apache中的ab工具，失败请求数很多。

使用apache中自带的测试工具ab， -n 100 -c 1,发现失败请求数很多，请问下是什么原因呢 ？

Document Path:          /fcgi-bin/dsservice.fcgi?dsservice=camerast
Document Length:        176 bytes

Concurrency Level:      1
Time taken for tests:   0.88070 seconds
Complete requests:      100
Failed requests:        91
   (Connect: 0, Length: 91, Exceptions: 0)
Write errors:           0
Total transferred:      30692 bytes
HTML transferred:       17692 bytes
Requests per second:    1135.46 [#/sec] (mean)
Time per request:       0.881 [ms] (mean)
Time per request:       0.881 [ms] (mean, across all concurrent requests)
Transfer rate:          329.28 [Kbytes/sec] received