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

[Jan Kiszka]

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.

At least Windows should no longer we a functional blocker thanks to
Paolo's work.

Jan

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