On 09/04/2009 08:04 PM, Brian Jackson wrote:
On Friday 04 September 2009 11:08:51 am Andrew Theurer wrote:
Brian Jackson wrote:
On Friday 04 September 2009 09:48:17 am Andrew Theurer wrote:
<snip>
Still not idle=poll, it may shave off 0.2%.
Won't this affect SMT in a negative way? (OK, I am not running SMT now,
but eventually we will be) A long time ago, we tested P4's with HT, and
a polling idle in one thread always negatively impacted performance in
the sibling thread.
FWIW, I did try idle=halt, and it was slightly worse.
I did get a chance to try the latest qemu (master and next heads). I
have been running into a problem with virtIO stor driver for windows on
anything much newer than kvm-87. I compiled the driver from the new git
tree, installed OK, but still had the same error. Finally, I removed
the serial number feature in the virtio-blk in qemu, and I can now get
the driver to work in Windows.
What were the symptoms you were seeing (i.e. define "a problem").
Device manager reports "a problem code 10" occurred, and the driver
cannot initialize.
Yes! I was getting this after I moved from 0.10.6 to 0.11.0-rc1. Now I know
how to fix it. Thank you. Thank you.
Vadim Rozenfeld informed me:
There is a sanity check in the code, which checks the I/O range and fails
if is not equal to 40h. Resent virtio-blk devices have I/O range equal to
0x400 (serial number feature). So, out signed viostor driver will fail
on the latest KVMs. This problem was fixed
and committed to SVN some time ago.
I assumed the fix was to the virtio windows driver, but I could not get
the driver I compiled from latest git to work either (only on
qemu-kvm-87). So, I just backed out the serial number feature in qemu,
and it worked. FWIW, the linux virtio-blk driver never had a problem.
There have been very few changes to the viostor windows git repo since it was
opened. Unless it was done before they were open sourced. In any case, it
doesn't seem to be working with what's publicly available, so I think maybe
there is something missing internal to external.
I don't believe it was merged to git yet.
The signed viostor driver works with qemu-kvm-87 only,
otherwise you need to remove SN from the virtio-blk code, or clip
IO range to the original size.
Cheers,
Vadim.
So, not really any good news on performance with latest qemu builds.
Performance is slightly worse:
qemu-kvm-87
user nice system irq softirq guest idle iowait
5.79 0.00 9.28 0.08 1.00 20.81 58.78 4.26
total busy: 36.97
qemu-kvm-88-905-g6025b2d (master)
user nice system irq softirq guest idle iowait
6.57 0.00 10.86 0.08 1.02 21.35 55.90 4.21
total busy: 39.89
qemu-kvm-88-910-gbf8a05b (next)
user nice system irq softirq guest idle iowait
6.60 0.00 10.91 0.09 1.03 21.35 55.71 4.31
total busy: 39.98
diff of profiles, p1=qemu-kvm-87, p2=qemu-master
<snip>
18x more samples for gfn_to_memslot_unali*, 37x for
emulator_read_emula*, and more CPU time in guest mode.
One other thing I decided to try was some cpu binding. I know this is
not practical for production, but I wanted to see if there's any benefit
at all. One reason was that a coworker here tried binding the qemu
thread for the vcpu and the qemu IO thread to the same cpu. On a
networking test, guest->local-host, throughput was up about 2x.
Obviously there was a nice effect of being on the same cache. I
wondered, even without full bore throughput tests, could we see any
benefit here. So, I bound each pair of VMs to a dedicated core. What I
saw was about a 6% improvement in performance. For a system which has
pretty incredible memory performance and is not that busy, I was
surprised that I got 6%. I am not advocating binding, but what I do
wonder: on 1-way VMs, if we keep all the qemu threads together on the
same CPU, but still allowing the scheduler to move them (all of them at
once) to different cpus over time, would we see the same benefit?
One other thing: So far I have not been using preadv/pwritev. I assume
I need a more recent glibc (on 2.5 now) for qemu to take advantage of
this?
Getting p(read|write)v working almost doubled my virtio-net throughput in
a Linux guest. Not quite as much in Windows guests. Yes you need
glibc-2.10. I think some distros might have backported it to 2.9. You
will also need some support for it in your system includes.
Thanks, I will try a newer glibc, or maybe just move to a newer Linux
installation which happens to have a newer glic.
Fwiw... In Debian, I had to get glibc from the experimental tree. So some
distros might not even have it.
-Andrew
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