[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 11:06, Aurelien Jarno wrote:
>> 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.
> 
> That's too nearsighted. SMP will be commodity on practically _any_ arch
> within the next years. And if QEMU doesn't keep up with it, feature and
> performance-wise, it will loose market share.
> 

Oh commercial arguments now. I am looking for something that answer my
needs, not about market share.

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