kern/50604: ld(4) on top of virtio(4) performs very badly

[tron%zhadum.org.uk@localhost]

>Number:         50604
>Category:       kern
>Synopsis:       ld(4) on top of virtio(4) performs very badly
>Confidential:   no
>Severity:       serious
>Priority:       low
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri Jan 01 19:40:00 +0000 2016
>Originator:     tron%zhadum.org.uk@localhost
>Release:        NetBSD 7.99.25 2016-01-01 sources
>Organization:
Matthias Scheler                                 https://zhadum.org.uk/
>Environment:
System: NetBSD lyssa.zhadum.org.uk 7.99.25 NetBSD 7.99.25 (GENERIC) #0: Fri Jan 1 15:28:41 GMT 2016 tron%lyssa.zhadum.org.uk@localhost:/src/sys/compile/GENERIC amd64
Architecture: x86_64
Machine: amd64
>Description:
I've got a NetBSD-current virtual machine running as a KVM guest.
The host is a Debian 8.2 system (Linux kernel 3.16.0, QEMU 2.1).

Today I tried to switch on all of the virtual machine's three disks from
"IDE" to "VirtIO" to improve performance. But to my big surprise I got
a massive performance degradation instead. I've used un-tar-ring the
Perl 5.22.1 sources as a crude benchmark. Here are the results:

IDE:		1.67s user 0.83s system 50% cpu 4.935 total
VirtIO:		1.74s user 0.75s system 2% cpu 1:27.43 total

It seems that virtio(4) is twenty times slower. As I've never experienced
similar problems with Linux guests (at work) I guess that this is a problem
with NetBSD's virtio(4) driver.

>How-To-Repeat:
cd /some/filesystem/on/ld/at/virtio
time sh -c "tar xzf /usr/pkgsrc/distfiles/perl-5.22.1.tar.bz2; sync"

>Fix:
Not known