Reason HP uses only 128GB RAM
whereas Solaris uses 256GB RAM, is because then the HP machine can use faster PC2-6400 memory. If HP wanted 256GB RAM, then they could have only used slower RAM. Therefore HP opted for faster RAM, but smaller size. HP also uses 2.8GHz CPUs.
HP is 7 unit machine. Sun is 4 unit machine.
SUN uses slower PC2-5300 RAM sticks and 2.6GHz CPUs and is still 25% faster on SAP. That is because the cores have 99% utilization under Solaris. On HPs Linux machine, the cores are only 87% utiliized, which shows that Linux has problems handling many cores well, on a single machine. On large clusters (which is basically a network with several computers), Linux performs well.
SAP white papers
If you have enough memory to cache everything, then you will not gain anything from adding more memory. Apparentely, HP saw that 128GB RAM was faster than 256GB RAM. HP has the resources to add another 128GB RAM if they wanted to. And they know how to tweak their system to get the most performance. HP got the fastest result with 128GB ram. Because the memory was faster.
There are benches with equal amount of RAM on both Solaris and Linux, and those benches confirms that Solaris machine is faster: Compare SAP benchmarks 2009035 and 2009030 to see that.
Regarding Sun T5440 being slower. I have no problems with that. The Niagara is not best on all work loads. If benches show that Niagara is slower, then there is nothing to argue about. Instead, choose SUNs 4 unit which delivers 25% more performance with Solaris, than Linux.
The fun thing is that I dont argue about benches. Whereas there are people here, that refuses to accept benchmarks and white papers. They say "the benches are cherry picked by SUN, or created by SUN to show the strength" etc. The fun thing is, there are so many benches where SUN is fastest. Has SUN created ALL benches? TPC-C, specint, etc? I dont argue about benches. Hard numbers are hard numbers. There are people that wont accept hard numbers. There is one moron that says something like "in MY OPINION the Niagara is slower than Power6, despite benches showing the opposite, I have seen those benches, yes. But in my opinion". Then you can say "in my opinion that 2m guy is shorter than the 1.7m guy" - equally weird.