Re: [Qemu-devel] Re: Performance Monitoring

["Cheif Jones" <avivgnet@gmail.com>]

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Vince,

Your suggested patch is a good solution. One thing bothers me that there is
a TB caching mechanism on top of the opcode translation mechanism. If the TB
cache is not disabled, your patch might give inaccurate results (e.g a MIPS
loop is translated to host CPU, gets cached, and executed N times from cache
without being re-translated). The TB cache is implemented in tb_find_*()
FYI.

Am i missing something?

Chief

On Fri, May 23, 2008 at 6:38 AM, Vince Weaver <vince@csl.cornell.edu> wrote:

>
> > I would like to run an OS, say Linux, and take a sample for a small
> period
> > of time (seconds) while some app(s) are running and get a list of opcode
> > names and how many times they were executed. I'm not interested in CPI at
> > the moment.
>
> What you are trying to do is relatively straightforward, especially if you
> are going to be running binaries from a RISC type machine.
>
> The way I'd recommend doing it is getting Qemu to output the raw
> instruction stream, and then write an external program that
> decodes the instructions and counts what kinds are in each.  This
> is fairly straightforward to do on an arch like MIPS; it would be
> very complicated on something like x86.
>
> I have some code I can dig up that does this kind of thing (I used
> it to run a branch predictor simulator).  I'll include it at the end ot
> this e-mail.
>
> >    - Paul mentioned "With either alternative you'll still have issues
> with
> >    exceptions. MMU faults abort a TB early, so will screw up your
> statistics.
> >    One possibility is to terminate a TB on every memory access, like we
> do for
> >    watchpoints." - is this an issue addressed by your patch?
>
> I've actually only tested my method of generating things with the
> userspace linux-user type method of emulation, I haven't tested it at all
> when doing full-system simulation.  I'd imagine it would still work.
>
>
> Here's the code.  It's based on a pre-TCG version of Qemu so you can't use
> it on the latest snapshots.  It also only works with MIPS, but it
> probably will be similar with other architectures.  The code
> buffers a large block of values before writing it out (for performance).
> To avoid creating huge traces to disk (and they will be huge) you
> can write to a named pipe (mkfifo) and have your analysis routine
> run at the same time reading in from the same pipe.
>
> Hopefully if I am doing something horribly wrong with this code, someone
> will correct me.  I've been using it for a while now though and have been
> getting good results when compared to hw perf counters.
>
>
> This adds code to dump the pc and instruction every executed instruction:
>
> --- ./target-mips/translate.c   2008-04-23 12:23:55.000000000 -0400
> +++ ./target-mips/translate.c   2008-05-22 23:31:13.000000000 -0400
> @@ -6696,6 +6696,7 @@
>             gen_opc_instr_start[lj] = 1;
>         }
>         ctx.opcode = ldl_code(ctx.pc);
> +        gen_op_dump_brpred(ctx.pc,ctx.opcode);
>         decode_opc(env, &ctx);
>         ctx.pc += 4;
>
>
> Add this to "op.c"
>
> void op_dump_brpred(void) {
>   helper_dump_brpred(PARAM1,PARAM2);
> }
>
> Add this to "helper.c":
>
> static int brpred_fd=-1,brpred_ptr=0;
>
> static char error_message[]="Write error!\n";
>
> struct brpredtype {
>   unsigned int addr;
>   unsigned int insn;
> } __attribute__((__packed__));
>
> #define TRACE_UNITS 4096
>
> static struct brpredtype brpred_buf[TRACE_UNITS];
>
> void helper_dump_brpred(unsigned long address,unsigned long insn) {
>
>     int result;
>
>     if (brpred_fd<0) {
>        brpred_fd=creat("trace.bpred",0666);
>     }
>
>     brpred_buf[memtrace_ptr].addr=address;
>     brpred_buf[memtrace_ptr].insn=insn;
>
>     brpred_ptr++;
>
>     if (brpred_ptr>TRACE_UNITS) {
>        brpred_ptr=0;
>        result=write(brpred_fd,brpred_buf,
>                     TRACE_UNITS*sizeof(struct brpredtype));
>        if (result!=TRACE_UNITS*sizeof(struct brpredtype)) {
>           write(2,error_message,13);
>        }
>     }
> }
>
>
>
>
>

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