Re: [Qemu-devel] 4 CPUs with vexpress is slow?

[Frederic Konrad <fred.konrad@greensocs.com>]

On 22/06/2015 22:57, Peter Maydell wrote:
> On 22 June 2015 at 19:18, Frederic Konrad <fred.konrad@greensocs.com> wrote:
>> Testing MTTCG patch-set performance I found strange slowness with _upstream_
>> qemu (46bca5404b08201bb9df1ac32bc88fc7e6db1f74).
>>
>> Basically booting a vexpress-a15 with "-smp 4" takes approximately forever
>> if I use a
>> vexpress-v2p-ca15-tc1.dtb (2x A15) dtb and 39 secs if I hack this dtb to
>> have 4 CPUs.
> Wait, so if you change the dtb it boots in a non-infinite time,
> but MTTCG boots both dtbs?
Yes MTTCG boots both dtbs.

>> This is definitely strange because if I use "-smp 2" with the same guest
>> image it takes
>> only 4 secs. And MTTCG patch-set seems to fix the issue as it boot in 6 secs
>> with
>> "-smp 4".
>>
>> Is that a known issue or maybe it's my guest (linux 3.13.5)?
> Dunno. I basically never run with SMP configs because they're
> always slower than uniprocessor... Does your kernel boot on
> KVM? It may be that it's configured to assume 2xSMP somehow,
> given the h/w we're modelling here is 2xSMP.
>
> Important question: when you boot this config, does /proc/cpuinfo
> say you have two cores booted, or all four? If we've ended up with
> the other two cores in the config spinning then that would explain
> why MTTCG is doing better here, but it's not a very interesting
> config to try to optimise :-)
All four cores are in /proc/cpuinfo.

>
> There is an issue where IPIs may be inefficient:
> http://lists.nongnu.org/archive/html/qemu-devel/2015-06/msg03824.html
> but I'd expect that to merely slow things down a bit, not send
> the boot time off to infinity.

This is definitely a bug :
Seems that removing "--nographic" option removes the problem it just 
boots as per
normal..

Fred

>
> -- PMM