens,

On Fri, Aug 11, 2017 at 08:26:59AM -0600, Jens Axboe wrote:
> 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.

So, you want to go with per-cpu bio recycling approach to
remove rw_page?

So, do you want me to hold this patchset?