[Qemu-devel] Re: [PATCH 2/7] Enable I/O thread and VNC threads by default

[Jan Kiszka]

On 2011-02-08 10:58, Aurelien Jarno wrote:
> Jan Kiszka a écrit :
>> On 2011-02-08 10:05, Aurelien Jarno wrote:
>>> Jan Kiszka a écrit :
>>>> On 2011-02-08 09:08, Paolo Bonzini wrote:
>>>>> On 02/08/2011 08:26 AM, Aurelien Jarno wrote:
>>>>>> I forget to remember when we decided that AIO should be implemented on
>>>>>> any host OS. Any pointer?
>>>>> To be fair, I/O-heavy workloads are almost unusable without AIO.  For 
>>>>> Window targets, they also crash under SMP due to the Windows AP 
>>>>> watchdog.  But then TCG and SMP do not go very well together anyway.
>>>>>
>>>>> However, I think deprecating Win32 support would be a very bad idea.
>>>> It would be too early at this point.
>>>>
>>>> But if Windows is once the only reason to keep tons of hardly tested
>>>> code paths around or to invest significant additional effort to change
>>>> logic or interfaces in this area, than I would prefer that step. I'm
>>>> hacking on IOTHREAD vs. !IOTHREAD for some weeks now, and all those
>>>> subtle differences are really a PITA and source of various breakages.
>>>>
>>>> People interested in that platform should finally realize that its fate
>>>> is coupled to reducing the #ifdefs as well as the design differences we
>>>> see right now and even more in the future.
>>>>
>>> The guilty here is IOTHREAD. Windows support predates IOTHREAD concept,
>>> it's just that people who introduce IOTHREAD didn't care about Windows
>>> support at all and added these #ifdef. Disabling Windows support because
>>> of that is not fair.
>>
>> The TCG execution model won't scale long-term. It's already a main to
>> boot a quad or just dual core VM, even more when your host has at least
>> as many real cores. I'm sure we'll see multi-threaded TCG CPUs in the
>> future, and the iothread will just be one of 7, 17 or 257 threads.
>>
> 
> And what's the issue with that? People don't always look for performance
> when using QEMU. They even often try to emulate old machines (and non
> x86 ones), which anyway only have one CPU. This won't change in 5 years,
> the only thing is that those machines will be 5 years older.
> 
> People have to keep in mind that QEMU doesn't mean only virtualization
> and doesn't mean only x86.

I'm not talking about virtualization here. I'm talking about usable
emulation of today's (!) embedded multi-core platforms. It matters a lot
if your test roundtrip for booting into a SMP guest and running some
apps is a few 10 seconds, a few minutes or even not practically working.
Ever tried to boot a 16 core VM in emulation mode? I did, for fun. I
just hope I'll never depend on this for work.

Jan

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