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 <minc...@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. [1]: https://lkml.org/lkml/2017/8/3/803 _______________________________________________ Linux-nvdimm mailing list Linux-nvdimm@lists.01.org https://lists.01.org/mailman/listinfo/linux-nvdimm