Re: [PATCH v4 1/2] bio: limit bio max size

[Ming Lei]

On Tue, Feb 02, 2021 at 01:12:04PM +0900, Changheun Lee wrote:
> > On Mon, Feb 01, 2021 at 11:52:48AM +0900, Changheun Lee wrote:
> > > > On Fri, Jan 29, 2021 at 12:49:08PM +0900, Changheun Lee wrote:
> > > > > bio size can grow up to 4GB when muli-page bvec is enabled.
> > > > > but sometimes it would lead to inefficient behaviors.
> > > > > in case of large chunk direct I/O, - 32MB chunk read in user space -
> > > > > all pages for 32MB would be merged to a bio structure if the pages
> > > > > physical addresses are contiguous. it makes some delay to submit
> > > > > until merge complete. bio max size should be limited to a proper size.
> > > > > 
> > > > > When 32MB chunk read with direct I/O option is coming from userspace,
> > > > > kernel behavior is below now. it's timeline.
> > > > > 
> > > > >  | bio merge for 32MB. total 8,192 pages are merged.
> > > > >  | total elapsed time is over 2ms.
> > > > >  |------------------ ... ----------------------->|
> > > > >                                                  | 8,192 pages merged a bio.
> > > > >                                                  | at this time, first bio submit is done.
> > > > >                                                  | 1 bio is split to 32 read request and issue.
> > > > >                                                  |--------------->
> > > > >                                                   |--------------->
> > > > >                                                    |--------------->
> > > > >                                                               ......
> > > > >                                                                    |--------------->
> > > > >                                                                     |--------------->|
> > > > >                           total 19ms elapsed to complete 32MB read done from device. |
> > > > > 
> > > > > If bio max size is limited with 1MB, behavior is changed below.
> > > > > 
> > bio_iov_iter_get_pages> > >  | bio merge for 1MB. 256 pages are merged for each bio.
> > > > >  | total 32 bio will be made.
> > > > >  | total elapsed time is over 2ms. it's same.
> > > > >  | but, first bio submit timing is fast. about 100us.
> > > > >  |--->|--->|--->|---> ... -->|--->|--->|--->|--->|
> > > > >       | 256 pages merged a bio.
> > > > >       | at this time, first bio submit is done.
> > > > >       | and 1 read request is issued for 1 bio.
> > > > >       |--------------->
> > > > >            |--------------->
> > > > >                 |--------------->
> > > > >                                       ......
> > > > >                                                  |--------------->
> > > > >                                                   |--------------->|
> > > > >         total 17ms elapsed to complete 32MB read done from device. |
> > > > 
> > > > Can you share us if enabling THP in your application can avoid this issue? BTW, you
> > > > need to make the 32MB buffer aligned with huge page size. IMO, THP perfectly fits
> > > > your case.
> > > > 
> > > 
> > > THP is enabled already like as below in my environment. It has no effect.
> > > 
> > > cat /sys/kernel/mm/transparent_hugepage/enabled
> > > [always] madvise never
> > 
> > The 32MB user buffer needs to be huge page size aligned. If your system
> > supports bcc/bpftrace, it is quite easy to check if the buffer is
> > aligned.
> > 
> > > 
> > > This issue was reported from performance benchmark application in open market.
> > > I can't control application's working in open market.
> > > It's not only my own case. This issue might be occured in many mobile environment.
> > > At least, I checked this problem in exynos, and qualcomm chipset.
> > 
> > You just said it takes 2ms for building 32MB bio, but you never investigate the
> > reason. I guess it is from get_user_pages_fast(), but maybe others. Can you dig
> > further for the reason? Maybe it is one arm64 specific issue.
> > 
> > BTW, bio_iov_iter_get_pages() just takes ~200us on one x86_64 VM with THP, which is
> > observed via bcc/funclatency when running the following workload:
> > 
> 
> I think you focused on bio_iov_iter_get_pages() because I just commented page
> merge delay only. Sorry about that. I missed details of this issue.
> Actually there are many operations during while-loop in do_direct_IO().
> Page merge operation is just one among them. Page merge operation is called
> by dio_send_cur_page() in while-loop. Below is call stack.
> 
> __bio_try_merge_page+0x4c/0x614
> bio_add_page+0x40/0x12c
> dio_send_cur_page+0x13c/0x374
> submit_page_section+0xb4/0x304
> do_direct_IO+0x3d4/0x854
> do_blockdev_direct_IO+0x488/0xa18
> __blockdev_direct_IO+0x30/0x3c
> f2fs_direct_IO+0x6d0/0xb80
> generic_file_read_iter+0x284/0x45c
> f2fs_file_read_iter+0x3c/0xac
> __vfs_read+0x19c/0x204
> vfs_read+0xa4/0x144
> 
> 2ms delay is not only caused by page merge operation. it inculdes many the
> other operations too. But those many operations included page merge should
> be executed more if bio size is grow up.

OK, got it.

Then I think you can just limit bio size in dio_bio_add_page() instead of
doing it for all.

-- 
Ming