Re: [PATCH 00/13] Parallelizing filesystem writeback

Darrick J. Wong
2025-07-02T18:43:13Z

On Wed, Jun 25, 2025 at 09:14:51PM +0530, Kundan Kumar wrote:
> >
> > 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?

Sounds good to me, spinning rust isn't known for iops.

Though: What about a raid0 of spinning rust? Do you see the same
declines for sequential IO?

--D

> - 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
>