Lenovo Thinkstation P920 Manual page 5

The following guidelines are recommended by Lenovo for obtaining the best
memory bandwidth from your P920 when filling DIMM slots:
 In single CPU configurations, only DIMM slots for CPU1 can be
utilized. For dual CPU configurations, all DIMM slots can be utilized.
 In dual CPU configurations, it is optimal to balance memory across
CPUs. This applies to both DIMM quantity and capacity. While the
system will function with unbalanced DIMMs, overall memory
performance could be affected.
 For best memory bandwidth performance, Lenovo recommends
optimizing the overall memory configuration to use 6 DIMMs per
CPU. DIMMS A1 and D1 (for each CPU) can be used where overall
memory capacity is prioritized over memory bandwidth.
 Once DIMM quantity and DIMM capacity have been balanced across
CPUs (if necessary), users should follow the recommended fill order
for each CPU found in Figure 2. Largest capacity DIMMs should be
installed first if using mixed DIMM capacities.