Poor virtio performance on Scaleway ARM systems

[Peter Jeremy]

I have been playing with the 4-core ARM64 VPSs on https://www.scaleway.com
and notice that the disk I/O performance (using virtio_blk) is abyssmal.
Using "dd if=/dev/{vda|vtbd0} of=/dev/null bs=256k count=4096", I get
400-500MBps, whilst under FreeBSD-12, I get about 5MBps.  I've checked on a
couple of instances and both Linux & FreeBSD on the same instance and get
similar results.  Linux & FreeBSD are both using a virtio block device
attached to the PCI bus.  Rebuilding FreeBSD to turn off all the debugging
has no effect.

The only oddity I've found is that FreeBSD is reporting very high interrupt
rates on gic0,p11, gic0,s4 and gic0,s5 whilst disk I/O is occurring.
Unfortunately, I can't tell what is attached to those interrupts (it's
not obvious from the dmesg and reported as "+").

I've done some searching and have only found general FUD ("FreeBSD virtio
isn't any good") and nothing specifically related to Scaleway.

Can anyone suggest where to look for a solution?

-- 
Peter Jeremy
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