Effects of Changing vfs.ufs.dirhash_maxmem in FreeBSD

I have a busy web frontend server many files in a single directory. The server is running FreeBSD 8 amd64. Here is the result of changing the sysctl vfs.ufs.dirhash_maxmem to 67108864. Note the change in system (red) area. The vertical arrow shows when the change was made: