On 10.10.2011, at 20:53, Anthony Liguori wrote: > On 10/10/2011 12:08 PM, Daniel P. Berrange wrote: >> I've been investigating where time disappears to when booting Linux guests. >> >> Initially I enabled DEBUG_BIOS in QEMU's hw/pc.c, and then hacked it so >> that it could print a timestamp before each new line of debug output. The >> problem with that is that it slowed down startup, so the timings I was >> examining all changed. >> >> What I really wanted was to use QEMU's trace infrastructure with a simple >> SystemTAP script. This is easy enough in the QEMU layer, but I also need >> to see where time goes to inside the various BIOS functions, and the >> options ROMs such as LinuxBoot. So I came up with a small hack to insert >> "probes" into SeaBios and LinuxBoot, which trigger a special IO port >> (0x404), which then cause QEMU to emit a trace event. >> >> The implementation is really very crude and does not allow any arguments >> to be passed each probes, but since all I care about is timing information, >> it is good enough for my needs. >> >> I'm not really expecting these patches to be merged into QEMU/SeaBios >> since they're just a crude hack& I don't have time to write something >> better. I figure they might be useful for someone else though... >> >> With the attached patches applied to QEMU and SeaBios, the attached >> systemtap script can be used to debug timings in QEMU startup. >> >> 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. Alex
