[LU-13814] DIO performance: cl_page struct removal for DIO path

• Type: Improvement
• Resolution: Unresolved
• Priority: Major
• Assignee: Patrick Farrell
• Reporter: Patrick Farrell

Description

When doing DIO at ~10 GiB/s (see LU-13798, LU-13799), about 70-75% of the time is still spent on working with the cl_page struct.

This means allocating it, setting it up, and then moving it around & managing it.  We use the cl_page to track the vm pages, and in doing so, we put it on lists and move it from list to list, and update the state of the cl_page... (literally, cl_page_state)

It's possible to improve this by doing cl_page allocations in batch, this results in roughly a 30% drop in time spent in cl_page work, and makes it possible to get close to 15 GiB/s.

Fundamentally, none of this is necessary for DIO.  The cl_page struct is for tracking per-page information, but all of the pages in a DIO submission (at the ll_direct_rw_page level) are the same - They have the same owner, the same page flags, they are part of the same stripe...  If we do unaligned DIO, the first and last page can have a starting & ending offset, but that's it, and we can associate that with the DIO itself, not the individual pages.

So the proposal is to switch from using the cl_page struct to track pages in a DIO, and instead use the array of pages which describes the user buffer (ie, the kiocb and the results of ll_get_user_pages).

The brw_page member of the cl_page struct seems like it will still be necessary, but this isn't such a big deal - We can allocate those separately, at a fraction of the cost of setting up and managing the full cl_page abstraction.

Back of the envelope calculations suggest that this would save about 60-75% of the time in submitting DIO in the current optimized path, which performs at 10 GiB/s.

 

That calculation suggests we could reach single threaded DIO performance in the 25-40 GiB/s range.  Presumably some other issues will prevent hitting such high rates, but I think it is reasonable to think we could reach 20+ GiB/s, with sufficient network hardware.  (We will likely have to accept "idle CPU time in the submitting thread while waiting for the network" as a proxy indicator, since networks in the 30 GiB/s/node range are not readily available for testing.)

This improvement would of course also apply to buffered i/o via this path (see LU-13805), with the fast buffering version seeing a smaller benefit (but still large).

This change would also likely make it easier (from a coding perspective) to move the buffer allocation & memcopy() in the ptlrpcd threads, which is a key part of improving the performance of the fast buffering.

Issue Links

• is related to: LU-13802 New i/o path: Buffered i/o as DIO
• is related to: LU-13799 DIO/AIO efficiency improvements
• is related to: LU-13805 i/o path: Unaligned direct i/o
• is related to: LU-17885 use transfer page arrays for DIO RPCs
• is related to: LU-13798 Improve direct i/o performance with multiple stripes: Submit all stripes of a DIO and then wait
• is related to: LU-17194 parallelize DIO submit

Comments

[Gerrit Updater]

"Oleg Drokin <[e-mail]>" merged in patch https://review.whamcloud.com/c/fs/lustre-release/+/52161/
Subject: LU-13814 osc: reduce queue use in __osc_dio_submit
Project: fs/lustre-release
Branch: master
Current Patch Set:
Commit: 14575563dea23fbb3417b64fd164cbc3da866469

[Gerrit Updater]

"Oleg Drokin <[e-mail]>" merged in patch https://review.whamcloud.com/c/fs/lustre-release/+/52160/
Subject: LU-13814 osc: begin converting queue_dio_pages
Project: fs/lustre-release
Branch: master
Current Patch Set:
Commit: be8e3958845f503beaf094c94183d931424fb6d1

[Gerrit Updater]

"Oleg Drokin <[e-mail]>" merged in patch https://review.whamcloud.com/c/fs/lustre-release/+/52159/
Subject: LU-13814 osc: convert osc_dio_submit to array
Project: fs/lustre-release
Branch: master
Current Patch Set:
Commit: ce6bcb9e53aa0f9c7e0593e16c31e88d4cd298aa

[Gerrit Updater]

"Oleg Drokin <[e-mail]>" merged in patch https://review.whamcloud.com/c/fs/lustre-release/+/52141/
Subject: LU-13814 osc: cleanup osc_completion
Project: fs/lustre-release
Branch: master
Current Patch Set:
Commit: 2910f279ca3a7e7388ea93000daf7c9d5b5493f9

[Gerrit Updater]

"Oleg Drokin <[e-mail]>" merged in patch https://review.whamcloud.com/c/fs/lustre-release/+/52140/
Subject: LU-13814 osc: add osc_queue_dio_pages
Project: fs/lustre-release
Branch: master
Current Patch Set:
Commit: 8efbad8ff4edc00b4b727f3bb6f6f0fead2c35e2

[Gerrit Updater]

"Oleg Drokin <[e-mail]>" merged in patch https://review.whamcloud.com/c/fs/lustre-release/+/52139/
Subject: LU-13814 osc: clarify osc_transfer_pinned usage
Project: fs/lustre-release
Branch: master
Current Patch Set:
Commit: fc9adb3e8f01cd3e880e2d0e18f50e44fa445a4f

[Gerrit Updater]

"Oleg Drokin <[e-mail]>" merged in patch https://review.whamcloud.com/c/fs/lustre-release/+/52136/
Subject: LU-13814 osc: add osc_dio_submit
Project: fs/lustre-release
Branch: master
Current Patch Set:
Commit: 87d17d0a16ab9ad63f828004df7265c279215772

[Gerrit Updater]

"Oleg Drokin <[e-mail]>" merged in patch https://review.whamcloud.com/c/fs/lustre-release/+/52113/
Subject: LU-13814 clio: convert lov submit to cl_dio_pages
Project: fs/lustre-release
Branch: master
Current Patch Set:
Commit: b32321feef7b1bf4eebe8bb3ea0cc6a945e4a285

[Gerrit Updater]

"Oleg Drokin <[e-mail]>" merged in patch https://review.whamcloud.com/c/fs/lustre-release/+/52112/
Subject: LU-13814 clio: rename 'cl_page_completion'
Project: fs/lustre-release
Branch: master
Current Patch Set:
Commit: f00863ff25b3be68b220dd2c6b6234fd6e3a0d8e

[Gerrit Updater]

"Oleg Drokin <[e-mail]>" merged in patch https://review.whamcloud.com/c/fs/lustre-release/+/52111/
Subject: LU-13814 clio: add cl_dio_pages_complete
Project: fs/lustre-release
Branch: master
Current Patch Set:
Commit: 4b6cbdf80bba74d1b226ca4db220ed04d0179534

[Gerrit Updater]

"Oleg Drokin <[e-mail]>" merged in patch https://review.whamcloud.com/c/fs/lustre-release/+/52100/
Subject: LU-13814 clio: add cl_sync_io_note batch
Project: fs/lustre-release
Branch: master
Current Patch Set:
Commit: 304fb2b16d1a4d583f9749b1029d00ff51202adc