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

[Changheun Lee]

> 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 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

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.

> 
> Thanks,
> Ming
> 
> 

---
Changheun Lee
Samsung Electronics.