I am making use of the access time to analyse some build process, but it is not working the way I want: the access time is updated the first time I read the file, then it stays the same for a long while, or until the next reboot. For instance:

$ ll -u some_file
-rw-r--r-- 1 root root 1.3M 2010-04-07 10:03 some_file
$ grep abcdef some_file
$ ll -u some_file
-rw-r--r-- 1 root root 1.3M 2010-04-07 11:24 some_file
# The access time is updated

# waiting a few minutes...
$ grep abcdef some_file
$ ll -u some_file
-rw-r--r-- 1 root root 1.3M 2010-04-07 11:24 some_file
# The access time has not been updated :(

I suppose that the file is buffered by Linux in the free memory, the only this copy is accessed the subsequent times for speed reasons. A solution would be to discard the buffers in memory. After searching some forums, I found:

sync
echo 1 > /proc/sys/vm/drop_caches
echo 2 > /proc/sys/vm/drop_caches
echo 3 > /proc/sys/vm/drop_caches

But it is not working, it seems that it only sync up the write buffers, not the read ones. May be it is due to some custom kernel configuration on my distro (fedora 9)?

Or I am missing something here? Is there a way to achieve this access time refresh?

Note also that I do not want to simulate some writes on my entire file tree. Because I am using some makefile based build system, this will cause the entire project to be build again.