Re: [Qemu-devel] profiling qemu

[Lluís Vilanova]

Artyom Tarasenko writes:
[...]
> QEMU 1.0.50 monitor - type 'help' for more information
> (qemu) profile
> unknown command: 'profile'
> (qemu) info profile
> async time  38505498320 (38.505)
> qemu time   35947093161 (35.947)

> Is there a way to find out more?

Command "info jit" also has some information added when compiled with profiling
support.

Search for CONFIG_PROFILER to see which code is activated during profiling.


> Next I tried gprof:

> build-prof $  gprof sparc64-softmmu/qemu-system-sparc64 gmon.out
> Flat profile:

> Each sample counts as 0.01 seconds.
>   %   cumulative   self              self     total
>  time   seconds   seconds    calls  Ts/call  Ts/call  name
> 100.00      5.06     5.06                             main

> Hmm. Not very informative. Is there a way to find out more details?

Did you run QEMU for a reasonable amount of time? gprof uses sampling to capture
its execution time statistics, so a small execution of QEMU will not be able to
capture any meaningful information.


[...]
> Here it looks like "compute_all_sub" and "compute_all_sub_xcc" are
> good candidates for optimizing: together they take the same amount of
> time as cpu_sparc_exec. I guess both operations would be trivial in
> the x86_64 assembler. What would be the best strategy to make TCG take
> the advantage of running on a x86_64 host?

A quick look into the code reveals that these two are called from a TCG helper
(helper_compute_psr), so I see two approaches here applicable to the most
frequently used "sub-operations" in helper_compute_psr:

* Define new simpler helpers for those sub-operations that can be declared with
  TCG_CALL_CONST and generate the new psr/xcc values in temporal registers. You
  must make sure any other code will still be able to use the new psr/xcc
  values.

* Reimplement these sub-operations in pure TCG code.


But first, make sure you run a proper benchmark to establish where are the
hotspots in the sparc code for QEMU. The problem here is to establish what a
proper benchmark is :)


Lluis

-- 
 "And it's much the same thing with knowledge, for whenever you learn
 something new, the whole world becomes that much richer."
 -- The Princess of Pure Reason, as told by Norton Juster in The Phantom
 Tollbooth