Intel’s six-core Dunnington Xeon CPUs have already been doing the rounds since September 2008, but AMD’s rival Opteron processors have now finally caught up. The company recently demonstrated its own six-core ‘Istanbul’ CPUs, and the demo included showing off a 24-core, quad-socket server based on the new CPUs.

The Tech Report attended the demo, and revealed some interesting findings. The first factor to note is that Istanbul CPUs will be fully compatible with existing Socket F Opteron systems. To perform a drop-in upgrade to Istanbul processors, the server's motherboard will just need a BIOS update and support for dual power planes.

Secondly, the performance of the new CPUs appears to scale well, more so than you would expect from the simple addition of more cores. AMD compared a 16-core system based on four Shanghai-based Opterons with a 24-core system based on four Istanbul-based chips, on which you could see all 24 cores in Task Manager on Windows Server 2008. To show the difference between the two machines, AMD ran the Stream benchmark on both machines, in which the Shanghai system managed a throughput of around 25,000MB/sec, while the Istanbul system managed a highly impressive 42,000MB/sec.

What’s interesting here is that Stream isn’t really designed to test CPU performance; it’s mainly designed to test memory bandwidth. The Tech Report says that the huge jump in performance may well be down to a new feature in the CPU that AMD calls HT Assist, which is designed to stop the HyperTransport links between CPU sockets getting clogged up with unnecessary coherency synchronisation requests. According to the site, HT Assist basically stores an index of the CPU’s caches in the processor’s L3 cache, and it can then filter probe requests rather than sending them to all the sockets in the server. The site also says that you’ll be able to set the amount of space dedicated to probe filtering in the BIOS.

A range of six Istanbul CPUs are currently planned, including both low-power and high-performance models, and AMD apparently hopes that the easy upgrade option will make the new chips particularly appealing to cash-strapped businesses looking for a cheap server upgrade.

Of course, the big test for the new Opterons will be a comparison with Intel’s Dunnington-based Xeons, but it could be a while before we see that test – Istanbul is currently scheduled to be released at some point in the second half of 2009. Could Istanbul potentially rival Dunnington in the server market? Let us know your thoughts in the forums.