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

[Sergio Lopez]

Michael S. Tsirkin <address@hidden> writes:

> 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 <address@hidden> 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.

I just tried disabling everything that menuconfig allowed me to. Saves
~27KiB and doesn't improve boot time.

> Trying with ACPI_REDUCED_HARDWARE_ONLY would also be educational.

I also tried enabling this one in my original config. It saves ~11.5KiB,
and has on impact on boot time either.

>> 
>> > 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.
>> > 
>> 
>>

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