[SeaBIOS] [Qemu-devel] Hack integrating SeaBios / LinuxBoot option rom with QEMU trace backends
Daniel P. Berrange
berrange at redhat.com
Tue Oct 11 11:18:19 CEST 2011
On Tue, Oct 11, 2011 at 11:08:33AM +0200, Avi Kivity wrote:
> On 10/10/2011 09:01 PM, Alexander Graf wrote:
> >>> For example, one execution of QEMU produced the following log:
> >>>
> >>> $ stap qemu-timing.stp
> >>> 0.000 Start
> >>> 0.036 Run
> >>> 0.038 BIOS post
> >>> 0.180 BIOS int 19
> >>> 0.181 BIOS boot OS
> >>> 0.181 LinuxBoot copy kernel
> >>> 1.371 LinuxBoot copy initrd
> >>
> >> Yeah, there was a thread a bit ago about the performance of the interface to read the kernel/initrd. I think at it was using single byte access instructions and there were patches to use string accessors instead? I can't remember where that threaded ended up.
> >
> >IIRC we're already using string accessors, but are still slow. Richard had a nice patch cooked up to basically have the fw_cfg interface be able to DMA its data to the guest. I like the idea. Avi did not.
> >
> >And yes, bad -kernel performance does hurt in some workloads. A lot.
> >
> >
>
> The rep/ins implementation is still slow, optimizing it can help.
>
> What does 'perf top' say when running this workload?
To ensure it only recorded the LinuxBoot code, I created a 100 MB
kernel image which takes approx 30 seconds to copy. Here is the
perf output for approx 15 seconds of that copy:
1906.00 15.0% read_hpet [kernel]
1029.00 8.1% x86_emulate_insn [kvm]
863.00 6.8% test_cc [kvm]
661.00 5.2% emulator_get_segment [kvm]
631.00 5.0% kvm_mmu_pte_write [kvm]
535.00 4.2% __linearize [kvm]
431.00 3.4% do_raw_spin_lock [kernel]
356.00 2.8% vmx_get_segment [kvm_intel]
330.00 2.6% vmx_segment_cache_test_set [kvm_intel]
308.00 2.4% segmented_write [kvm]
291.00 2.3% vread_hpet [kernel].vsyscall_fn
251.00 2.0% vmx_get_cpl [kvm_intel]
230.00 1.8% trace_kvm_mmu_audit [kvm]
207.00 1.6% kvm_write_guest [kvm]
199.00 1.6% emulator_write_emulated [kvm]
187.00 1.5% emulator_write_emulated_onepage [kvm]
185.00 1.5% kvm_write_guest_page [kvm]
177.00 1.4% vmx_get_segment_base [kvm_intel]
158.00 1.2% fw_cfg_io_readb qemu-system-x86_64
148.00 1.2% register_address_increment [kvm]
142.00 1.1% emulator_write_phys [kvm]
134.00 1.1% acpi_os_read_port [kernel]
Daniel
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