AMD has provided details of the cost savings that the multichip design of its Epyc server processors delivers. By building the CPUs with 32 cores from 4 modules, the costs are 41 percent lower than if one large chip were used.
Epyc processors are made up of four dies of 213mm², each containing eight cores. A processor therefore consists of four modules. CPUs with fewer cores have the same configuration, but with fewer active cores per die. That way AMD can also use dies with disabled cores. In a presentation on Hot Chips, AMD announced the exact savings that this provides compared to a monolithic design, ServeTheHome writes.
According to the calculations, the production of the current Epyc CPU with 32 cores costs 59 percent of what the same chip would cost if it were made in one piece. That chip would be 777mm² in size. The mcm design also involves additional costs, as additional transistors are made for the Infinity Fabric interconnects connecting the dies. There are also several redundant features. For example, each server has a server controller hub, while only one is being used. Furthermore, the total package with 852 mm² is about ten percent larger than if one large chip were made. Despite this, the mcm design is significantly cheaper.
Competitor Intel makes all its CPUs according to a monolithic design, including the Xeon models with many cores. The production of those processors is therefore probably more expensive. In theory, a monolithic chip performs slightly better, because there is no loss by adding interconnects between the different dies.
However, the cost savings and scalability of an mcm design are major advantages, making it likely that such constructions will be used more in the future. Nvidia, among others, is researching an MCM design for its GPUs and AMD has previously indicated that its Infinity Fabric can be used for GPUs.