Re: [Qemu-devel] [PATCH v3 0/4] Introduce the microvm machine type

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jul 25, 2019 at 10:58:22AM -0400, Michael S. Tsirkin wrote:
> On Thu, Jul 25, 2019 at 04:42:42PM +0200, Sergio Lopez wrote:
> > 
> > Paolo Bonzini <pbonzini@redhat.com> writes:
> > 
> > > On 25/07/19 15:26, Stefan Hajnoczi wrote:
> > >> The microvm design has a premise and it can be answered definitively
> > >> through performance analysis.
> > >> 
> > >> If I had to explain to someone why PCI or ACPI significantly slows
> > >> things down, I couldn't honestly do so.  I say significantly because
> > >> PCI init definitely requires more vmexits but can it be a small
> > >> number?  For ACPI I have no idea why it would consume significant
> > >> amounts of time.
> > >
> > > My guess is that it's just a lot of code that has to run. :(
> > 
> > I think I haven't shared any numbers about ACPI.
> > 
> > I don't have details about where exactly the time is spent, but
> > compiling a guest kernel without ACPI decreases the average boot time in
> > ~12ms, and the kernel's unstripped ELF binary size goes down in a
> > whooping ~300KiB.
> 
> At least the binary size is hardly surprising.
> 
> I'm guessing you built in lots of drivers.
> 
> It would be educational to try to enable ACPI core but disable all
> optional features.

Trying with ACPI_REDUCED_HARDWARE_ONLY would also be educational.


> 
> > On the other hand, removing ACPI from QEMU decreases its initialization
> > time in ~5ms, and the binary size is ~183KiB smaller.
> 
> Yes - ACPI generation uses a ton of allocations and data copies.
> 
> Need to play with pre-allocation strategies. Maybe something
> as simple as:
> 
> diff --git a/hw/i386/acpi-build.c b/hw/i386/acpi-build.c
> index f3fdfefcd5..24becc069e 100644
> --- a/hw/i386/acpi-build.c
> +++ b/hw/i386/acpi-build.c
> @@ -2629,8 +2629,10 @@ void acpi_build(AcpiBuildTables *tables, MachineState *machine)
>      acpi_get_pci_holes(&pci_hole, &pci_hole64);
>      acpi_get_slic_oem(&slic_oem);
>  
> +#define DEFAULT_ARRAY_SIZE 16
>      table_offsets = g_array_new(false, true /* clear */,
> -                                        sizeof(uint32_t));
> +                                        sizeof(uint32_t),
> +                                        DEFAULT_ARRAY_SIZE);
>      ACPI_BUILD_DPRINTF("init ACPI tables\n");
>  
>      bios_linker_loader_alloc(tables->linker,
> 
> will already help a bit.
> 
> > 
> > IMHO, those are pretty relevant savings on both fronts.
> > 
> > >> Until we have this knowledge, the premise of microvm is unproven and
> > >> merging it would be premature because maybe we can get into the same
> > >> ballpark by optimizing existing code.
> > >> 
> > >> I'm sorry for being a pain.  I actually think the analysis will
> > >> support microvm, but it still needs to be done in order to justify it.
> > >
> > > No, you're not a pain, you're explaining your reasoning and that helps.
> > >
> > > To me *maintainability is the biggest consideration* when introducing a
> > > new feature.  "We can do just as well with q35" is a good reason to
> > > deprecate and delete microvm, but not a good reason to reject it now as
> > > long as microvm is good enough in terms of maintainability.  Keeping it
> > > out of tree only makes it harder to do this kind of experiment.  virtio
> > > 1 seems to be the biggest remaining blocker and I think it'd be a good
> > > thing to have even for the ARM virt machine type.
> > >
> > > FWIW the "PCI tax" seems to be ~10 ms in QEMU, ~10 ms in the firmware(*)
> > > and ~25 ms in the kernel.  I must say that's pretty good, but it's still
> > > 30% of the whole boot time and reducing it is the hardest part.  If
> > > having microvm in tree can help reducing it, good.  Yes, it will get
> > > users, but most likely they will have to support pc or q35 as a fallback
> > > so we could still delete microvm at any time with the due deprecation
> > > period if it turns out to be a failed experiment.
> > >
> > > Whether to use qboot or SeaBIOS for microvm is another story, but it's
> > > an implementation detail as long as the ROM size doesn't change and/or
> > > we don't do versioned machine types.  So we can switch from one to the
> > > other at any time; we can also include qboot directly in QEMU's tree,
> > > without going through a submodule, which also reduces the infrastructure
> > > needed (mirrors, etc.) and makes it easier to delete it.
> > >
> > > Paolo
> > >
> > > (*) I measured 15ms in SeaBIOS and 5ms in qboot from the first to the
> > > last write to 0xcf8.  I suspect part of qboot's 10ms boot time actually
> > > end up measured as PCI in SeaBIOS, due to different init order, so the
> > > real firmware cost of PAM and PCI initialization should be 5ms for qboot
> > > and 10ms for SeaBIOS.
> > 
> 
>