This CL makes it possible for the Dart VM to start or stop `perf` while
doing GC, which gives a significantly more stable output.
Example usage:
Bash script that creates the correct file handles and calls dart under
`perf stat` based on the arguments given to the script:
```
DART=$1
shift
AOTSNAPSHOT=$1
shift
ctl_dir=/tmp/
ctl_fifo=${ctl_dir}perf_ctl.fifo
test -p ${ctl_fifo} && unlink ${ctl_fifo}
mkfifo ${ctl_fifo}
exec {ctl_fd}<>${ctl_fifo}
ctl_ack_fifo=${ctl_dir}perf_ctl_ack.fifo
test -p ${ctl_ack_fifo} && unlink ${ctl_ack_fifo}
mkfifo ${ctl_ack_fifo}
exec {ctl_fd_ack}<>${ctl_ack_fifo}
perf_ctl_fd=$ctl_fd perf_ctl_fd_ack=$ctl_fd_ack perf stat --delay=-1 --control fd:${ctl_fd},${ctl_fd_ack} -B -e "task-clock:u,context-switches:u,cpu-migrations:u,page-faults:u,cycles:u,instructions:u,branch-misses:u" $DART --perf_ctl_fd=${ctl_fd} --perf_ctl_fd_ack=${ctl_fd_ack} --perf_ctl_usage=1 --deterministic $AOTSNAPSHOT $@
exec {ctl_fd_ack}>&-
unlink ${ctl_ack_fifo}
exec {ctl_fd}>&-
unlink ${ctl_fifo}
```
This will start `perf stat` paused (which would require the run dart
aot-compiled script to start it when it wants to) and where the VM
pauses `perf` while doing GC.
In practise, looking at instruction counts reported by `perf stat`, I've
seen the difference between runs go from 2+ mio (and in some instances
23+ mio) to around 30,000 (!) on runs of the analyzer (tool
"stable_analysis").
TEST=manually
Change-Id: Iab955a5dd35e47f22c4f693ae50effc8ee633897
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/468260
Reviewed-by: Slava Egorov <vegorov@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>
Jens Johansen