Where is my memory going in Ubuntu 14.04?

I'm running Ubuntu Trusty 14.04 on a new machine with 8GB of RAM, and it seems to be locking up periodically and spewing "Kill process 21500 (python) score 1 or sacrifice child" errors with lots of different processes (there are 182 such messages in syslog from today). I've installed Munin and have been watching the graphs, and it looks like the "apps" memory is going through the roof...

http://loath.org/aneel/random/munin-memory.png

free agrees, claiming I'm using more than 7G of memory:

    # free -h
                 total used free shared buffers cached
    Mem: 7.5G 7.3G 162M 147M 1.7M 282M
    -/+ buffers/cache: 7.1G 446M
    Swap: 15G 571M 14G

However, I'm having trouble determining what exactly is eating all of that memory. Running top or htop doesn't seem to single anything out, and ps_mem.py (https://raw.github.com/pixelb/ps_mem/master/ps_mem.py) claims that I'm using much less than the system thinks...

    ...
      8.3 MiB + 279.5 KiB = 8.6 MiB Plex Media Server
      9.8 MiB + 312.0 KiB = 10.1 MiB Xorg
     13.7 MiB + 233.0 KiB = 13.9 MiB transmission-daemon
     12.6 MiB + 1.7 MiB = 14.4 MiB compiz
     68.5 MiB + 1.3 MiB = 69.8 MiB python (4)
     96.2 MiB + 2.6 MiB = 98.9 MiB plexhometheater (2)
    ---------------------------------
                            292.0 MiB
    =================================

This other formula for totaling the memory roughly agrees:

    # ps -e -orss=,args= | sort -b -k1,1n | awk '{total = total + $1}END{print total}'
    318460

If the processes only total 292-318 MiB, where's the rest of the memory going?

Slabtop doesn't look like I have a huge cache or anything...

     Active / Total Objects (% used) : 249458 / 272022 (91.7%)
     Active / Total Slabs (% used) : 9901 / 9901 (100.0%)
     Active / Total Caches (% used) : 77 / 105 (73.3%)
     Active / Total Size (% used) : 53500.41K / 60447.70K (88.5%)
     Minimum / Average / Maximum Object : 0.01K / 0.22K / 8.00K

      OBJS ACTIVE USE OBJ SIZE SLABS OBJ/SLAB CACHE SIZE NAME
     41496 40465 97% 0.15K 1596 26 6384K btrfs_extent_map
     31752 29947 94% 0.19K 1512 21 6048K kmalloc-192
     28800 24573 85% 0.06K 450 64 1800K kmalloc-64
     25740 25740 100% 0.11K 715 36 2860K sysfs_dir_cache
     24843 19234 77% 0.19K 1183 21 4732K dentry

I'm running Trusty and not Saucy because I'm using btrfs, and wanted as fresh a kernel as possible.

What other tests can I do to understand my memory usage? Thanks!