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

[Ming Lei <ming.lei@redhat.com>]

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:

[root@ktest-01 test]# cat fio.job
[global]
bs=32768k
rw=randread
iodepth=1
ioengine=psync
direct=1
runtime=20
time_based

group_reporting=0
ramp_time=5

[diotest]
filename=/dev/sde


[root@ktest-01 func]# /usr/share/bcc/tools/funclatency bio_iov_iter_get_pages
Tracing 1 functions for "bio_iov_iter_get_pages"... Hit Ctrl-C to end.
^C
     nsecs               : count     distribution
         0 -> 1          : 0        |                                        |
         2 -> 3          : 0        |                                        |
         4 -> 7          : 0        |                                        |
         8 -> 15         : 0        |                                        |
        16 -> 31         : 0        |                                        |
        32 -> 63         : 0        |                                        |
        64 -> 127        : 0        |                                        |
       128 -> 255        : 0        |                                        |
       256 -> 511        : 0        |                                        |
       512 -> 1023       : 0        |                                        |
      1024 -> 2047       : 0        |                                        |
      2048 -> 4095       : 0        |                                        |
      4096 -> 8191       : 0        |                                        |
      8192 -> 16383      : 0        |                                        |
     16384 -> 32767      : 0        |                                        |
     32768 -> 65535      : 0        |                                        |
     65536 -> 131071     : 0        |                                        |
    131072 -> 262143     : 1842     |****************************************|
    262144 -> 524287     : 125      |**                                      |
    524288 -> 1048575    : 6        |                                        |
   1048576 -> 2097151    : 0        |                                        |
   2097152 -> 4194303    : 1        |                                        |
   4194304 -> 8388607    : 0        |                                        |
   8388608 -> 16777215   : 1        |                                        |
Detaching...



-- 
Ming