On 22 June 2015 at 19:18, Frederic Konrad <fred.kon...@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? > 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 :-) 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. -- PMM
