Barcelona Architecture: AMD on the Counterattack

A Faster Memory Controller

When AMD integrated a memory controller on-die, we knew that every time we saw a new AMD processor, we'd get a slightly enhanced memory controller. In Barcelona, the tweaks are significant and should provide for a tangible improvement in memory performance.

One strength of Intel's FB-DIMM architecture used in Xeon servers is that you can execute read and write requests to the AMB simultaneously. With standard DDR2 memory, you can do one or the other, and there's a penalty for switching between the two types of operations. If you have a fairly random mixture of reads and writes you can waste a lot of time switching between the two rather than performing all of your reads sequentially then switching over to writes. The K8's memory controller made some allowances for preferring reads over writes since they take less time, but in Barcelona the memory controller is far more intelligent.

Now, instead of executing writes as soon as they show up, writes are stored in a buffer and once the buffer reaches a preset threshold the controller bursts the writes sequentially. What this avoids is the costly read/write switch penalty, helping improve bandwidth efficiency and reduce latency.

The K8 core (Socket-940/939/AM2) featured a single memory controller that was 128-bits wide, but in Barcelona AMD has split up the DRAM controller into two separate 64-bit controllers. Each controller can be operated independently and thus you get some improvements in efficiency, especially when dealing with quad core implementations where the individual cores working on independent threads all have their own memory access patterns.

Barcelona's Northbridge is also set up to handle higher bandwidth than before. Deeper buffers are present, allowing for higher bandwidth utilization, and the Northbridge itself is ready for use with future memory technologies (e.g. DDR3). We'd expect one or two revisions past Barcelona will be when AMD switches memory technologies, but the new core will initially debut with DDR2 support.