在esxi虚拟化debian squeeze上运行时，Perf报告没有显示任何输出
root@my:~# uname -a
Linux my 2.6.32-5-amd64 #1 SMP Fri May 10 08:43:19 UTC 2013 x86_64 GNU/Linux
root@my:~# dpkg --get-selections | grep linux-tools
linux-tools-2.6.32
root@my:~# perf stat true
Performance counter stats for 'true':
0.723921 task-clock-msecs # 0.261 CPUs
1 context-switches # 0.001 M/sec
0 CPU-migrations # 0.000 M/sec
99 page-faults # 0.137 M/sec
6442450941 cycles # 8899383.967 M/sec
6442450941 instructions # 1.000 IPC
0 cache-references # 0.000 M/sec
0 cache-misses # 0.000 M/sec
0.002775829 seconds time elapsed
root@my:~# perf record -a -g -f sleep 20
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.042 MB perf.data (~1826 samples) ]
root@my:~# perf report
# Samples: 0
#
# Overhead Command Shared Object Symbol
# ........ ............. ............. ......
#
#
# (For a higher level overview, try: perf report --sort comm,dso)
#
为什么录音中没有样本？