Re: [PATCH 00/13] Parallelizing filesystem writeback

Kundan Kumar
2025-06-25T15:45:03Z

>
> Makes sense. It would be good to test this on a non-SMP machine, if
> you can find one ;)
>

Tested with kernel cmdline with maxcpus=1. The parallel writeback falls
back to 1 thread behavior, showing nochange in BW.

- On PMEM:
Base XFS : 70.7 MiB/s
Parallel Writeback XFS : 70.5 MiB/s
Base EXT4 : 137 MiB/s
Parallel Writeback EXT4 : 138 MiB/s

- On NVMe:
Base XFS : 45.2 MiB/s
Parallel Writeback XFS : 44.5 MiB/s
Base EXT4 : 81.2 MiB/s
Parallel Writeback EXT4 : 80.1 MiB/s

>
> Please test the performance on spinning disks, and with more filesystems?
>

On a spinning disk, random IO bandwidth remains unchanged, while sequential
IO performance declines. However, setting nr_wb_ctx = 1 via configurable
writeback(planned in next version) eliminates the decline.

echo 1 > /sys/class/bdi/8:16/nwritebacks

We can fetch the device queue's rotational property and allocate BDI with
nr_wb_ctx = 1 for rotational disks. Hope this is a viable solution for
spinning disks?

- Random IO
Base XFS : 22.6 MiB/s
Parallel Writeback XFS : 22.9 MiB/s
Base EXT4 : 22.5 MiB/s
Parallel Writeback EXT4 : 20.9 MiB/s

- Sequential IO
Base XFS : 156 MiB/s
Parallel Writeback XFS : 133 MiB/s (-14.7%)
Base EXT4 : 147 MiB/s
Parallel Writeback EXT4 : 124 MiB/s (-15.6%)

-Kundan