Quoting Gavin McDonald:
We run dual-Xeons w/ Hyperthreading on an IBM eServer, and with the smp
kernel we get (the appearance of?) 4 CPUs.
Actually, this makes me wonder... Am I truly getting 4 threads running
synchronously? Maybe I should watch 'top' output more closely, and see if
the individual loads are paired,
or evenly spread...
Any thoughts, all?
Intel's HT (HyperThread) technology is a clever hack that makes the P4 a
very nice CPU out of an inefficiency in the original design. In the
original P4 the prefetch queues were so deep that they caused execution
stalls, and it was actually slower than a similarly-clocked P3 would
be. Then Intel added circuitry to run two independent execution threads
simultaneously. The stalls on one side give execution time to the other
CPU thread; they just become opportunities for parallelism. Now one P4
chip acts in every respect like two, and the increase in speed is
comparable to a true two-CPU brew. Of course, the use of only one
physical chip means a simpler motherboard with less power consumption.
So a P4 liability became a huge asset.
From the OS's point of view, a HT CPU is two CPUs. The hyper-threading
is done electrically, in the chip itself, so it presents in every
respect as two physical CPUs. It is not a mere appearance of multiple
CPUs.
As for how well Linux (or any other OS) uses those extra CPUs, that's
another question. If it uses actual multiple CPUs well it'll use the HT
P4 well, and contrariwise.
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