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

[Ronen Hod <rhod@redhat.com>]

On 01/01/2012 12:16 PM, Dor Laor wrote:
> 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.

You might want to take into account migration considerations. I.e., the 
target host's optimal setup.
Also, we need to beware of too much automation, since hardware changes 
might void Windows license activations.
Some of the parameters will depend on dynamic factors such as the total 
guest's nCPUs, mem, sharing (KSM), or whatever.
As a minimum, we can automatically suggest the qemu parameters and the 
host setup.

Ronen.

>>>
>>> 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
>>>
>>
>>
>
>