Hi Julian,

 I'll let Neel take care of the time questions.

3. Even moderate guest disk I/O completely kills guest network
performance.  For example, whenever security(8) (security(7) in FreeBSD)
runs, guest network throughput drops from 150+ Mbps to ~20 Mbps, and
jitter from ping jumps from <0.01 ms to 100+ ms.  If I try to build
something in the guest, then network becomes almost unusable.

The network performance degradation only affects the guest that's
generating the I/O; high I/O on guest B doesn't affect guest A, nor
would high I/O on the host.

I'm using both virtio-blk and virio-net drivers, and the guests' disk
images are backed by zvol+geli.  Removing geli has no effect.

There are some commits in CURRENT that suggests improved virtio
performance, but I'm not comfortable running CURRENT.  Is there a
workaround I could use for 10.1?

In 10.1, virtio-blk i/o is done sychronously in the context of the guest vCPU exit. If it's a single vCPU guest, or the virtio-net interrupt happens to be delivered to that vCPU, performance will suffer.

A workaround is to use ahci-hd for the disk emulation and not virtio-blk. The AHCI emulation does i/o in a dedicated thread and doesn't block the vCPU thread.

4. virtio-blk always reports the virtual disk as having 512-byte
sectors, and so I get I/O errors on OpenBSD guests when the disk image
is backed by zvol+geli with 4K sector size.  Curiously, this only seems
to affect zvol+geli; with just zvol it seems to work.  Also, it works
either way on Linux guests.

ATM I changed the zvol / geli sector size to 512 bytes, which probably
made #2 worse.  I think this bug / feature is addressed by:
but again is there a workaround to force a specific sector size for 10.1?

The only workaround for 10.1 would be to use ahci-hd instead of virtio-blk. The correct sector size will be reported there.

5. This may be better directed at OpenBSD but I'll ask here anyway: if I
enable virtio-rnd then OpenBSD would not boot with "couldn't map
interrupt" error.  The kernel in bsd.rd will boot, but not the installed
kernel (or the one built from STABLE; I forgot).  Again, Linux seems
unaffected, but I couldn't tell if it's actually working.

Try using the -W option to bhyve. This will force the bhyve virtio code to advertize (non-standard) MSI interrupt capability which OpenBSD will then use to allocate vectors.



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