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The funny bit

The one funny bit in the Poulson design are the two half-QPI links at the bottom of the chip. These were added, accoording to Riedlinger, to provide glueless interconnections for eight-socket machines. Iit would not be surprising to see them used as interconnects for linking even larger systems into a single SMP image with a modified "Boxboro" chipset from Intel or a tweaked sx3000 chipset from HP, which is used in its Integrity Superdome 2 servers.

The main thing to remember about both the QPI and SMI links is that the run at 6.4 GT/sec, which is the full speed supported by the current Boxboro chipset from Intel and that is 50 per cent more bandwidth than the Tukwilas, with their 4.8 GT/sec speed, had. The Poulson processors are socket compatible with the Tukwilas, so they can plug into existing systems, which already had that faster transfer rate baked in last year. They also run at 170 watts under normal loads, compared to 185 watts for the Tukwilas.

Here is a slightly better schematic of the Poulson cores themselves:

Intel Poulson Itanium core schematic

You can see all of the components clearly in this one. The Tukwila Itaniums had an eleven stage pipeline, but the Poluson cores have shrunk that down to nine stages The pipeline does instructions in order (remember, the EPIC architecture has compilers do the parallelization work so the hardware doesn't have to do out-of-order execution.) The front-ends and back-ends of this Poulson pipeline are decoupled, with the front-end able to fetch six instructions per clock and the back-end able to retire a dozen instructions per clock. The prior Itaniums could only issue six instructions per cycle. Each core has HyperThreading, but Intel has not said how many virtual threads will be on each core. (There's some chatter that Intel will jump up from two to four threads per core.)

The Poulson core has six arithmetic logic units, two integer units, two floating point units, two memory units, and three branch units that are distributed across twelve ports. Each core has 16 KB of L1 instruction and 16 KB of L1 data cache, with 512 KB of L2 instruction cache and 256 KB of L2 data cache. Then there is 4 MB of L3 cache per core, but these L3 caches are shared across the cores, unlike the L1 and L2 caches. Those caches all have ECC error correction.

Here's how the caches lay out on the Poulson chip:

Intel Poulson Itanium cache layout

If you play a lot of Tetris, you can see that there are actually two Poulson core types, with one being a vertical mirror image of the other. There is also a horizontal reflection core (so I guess that makes four in total). This is so Intel can get the ring connect electronics and L2 data caches to line up along the inside of the chip, near the L3 caches and ring interconnect. The L2 instruction caches can be on the outside of the cores, and far away from the the L2 data and L3 caches, because the Poulson core is optimized for single-cycle integer execution against the L1 data cache. This is the kind of work that big database and ERP systems do.

Each memory controller on the Poulson chip supports two SMI ports operating in lockstep at the 6.4 GT/sec speed, and they provide a maximum of 512 GB of memory per socket. So a glueless eight-socket box will be able to support up to 8 TB of main memory, which is a hell of a lot of DDR3 DRAM. (I was going to say I was glad that I didn't have to pay for it, but we all do pay for it, one way or another, when a corporation or government buys a server, don't we?) Those memory controllers have double device data correction scrubbing.

Riedlinger said that there was an aggregate of 45 GB/sec of bandwidth from those SMI ports to memory cards in the system and 128 GB/sec of bandwidth out of those four and two halves of QPI ports. The L3 cache ring interconnect has a whopping 700 GB/sec of bandwidth.

The Poulson also has a ten-port crossbar router for managing I/O and memory traffic across the chip. Here's what that looks like:

Intel Poulson Itanium system interface

The Poulson chip has seven different power planes. Four for each pair of cores, one each for the systen I/O at the top and bottom of the chip, and one for the L3 cache and ring interconnect. By having these separate power domains, Riedlinger said that Intel could ratchet up and down the speed of core pairs without impacting the overall thermals of the chip, providing about 5 percent extra performance on the workloads it tested without impacting electricity consumption or heat dissipation. This is about as close a thing to a free lunch as you can get in the chip racket. ®

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