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