Re: [Qemu-devel] Hack integrating SeaBios / LinuxBoot option rom with QEMU trace backends

["Daniel P. Berrange" <berrange@redhat.com>]

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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