Tomcat process killed by Linux kernel after running out of swap space; don't get any JVM OutOfMemory error

I was performing load testing against a tomcat server. The server has 10G physical memory and 2G swap space. The heap size (xms and xmx) was set to 3G before, and the server just worked fine. Since I still saw a lot free memory left and the performance was not good, I increased heap size to 7G and ran the load testing again. This time I observed physical memory was eaten up very quickly, and the system started consuming swap space. Later, tomcat crashed after running out of swap space. I included -XX:+HeapDumpOnOutOfMemoryError when starting tomcat, but I didn't get any heap dump. When I checked /var/log/messages, I saw kernel: Out of memory: Kill process 2259 (java) score 634 or sacrifice child.

To provide more info, here's what I saw from Linux top command when heap size set to 3G and 7G

xms&xmx = 3G (which worked fine):

• Before starting tomcat:

  Mem:  10129972k total,  1135388k used,  8994584k free,    19832k buffers
  Swap:  2097144k total,        0k used,  2097144k free,    56008k cached
  

• After starting tomcat:

  Mem:  10129972k total,  3468208k used,  6661764k free,    21528k buffers
  Swap:  2097144k total,        0k used,  2097144k free,   143428k cached
  PID  USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
  2257 tomcat    20   0 5991m 1.9g  19m S 352.9 19.2   3:09.64 java
  

• After starting load for 10 min:

  Mem:  10129972k total,  6354756k used,  3775216k free,    21960k buffers
  Swap:  2097144k total,        0k used,  2097144k free,   144016k cached
  PID  USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
  2257 tomcat    20   0 6549m 3.3g  10m S 332.1 34.6  16:46.87 java
  

xms&xmx = 7G (which caused tomcat crash):

• Before starting tomcat:

  Mem:  10129972k total,  1