Re: [Qemu-devel] [PATCH 1/2] accel: allows to select the "best" accelerator

[Thomas Huth]

On 04.10.2016 19:16, Paolo Bonzini wrote:
> 
> 
> On 04/10/2016 16:18, Thomas Huth wrote:
>>>>>> Using only tcg has also some disadvantages: For some tests, it's
>>>>>> interesting to know whether they also work properly with KVM (e.g.
>>>>>> migration tests), and only using tcg by default slows down the "make
>>>>>> check" quite a bit - which might become an issue now that we're adding
>>>>>> more and more tests.
>>>>
>>>> Which tests are you seeing a slow-down for ?
>> Well, everything that is using accel=tcg in tests/ could be accelerated.
>> For example, the new ipv6/ppc64 unit test is quite slow with TCG:
>>
>> sudo QTEST_QEMU_BINARY=ppc64-softmmu/qemu-system-ppc64 time tests/pxe-test
>>
>>  48.46user 0.07system 0:48.48elapsed
> 
> Could that point to a firmware bug?  10 network-bound seconds for a boot
> makes some sense, but 10 CPU-bound seconds don't...

SLOF is incredibly slow with TCG - most parts are written in Forth that
gets interpreted during runtime, and that seems to perform quite badly
with TCG for some reasons. We're in progress of speeding up the boot
process of SLOF a little bit (see
https://github.com/aik/SLOF/commit/b3fde41bc75269df2 for example), but
it will likely always be slower than a firmware that has been written in
C only.

 Thomas