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

[Jan Kiszka]

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.

Jan

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