i/OS Gets Short Sheeted with Power7 Thread Counts
Published: February 22, 2010
by Timothy Prickett Morgan
It is no surprise at all when hardware features outrun software's ability to keep up with those features. This is the computer business, and it has always been this way. If we waited for software to be perfectly aligned with hardware before the hardware was delivered, the pace of change in the IT business would be cut in half. Some might argue that this would be a marked improvement over the current way of doing things, but they would not be employees of IT vendors, who have been engaged in a feature and scalability arms race long since before I was born.
So it is with the new Power7-based hardware, which has gotten way out in front of the operating systems that run atop it. As The Four Hundred has been warning you since last year, the Power7 design includes four times as many cores per chip than the Power6 and Power6+ chips it replaces, and has twice as many threads (virtualized instruction pipelines) per core as well. The instruction pipelines are also different to take into account all those extra cores and threads and other new features in the chips.
IBM's announcement letters from two weeks ago did not get into this, but the current crop of operating systems on Power7-based Power Systems--i 6.1.1, AIX 5.3 and 6.1, and Linux 2.6--are going to strain themselves to make use of the cores and threads in even the midrange Power 750, 770, and 780 servers. (The Power 755 is a supercomputer cluster node and is not particularly interesting for general business customers.) The Power 770 and 780 machines top out at eight processor cards, for a total of 64 cores and 256 threads. The Power 750 tops out at 32 cores and 128 threads. Now, check out this chart IBM gave to business partners briefed on the Power7 launch:
As you can see, on Power6 iron (and the Power7's Power6 mode), i 6.1 and its interim tweaked i 6.1.1 version can support only 32 cores and a maximum of 64 threads, and there is a special support patch that doubles this up to 64 cores and 128 threads. So this special support can, in theory, span all the cores in the Power 770 and 780 boxes, right? Wrong. With i 6.1.1 on Power7 iron, the standard support is the same as this special support, but this operating system can only span 32 cores and 128 threads. That's only half the box, which means to get the full oomph of the machine supporting i/OS, you need to partition the machine into two halves. And it also means that the full processing power of the machine cannot be brought to bear on a single database and its applications.
The future i 7.1 release, which I have heard is coming out in April ahead of the COMMON midrange tradeshow in Orlando, Florida, will have exactly the same core and thread support on Power6 iron, and will only span half a Power 770 or 780, except if you can get your hands on the special support patches, which push it all the way out to 64 cores and 256 threads.
As you can also see from the charts, AIX 5.3 and 6.1 at earlier technology levels (what we would call a sub-release in the AS/400 world) do a little better than i 6.1 or i 6.1.1 on Power6 iron, supporting 64 cores and 128 threads--the maximum on the Power 595 server. And the newer technology levels don't improve upon this on Power6 iron, either. But with AIX 6.1 TL4 and TL5, in Power7 mode, Power7-based servers will be able to have an AIX operating system that spans the full 64 cores and 256 threads of the Power 770 and 780 boxes. However, customers will have to move to AIX 7.1 (which I hear is coming in October) to get the full 256 cores and 1,024 threads that IBM has promised for the high-end Power7 box.
No word on when i 7.1 might be able to span all the 256 cores and 1,024 threads in what I presume will be called the Power 795.
Here's what the core and thread counts look like for Linux running on Power7 iron:
Red Hat Enterprise Linux 5 does not support Power7 hardware at all, and it looks like you'll have to wait until RHEL 6 comes out (probably sometime around the middle of the year) to run RHEL on Power7 iron. (This seems odd to me, and Red Hat doesn't do this on X64 iron. But then again, it is cutting Itanium with RHEL 6, which is a bad thing.) Novell's SUSE Linux Enterprise Server 10, like RHEL 5, supports 64 cores and 128 threads in Power6 mode. SLES 11 supports 256 cores and 1,024 threads right now, and RHEL 6 will do the same, on Power7 iron in Power7 mode.
In other words, operating systems not created by IBM as well as AIX will support more threads than i/OS. Yeah, that's annoying.
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