Re: [PATCH 00/13] Parallelizing filesystem writeback

["Darrick J. Wong" <djwong@kernel.org>]

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
>