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

[Minchan Kim]

On Mon, Aug 14, 2017 at 09:14:03AM -0600, Jens Axboe wrote:
> On 08/14/2017 09:06 AM, Minchan Kim wrote:
> > On Mon, Aug 14, 2017 at 08:36:00AM -0600, Jens Axboe wrote:
> >> On 08/14/2017 02:50 AM, Minchan Kim wrote:
> >>> 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?
> >>
> >> I don't want to hold this series up, but I do think the recycling is
> >> a cleaner approach since we don't need to special case anything. I
> >> hope I'll get some time to dust it off, retest, and post soon.
> > 
> > I don't know how your bio recycling works. But my worry when I heard
> > per-cpu bio recycling firstly is if it's not reserved pool for
> > BDI_CAP_SYNCHRONOUS(IOW, if it is shared by several storages),
> > BIOs can be consumed by slow device(e.g., eMMC) so that a bio for
> > fastest device(e.g., zram in embedded system) in the system can be
> > stucked to wait on bio until IO for slow deivce is completed.
> > 
> > I guess it would be a not rare case for swap device under severe
> > memory pressure because lots of page cache are already reclaimed when
> > anonymous page start to be reclaimed so that many BIOs can be consumed
> > for eMMC to fetch code but swap IO to fetch heap data would be stucked
> > although zram-swap is much faster than eMMC.
> > As well, time to wait to get BIO among even fastest devices is
> > simple waste, I guess.
> 
> I don't think that's a valid concern. First of all, for the recycling,
> it's not like you get to wait on someone else using a recycled bio,
> if it's not there you simply go to the regular bio allocator. There
> is no waiting for free. The idea is to have allocation be faster since
> we can avoid going to the memory allocator for most cases, and speed
> up freeing as well, since we can do that in batches too.

I doubt how it performs well because at the beginning of this
thread[1], Ross said that with even dynamic bio allocation without
rw_page, there is no regression in some testing.
[1] http://lkml.kernel.org/r/<20170728165604.10455-1-ross.zwisler@xxxxxxxxxxxxxxx>

> 
> Secondly, generally you don't have slow devices and fast devices
> intermingled when running workloads. That's the rare case.

Not true. zRam is really popular swap for embedded devices where
one of low cost product has a really poor slow nand compared to
lz4/lzo [de]comression.

> 
> > To me, bio suggested by Christoph Hellwig isn't diverge current
> > path a lot and simple enough to change.
> 
> It doesn't diverge it a lot, but it does split it up.
> 
> > Anyway, I'm okay with either way if we can remove rw_page without
> > any regression because the maintainance of both rw_page and
> > make_request is rather burden for zram, too.
> 
> Agree, the ultimate goal of both is to eliminate the need for the
> rw_page hack.

Yeb.