如何知道apache或者nginx缓存功能是否生效?

配置了apache的mem cache功能,配置完成启动也没有问题,但是好像缓存功能并没有生效.
配置如下:

<IfModule mod_cache.c>
CacheDefaultExpire 3600
CacheMaxExpire 86400
CacheLastModifiedFactor 0.1
CacheIgnoreCacheControl on
CacheIgnoreNoLastMod On
CacheStoreNoStore       On
CacheStorePrivate on
CacheIgnoreHeaders Set-Cookie




<IfModule mod_mem_cache.c>

CacheEnable mem /
MCacheSize 4096
MCacheMaxObjectCount 100
MCacheMinObjectSize 1
MCacheMaxObjectSize 2048

</IfModule>

用strace跟踪apache进程仍然是读磁盘,重复访问同一个文件仍一样:

[pid  8811] read(9, "GET /file/b.gif HTTP/1.1\r\nAccept"..., 8000) = 285
[pid  8811] gettimeofday({1277367291, 554603}, NULL) = 0
[pid  8811] gettimeofday({1277367291, 554763}, NULL) = 0
[pid  8811] gettimeofday({1277367291, 554802}, NULL) = 0
[pid  8811] stat64("/data/file/b.gif", {st_mode=S_IFREG|0644, st_size=7116, ...}) = 0
[pid  8811] open("/data/file/b.gif", O_RDONLY|O_LARGEFILE) = 10
[pid  8811] fcntl64(10, F_GETFD)        = 0
[pid  8811] fcntl64(10, F_SETFD, FD_CLOEXEC) = 0
[pid  8811] setsockopt(9, SOL_TCP, TCP_CORK, [1], 4) = 0
[pid  8811] writev(9, [{"HTTP/1.1 200 OK\r\nDate: Thu, 24 J"..., 297}], 1) = 297
[pid  8811] sendfile64(9, 10, [0], 7116) = 7116

我晕,CU的技术位越来越水了,还是高手不愿出招?
这样一个问题放了几天无人能回答?