Re: [PATCH 00/13] Parallelizing filesystem writeback

[Kundan Kumar <kundanthebest@gmail.com>]

>
> 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