cyb70289 commented on issue #6986:
URL: https://github.com/apache/arrow/pull/6986#issuecomment-616978252
This change introduces severe branch misses in certain conditions. See perf
logs below. I changed benchmark code to run only the problematic test case.
Without this patch
```bash
807.415826 task-clock (msec) # 0.979 CPUs utilized
83 context-switches # 0.103 K/sec
0 cpu-migrations # 0.000 K/sec
427 page-faults # 0.529 K/sec
2,285,801,407 cycles # 2.831 GHz
(83.17%)
2,313,785 stalled-cycles-frontend # 0.10% frontend cycles
idle (83.16%)
915,631,177 stalled-cycles-backend # 40.06% backend cycles
idle (82.93%)
9,997,208,858 instructions # 4.37 insn per cycle
# 0.09 stalled cycles
per insn (83.66%)
1,679,799,451 branches # 2080.464 M/sec
(83.66%)
106,599 branch-misses # 0.01% of all branches
(83.41%)
```
With this patch
```bash
902.557236 task-clock (msec) # 0.980 CPUs utilized
94 context-switches # 0.104 K/sec
0 cpu-migrations # 0.000 K/sec
427 page-faults # 0.473 K/sec
2,567,879,767 cycles # 2.845 GHz
(83.17%)
88,266,680 stalled-cycles-frontend # 3.44% frontend cycles
idle (83.17%)
20,826,862 stalled-cycles-backend # 0.81% backend cycles
idle (83.03%)
2,518,949,193 instructions # 0.98 insn per cycle
# 0.04 stalled cycles
per insn (83.62%)
847,459,928 branches # 938.954 M/sec
(83.61%)
75,187,208 branch-misses # 8.87% of all branches
(83.39%)
```
Absolute counts are not comparable as gtest runs different loops for each
test.
The point is branch-misses jumps from 0.01% to 8.87%, which causes high
frontend stall(cpu wait for fetching code to execute), and ipc(instructions per
cycle) drops from 4.37 to 0.98.
I didn't figure out which branch is miss predicted and why. My haswell cpu
is too old to support branch tracing. My guess is [this
line](https://github.com/apache/arrow/blob/5093b809d63ac8db99aec9caa7ad7e723f277c46/cpp/src/arrow/util/bit_util.cc#L285),
no concrete justification.
----------------------------------------------------------------
This is an automated message from the Apache Git Service.
To respond to the message, please log on to GitHub and use the
URL above to go to the specific comment.
For queries about this service, please contact Infrastructure at:
[email protected]
