On Mon, Aug 25, 2003 at 08:27:46AM -0600, Scott Long wrote:
Since HTT can lead to performance degradation in some (many?) cases, the second logical CPU's are halted by default. They are enabled, however, in order for interrupt routing to work right. Work is ongoing to make an HTT-aware scheduler, and make the enabling of the logical cores optional.
I've heard this several times and don't doubt it, but it would be nice to know more about the issue. What type of cases? What benchmarks have been run showing this?
Well, I haven't actually seen any case where there was a performance gain instead of degradation.
There are two problems with HTT. First, L1/L2 cache issues. Second, the virtual CPUs are not independent, and there are many cases where instructions in one virtual CPU stall the other. So take, for example, the case of a userland application on CPU0 stalling the kernel on CPU1.
The case where HTT presents gains are for applications compiled in a way to maximize HTT benefits and minimize HTT stalling. It would be perhaps be best to restrict HTT usage to threads of a same application, and avoid them at all when in kernel. Intel disagrees, of course, and I haven't been a low level person for many, many years, so YMMV. :-)
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