Re: [PATCH 0/3] remove rw_page() from brd, pmem and btt

[Minchan Kim <minchan@kernel.org>]

On Fri, Aug 04, 2017 at 11:24:49AM -0700, Dan Williams wrote:
> On Fri, Aug 4, 2017 at 11:21 AM, Ross Zwisler
> <ross.zwisler@linux.intel.com> wrote:
> > On Fri, Aug 04, 2017 at 11:01:08AM -0700, Dan Williams wrote:
> >> [ adding Dave who is working on a blk-mq + dma offload version of the
> >> pmem driver ]
> >>
> >> On Fri, Aug 4, 2017 at 1:17 AM, Minchan Kim <minchan@kernel.org> wrote:
> >> > On Fri, Aug 04, 2017 at 12:54:41PM +0900, Minchan Kim wrote:
> >> [..]
> >> >> Thanks for the testing. Your testing number is within noise level?
> >> >>
> >> >> I cannot understand why PMEM doesn't have enough gain while BTT is significant
> >> >> win(8%). I guess no rw_page with BTT testing had more chances to wait bio dynamic
> >> >> allocation and mine and rw_page testing reduced it significantly. However,
> >> >> in no rw_page with pmem, there wasn't many cases to wait bio allocations due
> >> >> to the device is so fast so the number comes from purely the number of
> >> >> instructions has done. At a quick glance of bio init/submit, it's not trivial
> >> >> so indeed, i understand where the 12% enhancement comes from but I'm not sure
> >> >> it's really big difference in real practice at the cost of maintaince burden.
> >> >
> >> > I tested pmbench 10 times in my local machine(4 core) with zram-swap.
> >> > In my machine, even, on-stack bio is faster than rw_page. Unbelievable.
> >> >
> >> > I guess it's really hard to get stable result in severe memory pressure.
> >> > It would be a result within noise level(see below stddev).
> >> > So, I think it's hard to conclude rw_page is far faster than onstack-bio.
> >> >
> >> > rw_page
> >> > avg     5.54us
> >> > stddev  8.89%
> >> > max     6.02us
> >> > min     4.20us
> >> >
> >> > onstack bio
> >> > avg     5.27us
> >> > stddev  13.03%
> >> > max     5.96us
> >> > min     3.55us
> >>
> >> The maintenance burden of having alternative submission paths is
> >> significant especially as we consider the pmem driver ising more
> >> services of the core block layer. Ideally, I'd want to complete the
> >> rw_page removal work before we look at the blk-mq + dma offload
> >> reworks.
> >>
> >> The change to introduce BDI_CAP_SYNC is interesting because we might
> >> have use for switching between dma offload and cpu copy based on
> >> whether the I/O is synchronous or otherwise hinted to be a low latency
> >> request. Right now the dma offload patches are using "bio_segments() >
> >> 1" as the gate for selecting offload vs cpu copy which seem
> >> inadequate.
> >
> > Okay, so based on the feedback above and from Jens[1], it sounds like we want
> > to go forward with removing the rw_page() interface, and instead optimize the
> > regular I/O path via on-stack BIOS and dma offload, correct?
> >
> > If so, I'll prepare patches that fully remove the rw_page() code, and let
> > Minchan and Dave work on their optimizations.
> 
> I think the conversion to on-stack-bio should be done in the same
> patchset that removes rw_page. We don't want to leave a known
> performance regression while the on-stack-bio work is in-flight.

Okay. It seems everyone get an agreement with on-stack-bio.
I will send my formal patchset including Ross's patches which
removes rw_page.

Thanks.

Thanks.
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