[Bug 231178] Very slow IO with small block size

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=231178

            Bug ID: 231178
           Summary: Very slow IO with small block size
           Product: Base System
           Version: CURRENT
          Hardware: arm64
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: arm
          Assignee: freebsd-arm at FreeBSD.org
          Reporter: robert.david.public at gmail.com

On raspberry pi3 aarch64, I have noticed very slow IO on any type of storage
device (USB,mmc) when the block size is slow.

Example execution of dd shows huge difference (>10x)



pi-backup ~ # dd if=/dev/mmcsd0 of=/dev/zero bs=1M
3781+1 records in
3781+1 records out
3965190144 bytes transferred in 226.929478 secs (17473226 bytes/sec)

pi-backup ~ # dd if=/dev/mmcsd0 of=/dev/zero bs=512
^C981585+0 records in
981585+0 records out
502571520 bytes transferred in 375.254570 secs (1339282 bytes/sec)

(Note I have stopped that, because it would take >half hour to read 4GB sd
card)

Similar results can be seen on USB hdd. This practically make speed of any fs
(UFS/ZFS) <1MB/s.


For comparison this is results from Linux 4.14

rpi64 ~ # dd if=/dev/mmcblk0 of=/dev/zero bs=512
7744512+0 records in
7744512+0 records out
3965190144 bytes (4.0 GB, 3.7 GiB) copied, 178.385 s, 22.2 MB/s

rpi64 ~ # dd if=/dev/mmcblk0 of=/dev/zero bs=1M
3781+1 records in
3781+1 records out
3965190144 bytes (4.0 GB, 3.7 GiB) copied, 168.359 s, 23.6 MB/s


I do not expect FreeBSD be easily on par with Linux, but on the other side
there seems to be huge bottleneck/contention somewhere.

Since it is not driver specific (USB/mmc), I expect something in core kernel
not optimized yet.

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