esxi虚拟化debian挤压的perf样本

Question

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？