Yuri,
What setting are you using for max-inodes? With 64 Meg, you probably
could increase it to 6000 or 8000. Cacheing under 2.1.xx is less
"sticky" than under 2.0.xx, so it recovers more quickly from changes.
But if the total number of files (or cache pages) greatly exceeds
memory, it won't remember very much from one execution to the next. Two
kernel source trees (src + dest) might be enough greater than your inode
count so that little is retained.
Regards,
Bill