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

[Jan Kiszka]

On 2011-02-08 11:26, Aurelien Jarno wrote:
> 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.
> 

"Market share" simply means user base, for commercial or for hobby,
academic, whatever use. QEMU has a nice position here ATM. Even
commercial competitors can help continuously comparing their solutions
with QEMU (I once enjoyed such a product presentation). However, time
does not stand still.

Jan

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