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SubjectRe: [PATCH v4 1/2] bio: limit bio max size
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> 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.

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