Re: [Qemu-devel] Better qemu/kvm defaults (was Re: [RFC PATCH 0/4] Gang scheduling in CFS)

[Anthony Liguori]

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?

> - 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.

> - 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 
licensed signed").

> - 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?

Regards,

Anthony Liguori

> HTH,
> Dor