Frederic Barrat <fbar...@linux.ibm.com> writes:
> On 28/06/2022 13:25, Matheus K. Ferst wrote: >> On 27/06/2022 15:25, Frederic Barrat wrote: >>> [ Resending as it was meant for the qemu-ppc list ] >>> >>> Hello, >>> >>> I've been looking at why our qemu powernv model is so slow when booting >>> a compressed linux kernel, using multiple vcpus and multi-thread tcg. >>> With only one vcpu, the decompression time of the kernel is what it is, >>> but when using multiple vcpus, the decompression is actually slower. And >>> worse: it degrades very fast with the number of vcpus! >>> >>> Rough measurement of the decompression time on a x86 laptop with >>> multi-thread tcg and using the qemu powernv10 machine: >>> 1 vcpu => 15 seconds >>> 2 vcpus => 45 seconds >>> 4 vcpus => 1 min 30 seconds >>> >>> Looking in details, when the firmware (skiboot) hands over execution to >>> the linux kernel, there's one main thread entering some bootstrap code >>> and running the kernel decompression algorithm. All the other secondary >>> threads are left spinning in skiboot (1 thread per vpcu). So on paper, >>> with multi-thread tcg and assuming the system has enough available >>> physical cpus, I would expect the decompression to hog one physical cpu >>> and the time needed to be constant, no matter the number of vpcus. <snip> >>> >>> Ironically, the behavior seen with single thread tcg is what I would >>> expect: 1 thread decompressing in 15 seconds, all the other threads >>> spinning for that same amount of time, all sharing the same physical >>> cpu, so it all adds up nicely: I see 60 seconds decompression time with >>> 4 vcpus (4x15). Which means multi-thread tcg is slower by quite a bit. >>> And single thread tcg hogs one physical cpu of the laptop vs. 4 physical >>> cpus for the slower multi-thread tcg. >>> >>> Does anybody have an idea of what might happen or have suggestion to >>> keep investigating? >>> Thanks for your help! >>> >>> Fred >>> >>> >> Hi Frederic, >> I did some boot time tests recently and didn't notice this behavior. >> Could you share your QEMU command line with us? Did you build QEMU >> with any debug option or sanitizer enabled? > > > You should be able to see it with: > > qemu-system-ppc64 -machine powernv10 -smp 4 -m 4G -nographic -bios > <path to skiboot.lid> -kernel <path to compresses kernel> -initrd > <path to initd> -serial mon:stdio > > > -smp is what matters. > > When simplifying the command line above, I noticed something > interesting: the problem doesn't show using the skiboot.lid shipped > with qemu! I'm using something closer to the current upstream head and > the idle code (the for loop in my initial mail) had been reworked in > between. So, clearly, the way the guest code is written matters. But > that doesn't explain it. > > I'm using a kernel in debug mode, so it's pretty big and that's why I > was using a compressed image. The compressed image is about 8 MB. If the debug mode on PPC enables live patching of kernel functions for instrumentation that can certainly slow things down. You would see that in tcg_optimize appearing in the perf log and "info jit" showing constantly growing translation buffers. > > The initrd shouldn't matter, the issue is seen during kernel > decompression, before the init ram is used. > > I can share my binaries if you'd like. Especially a recent version of > skiboot showing the problem. > > Fred -- Alex Bennée
