On 12/29/2011 06:16 PM, Anthony Liguori wrote:
On 12/29/2011 10:07 AM, Dor Laor wrote:
On 12/26/2011 11:05 AM, Avi Kivity wrote:
On 12/26/2011 05:14 AM, Nikunj A Dadhania wrote:
btw you can get an additional speedup by enabling x2apic, for
default_send_IPI_mask_logical().
In the host?
In the host, for the guest:
qemu -cpu ...,+x2apic
It seems to me that we should improve our default flags.
So many times users fail to submit the proper huge command-line
options that we
require. Honestly, we can't blame them, there are so many flags and so
many use
cases its just too hard to get it right for humans.
I propose a basic idea and folks are welcome to discuss it:
1. Improve qemu/kvm defaults
Break the current backward compatibility (but add a --default-
backward-compat-mode) and set better values for:
- rtc slew time
What do you specifically mean?
-rtc localtime,driftfix=slew
- cache=none
I'm not sure I see this as a "better default" particularly since
O_DIRECT fails on certain file systems. I think we really need to let
WCE be toggable from the guest and then have a caching mode independent
of WCE. We then need some heuristics to only enable cache=off when we
know it's safe.
cache=none is still faster then it has the FS support.
qemu can test-run O_DIRECT and fall back to cache mode or just test the
filesystem capabilities.
- x2apic, maybe enhance qemu64 or move to -cpu host?
Alex posted a patch for this. I'm planning on merging it although so far
no one has chimed up either way.
- aio=native|threads (auto-sense?)
aio=native is unsafe to default because linux-aio is just fubar. It
falls back to synchronous I/O if the underlying filesystem doesn't
support aio. There's no way in userspace to problem if it's actually
supported or not either...
Can we test-run this too? Maybe as a separate qemu mode or even binary
that given a qemu cmdline, it will try to suggest better parameters?
- use virtio devices by default
I don't think this is realistic since appropriately licensed signed
virtio drivers do not exist for Windows. (Please note the phrase
"appropriately licensed signed").
What's the percentage of qemu invocation w/ windows guest and a short
cmd line? My hunch is that plain short cmdline indicates a developer and
probably they'll use linux guest.
- more?
Different defaults may be picked automatically when TCG|KVM used.
2. External hardening configuration file kept in qemu.git
For non qemu/kvm specific definitions like the io scheduler we
should maintain a script in our tree that sets/sense the optimal
settings of the host kernel (maybe similar one for the guest).
What are "appropriate host settings" and why aren't we suggesting that
distros and/or upstream just set them by default?
It's hard to set the right default for a distribution since the same
distro should optimize for various usages of the same OS. For example,
Fedora has tuned-adm w/ available profiles:
- desktop-powersave
- server-powersave
- enterprise-storage
- spindown-disk
- laptop-battery-powersave
- default
- throughput-performance
- latency-performance
- laptop-ac-powersave
We need to keep on recommending the best profile for virtualization, for
Fedora I think it either enterprise-storage and maybe
throughput-performance.
If we have a such a script, it can call the matching tuned profile
instead of tweaking every /sys option.
Regards,
Anthony Liguori
HTH,
Dor
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