Cache memory with Chroots

Question

I have a server with many chroots, each one used by a different group of users. Most of them use Jupyter Notebooks that store variables in memory for as long the notebooks remain open. The output of free -h is the following;

+-------------------+-------+------+------+--------+---------+--------+
|                   | total | used | free | shared | buffers | cached |
+-------------------+-------+------+------+--------+---------+--------+
| memory            | 70G   | 65G  | 5.8G | 35G    | 25M     | 36G    |
| -/+ buffers/cache |       | 29G  | 41G  |        |         |        |
| cache             | 3.7G  | 3.7G |      |        |         |        |
+-------------------+-------+------+------+--------+---------+--------+

1. I cannot clean the cache using sync; echo 1 > /sys/proc/vm/drop_caches. Why?

2. Users in other chroots cannot used memory occupied by cache. Is there a reason for this?