Re: [Qemu-devel] Slow PXE boot in qemu.git (fast in qemu-kvm.git)

[Jan Kiszka]

On 2011-04-11 22:18, Jan Kiszka wrote:
> On 2011-04-11 22:14, Alex Williamson wrote:
>> On Mon, 2011-04-11 at 22:04 +0200, Jan Kiszka wrote:
>>> On 2011-04-11 21:15, Luiz Capitulino wrote:
>>>> On Mon, 11 Apr 2011 13:00:32 -0600
>>>> Alex Williamson <address@hidden> wrote:
>>>>
>>>>> On Mon, 2011-04-11 at 15:35 -0300, Luiz Capitulino wrote:
>>>>>> On Fri, 08 Apr 2011 19:50:57 -0500
>>>>>> Anthony Liguori <address@hidden> wrote:
>>>>>>
>>>>>>> On 04/08/2011 06:25 PM, Luiz Capitulino wrote:
>>>>>>>> Hi there,
>>>>>>>>
>>>>>>>> Summary:
>>>>>>>>
>>>>>>>>   - PXE boot in qemu.git (HEAD f124a41) is quite slow, more than 5 
>>>>>>>> minutes. Got
>>>>>>>>     the problem with e1000, virtio and rtl8139. However, pcnet *works* 
>>>>>>>> (it's
>>>>>>>>     as fast as qemu-kvm.git)
>>>>>>>>
>>>>>>>>   - PXE boot in qemu-kvm.git (HEAD df85c051) is fast, less than a 
>>>>>>>> minute. Tried
>>>>>>>>     with e1000, virtio and rtl8139 (I don't remember if I tried with 
>>>>>>>> pcnet)
>>>>>>>>
>>>>>
>>>>> I was having this problem too, but I think it's because I forgot to
>>>>> build qemu with --enable-io-thread, which is the default for qemu-kvm.
>>>>> Can you re-configure and build with that and see if it's fast?  Thanks,
>>>>
>>>> Yes, nice catch, it's faster with I/O thread enabled, even seem faster
>>>> than qemu-kvm.git.
>>>
>>> What's the performance under qemu-kvm with -no-kvm-irqchip?
>>>
>>>>
>>>> So, does this have to be fixed w/o I/O thread?
>>>
>>> If it's most probably an architectural deficit of non-io-thread mode, I
>>> would say let it rest in peace. But maybe it points to a generic issues
>>> that is just magnified by non-threaded mode.
>>
>> I've probably been told, but forget.  Why isn't io-thread enabled by
>> default?  Thanks,
> 
> TCG performance still sucks in io-threaded mode. I've three patches in
> my queue that reduces the overhead a bit further - for me to a
> reasonable level (will post them the next days). But, still, YMMV
> depending on the workload.

In fact, they were already prepared. So I've just sent them out.

Jan

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