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Hyper-Threading Speeds Linux
developerWorks writes "The Intel Xeon processor introduces a new technology called Hyper-Threading (HT) that makes a single processor behave like two logical processors. The technology allows the processor to execute multiple threads simultaneously, which can yield significant performance improvement. But, exactly how much improvement can you expect to see? This article gives the results the investigation into the effects of Hyper-Threading (HT) on the Linux SMP kernel. It compares the performance of a Linux SMP kernel that was aware of Hyper-Threading to one that was not." Ah, the joys of high performance.
Here is the associated press release from Intel about the HT in 3 Ghz P4's. I have seen screenshots of Windows task manager showing (2) CPU performance graphs.
SMP already can gain benefit from hyperthreading. However, an OS really needs special support to A) get the most out of hyperthreading and B) avoid worst-case scenarios, especially when you have both multiple physical CPUs and multiple logical CPUs per physical CPU.
For instance, if you have two processes running, you want to put them on different physical CPUs, and if you have a choice, grouping threads with the same memory image on a single processor improves cache usage.
Without this, hyperthreading may
The downside is that for code that isn't SMP/HT-aware, performance can actually degrade. Tom's Hardware ran tests of hyperthreading on the 3.06GHz P-4, and in almost every case, it performed better with hyperthreading disabled.
Breakfast served all day!
Holy intellectual dishonesty, Batman!
NT and Windows 2000 do not support HT and never will. NT will not becuase it's been end-of-lifed, and Windows 2000 will not because of Microsft policy. On a 2-CPU system with HyperThreading, NT and Windows 2000 will think they have real 4 CPUs (unsurprisingly, this is what a pre-HT version of Linux will see as well). HT support means the OS knows that it has, in this example, 2 real CPUS and 2 fakes, and the scheduler will weight the real CPUs accordingly.
XPPro SP1 is the first, and only shipping version of Windows to support HT.
One of the major impediments to increasing CPU performance has been increasing memory latency. Memory latency has grown worse as CPUs have gotten faster. Accessing RAM will now cause a >150 cycle latency, during which the processor sits IDLE.
Cache only partly mitigates this problem. Some applications, such as databases and OLTP, are heavily dependent on repeatedly accessing non-cached RAM. There is no way to cache all the relevant data, since virtually all databases are larger than can fit in any present cache, no matter how large, and there is sometimes no way to predict which data will be accessed. ALL of these applications have CPUs that spend much of their time being IDLE, waiting for memory to be returned.
SMT (hyperthreading) allows the processor to perform useful work during these otherwise idle periods, by allowing the cpu to switch to a thread that is not blocked on memory access. The "idle bubbles" in the execution pipeline can therefore be "filled in" by useful work that advances the state of relevant programs.
SMT can cause a degredation in performance beceause it can lead to "cache thrashing." In an SMT-naive kernel, two unrelated threads could be scheduled for the same physical CPU. These unrelated threads will likely share very little code or data. The two threads will therefore "compete" for the single shared cache, with each thread's data being repeatedly displaced by the other's.
This difficulty can be substantially mitigated by making the kernel aware of "virtual processors," and by implementing scheduleing algorithms to minimize the impact. The performance of hyperthreading will likely improve as kernels are better able to exploit it.