Description of problem:

The beaker matrix report doesn't have any limits on the number of jobs it will process.

Since you can select "" for whiteboard and most jobs submitted don't include a whiteboard this results in 2,285 jobs being selected.  Here are the details from David's analysis.

thread 3: frame 6 has "msg", which is 450MB of textual HTML response
data allocated on the heap being sent back through httpd output filters,
with <title>Job Matrix Report</title>

Specifically, 451515113 bytes (with commas, that's 451,515,113)

I've extracted the first and last megabyte of the message:

[root@ibm-x3950m2-02 ~]# ll *.html
-rw-r--r-- 1 root root 1048576 Apr 12 16:49 first-meg-of-msg.html
-rw-r--r-- 1 root root 1048576 Apr 12 19:25 last-meg-of-msg.html