I am looking for help with MPICH2. I installed mpich-3.0.4 on a Centos 6.5 PC with 24 cores. When I launch with mpiexec on 17 procs all procs are running 100%. When I launch on more than 17 procs the job gets slower and most processors run on less than 100%. I spent a lot of time google'ing this problem but did not find any answers. Somebody said it may be a memory problem but the computer has 32GB RAM and the same problem occurred when running a very small problem. On bigger computers I have run the same code on 2,048 processors with no problem at all. "lscpu" gives the following response: Architecture: x86_64 CPU op-mode(s): 32-bit, 64-bit Byte Order: Little Endian CPU(s): 24 On-line CPU(s) list: 0-23 Thread(s) per core: 2 Core(s) per socket: 6 Socket(s): 2 NUMA node(s): 2 Vendor ID: GenuineIntel CPU family: 6 Model: 45 Stepping: 7 CPU MHz: 1200.000 BogoMIPS: 4590.92 Virtualization: VT-x L1d cache: 32K L1i cache: 32K L2 cache: 256K L3 cache: 15360K NUMA node0 CPU(s): 0-5,12-17 NUMA node1 CPU(s): 6-11,18-23 "less /proc/cpuinfo" shows: processor : 0 vendor_id : GenuineIntel cpu family : 6 model : 45 model name : Intel(R) Xeon(R) CPU E5-2630 0 @ 2.30GHz stepping : 7 cpu MHz : 1200.000 cache size : 15360 KB physical id : 0 siblings : 12 core id : 0 cpu cores : 6 apicid : 0 initial apicid : 0 fpu : yes fpu_exception : yes cpuid level : 13 wp : yes