slabtop command (part of the package procps) shows top memory objects used by the kernel.
dstat can help you figure out what is happening. dstat -cdnpmgs --top-bio --top-cpu --top-mem
Also have a look at smem ("smem -kt"), it can show you nicely what is in your swap.
ZRAM if you have no HDD/SSD swap partition.
ZSWAP if you do have a HDD/SSD swap partition.
ZCACHE: It does what ZSWAP does and ALSO compresses and speeds the filesystem page cache. (It is internally much more complicated and is not in the mainline kernel as it is still under development).