G'Day,
I was profiling and noticed some odd instruction counts, and boiled it
down to a 16 NOP loop:
# perf annotate -i perf.data.noplooper --stdio
Percent | Source code & Disassembly of noplooper
--------------------------------------------------------
:
: Disassembly of section .text:
:
: 00000000004004ed <main>:
0.00 : 4004ed: push %rbp
0.00 : 4004ee: mov %rsp,%rbp
20.86 : 4004f1: nop
0.00 : 4004f2: nop
0.00 : 4004f3: nop
0.00 : 4004f4: nop
19.84 : 4004f5: nop
0.00 : 4004f6: nop
0.00 : 4004f7: nop
0.00 : 4004f8: nop
18.73 : 4004f9: nop
0.00 : 4004fa: nop
0.00 : 4004fb: nop
0.00 : 4004fc: nop
19.08 : 4004fd: nop
0.00 : 4004fe: nop
0.00 : 4004ff: nop
0.00 : 400500: nop
21.49 : 400501: jmp 4004f1 <main+0x4>
Despite millions of samples, many NOPs are never seen. (See the
Percent column.) I'm not using PEBS, but I suppose I should.
I think Andi mentioned this to me last year -- that instruction
profiling was no longer reliable. At the time I hadn't been bitten by
it, but now I have!
Is this due to parallel and out-of-order execution? (ie, we're
sampling the instruction pointer, but that's set to the resumption
instruction, not the instructions being processed in the backend?).
Thanks,
Brendan
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