Not being one to hijack threads, but I haven't heard of this performance hit
when using HT, I have what should all rights be a pretty fast server, dual
2.4 Xeons with HT 205gb raid 5 array, 1 gig of memory. And it is only 50% as
fast as my old server which was a dual AMD MP 1400's with a 45gb raid 5
array and 1gb of ram. I have read everything I could find on Pg performance
tweaked all the variables that were suggested and nothing. Which is why I
subscribed to this list, just been lurking so far but this caught my eye.
Not to get into a big Intel vs AMD argument but 50% sounds about right. Let's first assume that the QS rating for the MP1400 is relatively accurate and convert that to a 1.4GHz Xeon. 2.4/1.4 = +71%. Since processor performance does not increase linearly with clockspeed, 50% is in line with expectations. Then you throw in the fact that (1) QS ratings for slower AMD chips are understated (but overstated for the fastest chips), (2) AMD uses a point-to-point CPU/memory interface (much better for SMP) versus the P4/Xeon's shared bus, (3) Athlon architecture is more suited for DB work compared to the P4, I'd say you're lucky to see 50% more performance from a Xeon 2.4.
As for HT, I've seen quite a few benchmarks where HT hurts performance. The problem is it's not only app and workload specific but also system and usage specific. As it involves the internal rescheduling of processes, adding more simultaneous processes could help to a point and then start hurting or vice-versa.
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