Intel disables TSX in Haswell

I was going to do a blog about this earlier, but the timing was rather unfortunate, because I had just published another blog. Then it slipped my mind, until the news of the new Haswell EP-based Xeon CPUs that is. So here it is after all (so people can stop saying I only publish things about AMD’s bugs).

So, a while ago, Intel published an erratum relating to the new TSX instructions in Haswell. As you might recall, TSX was one of the most interesting things about Haswell in my opinion. Apparently there is a bug in the implementation, which causes unpredictable behaviour in some cases. Sounds like some kind of race condition. Since there apparently is no way to fix this in microcode, TSX will just be disabled by default. The upcoming Haswell-EX CPU should have a fixed TSX implementation, at which time it can be enabled again.

As for the new Xeons… well, I don’t think I’ll do a writeup on them. There are some interesting things, such as how the cache is organized in ‘clusters’, and how turbo mode is now so effective that even the 18-core model can perform very well in single/low-threaded workloads, making it the best of both worlds. But, all that is already explained in great detail in the various reviews, so I suggest those for further reading.