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HT Dual core processors - problems ahead?
Written by Aidan   
Tuesday, 05 April 2005 02:47

Dual core processors are looking good in many ways, but are unexpected problems cropping up for Intel and Microsoft? People who have had access to Intel's dual core processors running have noticed some interesting things. When running as dual core, the system operates as if it were a true SMP system. That's expected, as dual core is to all intents and purposes an SMP system. Throw Hyperthreading into the mix, and things get a bit more interesting.

The system sees four logic processors, as expected. However, Windows XP allocates processor time in a strict order, so the first thread will execute on the first logical CPU. The second thread will execute on the second logical CPU, and so on. The problem is that Windows XP sets the first physical processor up as logical CPUs one and two. 

As we already know, the second logical CPU on a Hyperthreading procesor is a neat trick to use the parts of the processor that aren't already in use. However, if two threads are trying to use the same resources on a Hyperthreading CPU, only one of them can have access, so the other has to stall whilst until the resource it's waiting on becomes free.

Spot the problem? The result is that Windows XP will favour running code on the first physical processor, and will be less likely to make use of the second physical processor. In many instances, this will actually lead to lower performance whilst code 'fights' over the resources of the first physical processor. People have already seen a 3-4% reduction in performance whilst running dual core and hyperthreading compared to dual core without HT.

The fix would require modifications to Windows scheduler. If history is any guide, then we might well have to wait for Longhorn before we see these modifications. Those who remember when HT was introduced, the Windows 2000 scheduler had some issues with HT. Microsoft did fix those issues, but not in Windows 2000. Instead, it required an upgrade to Windows XP. It might just be that history repeats itself, and those who want to maximise the performance of their dual core HT CPUs will upgrade to Lornhorn.

There is a workaround, but only the application developers are in a position to apply it. The workaround is that the application itself sets the processor affinity, thus bypassing the parts of the Windows scheduler that fail to take HT into account properly.

Want more? There's already people discussing AMD and Intel dual cores in the forums!


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