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Re: [Qemu-devel] Better qemu/kvm defaults (was Re: [RFC PATCH 0/4] Gang
Re: [Qemu-devel] Better qemu/kvm defaults (was Re: [RFC PATCH 0/4] Gang scheduling in CFS)
Thu, 29 Dec 2011 10:16:55 -0600
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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
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?
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.
- 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...
- 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
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?