Problem: A fresh installed Debian 8 (Jessie) 32bit 686 uses only one core of a 6 core Xeon CPU. amd64 kernels don’t have this problem.
Solution: The HP Proliant DL360 Gen9 has a BIOS option called “Processor x2APIC Support”. When you set this option to “Disabled” the Linux kernel uses all 6 cores. x2APIC is a new controller for multi core CPUs the works only for 64bit kernels (it seems).
Versions: CPU: Intel(R) Xeon(R) CPU E5-2620 v3, tested with Debian Linux Kernel: 3.16.7-ckt9-3~deb8u1 and self compiled kernel: linux-3.18.13 from kernel.org
Stats: compiling a Linux kernel on one core: 56 min
compiling on 6 cores (12 threads): 7 min