[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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

From: Anthony Liguori
Subject: Re: [Qemu-devel] Better qemu/kvm defaults (was Re: [RFC PATCH 0/4] Gang scheduling in CFS)
Date: Thu, 29 Dec 2011 10:16:55 -0600
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20110922 Lightning/1.0b2 Thunderbird/3.1.15

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?

- 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?


Anthony Liguori


reply via email to

[Prev in Thread] Current Thread [Next in Thread]