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

[Aurelien Jarno]

Jan Kiszka a écrit :
> 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.

Yes, it's slow. But is it a problem? You assume that people use QEMU
only for emulating SMP platforms. This is a wrong assumption. Beside the
x86 target, only sparc really supports SMP emulation.

-- 
Aurelien Jarno                          GPG: 1024D/F1BCDB73
address@hidden                 http://www.aurel32.net