Re: [PATCH 0/6] Improve visibility of writeback

From: Jan Kara
Date: Thu Mar 21 2024 - 14:07:19 EST


On Thu 21-03-24 16:12:52, Kemeng Shi wrote:
> on 3/21/2024 1:22 AM, Jan Kara wrote:
> > On Wed 20-03-24 19:02:16, Kemeng Shi wrote:
> >> This series tries to improve visilibity of writeback. Patch 1 make
> >> /sys/kernel/debug/bdi/xxx/stats show writeback info of whole bdi
> >> instead of only writeback info in root cgroup. Patch 2 add a new
> >> debug file /sys/kernel/debug/bdi/xxx/wb_stats to show per wb writeback
> >> info. Patch 4 add wb_monitor.py to monitor basic writeback info
> >> of running system, more info could be added on demand. Rest patches
> >> are some random cleanups. More details can be found in respective
> >> patches. Thanks!
> >>
> >> Following domain hierarchy is tested:
> >> global domain (320G)
> >> / \
> >> cgroup domain1(10G) cgroup domain2(10G)
> >> | |
> >> bdi wb1 wb2
> >>
> >> /* all writeback info of bdi is successfully collected */
> >> # cat /sys/kernel/debug/bdi/252:16/stats:
> >> BdiWriteback: 448 kB
> >> BdiReclaimable: 1303904 kB
> >> BdiDirtyThresh: 189914124 kB
> >> DirtyThresh: 195337564 kB
> >> BackgroundThresh: 32516508 kB
> >> BdiDirtied: 3591392 kB
> >> BdiWritten: 2287488 kB
> >> BdiWriteBandwidth: 322248 kBps
> >> b_dirty: 0
> >> b_io: 0
> >> b_more_io: 2
> >> b_dirty_time: 0
> >> bdi_list: 1
> >> state: 1
> >>
> >> /* per wb writeback info is collected */
> >> # cat /sys/kernel/debug/bdi/252:16/wb_stats:
> >> cat wb_stats
> >> WbCgIno: 1
> >> WbWriteback: 0 kB
> >> WbReclaimable: 0 kB
> >> WbDirtyThresh: 0 kB
> >> WbDirtied: 0 kB
> >> WbWritten: 0 kB
> >> WbWriteBandwidth: 102400 kBps
> >> b_dirty: 0
> >> b_io: 0
> >> b_more_io: 0
> >> b_dirty_time: 0
> >> state: 1
> >> WbCgIno: 4284
> >> WbWriteback: 448 kB
> >> WbReclaimable: 818944 kB
> >> WbDirtyThresh: 3096524 kB
> >> WbDirtied: 2266880 kB
> >> WbWritten: 1447936 kB
> >> WbWriteBandwidth: 214036 kBps
> >> b_dirty: 0
> >> b_io: 0
> >> b_more_io: 1
> >> b_dirty_time: 0
> >> state: 5
> >> WbCgIno: 4325
> >> WbWriteback: 224 kB
> >> WbReclaimable: 819392 kB
> >> WbDirtyThresh: 2920088 kB
> >> WbDirtied: 2551808 kB
> >> WbWritten: 1732416 kB
> >> WbWriteBandwidth: 201832 kBps
> >> b_dirty: 0
> >> b_io: 0
> >> b_more_io: 1
> >> b_dirty_time: 0
> >> state: 5
> >>
> >> /* monitor writeback info */
> >> # ./wb_monitor.py 252:16 -c
> >> writeback reclaimable dirtied written avg_bw
> >> 252:16_1 0 0 0 0 102400
> >> 252:16_4284 672 820064 9230368 8410304 685612
> >> 252:16_4325 896 819840 10491264 9671648 652348
> >> 252:16 1568 1639904 19721632 18081952 1440360
> >>
> >>
> >> writeback reclaimable dirtied written avg_bw
> >> 252:16_1 0 0 0 0 102400
> >> 252:16_4284 672 820064 9230368 8410304 685612
> >> 252:16_4325 896 819840 10491264 9671648 652348
> >> 252:16 1568 1639904 19721632 18081952 1440360
> >> ...
> >
> > So I'm wondering: Are you implementing this just because this looks
> > interesting or do you have a real need for the functionality? Why?
> Hi Jan, I added debug files to test change in [1] which changes the way how
> dirty background threshold of wb is calculated. Without debug files, we could
> only monitor writeback to imply that threshold is corrected.
> In current patchset, debug info has not included dirty background threshold yet,
> I will add it when discution of calculation of dirty background threshold in [1]
> is done.
> The wb_monitor.py is suggested by Tejun in [2] to improve visibility of writeback.
> The script is more convenient than trace to monitor writeback behavior of the running
> system.

Thanks for the pointer. OK, I agree this is useful so let's have a look
into the code :)

Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR