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Re: To hyper-thread or not to hyper-thread



Aaron Konstam wrote:
I acquired another computer recently that has a Pentium(R) 4  D CPU Dual
core that is capable of hyper-threading.

I was not satisfied with its performance so I looked carefully at its
configuration and found that hyper-threading was disabled. A little more
looking and I noticed that hyper-threading was disabled in the BIOS and
could not be turned on. So what are my options if I want to enable
hyper-threading and is it worth it?

One option I assume is to find another BIOS. Are there other options or
if it is disabled in the BIOS it is disabled?

Another question is how much boost in performance should I expect from
the dual core SMB functionality of the CPU?

On some loads you win a lot, and my favorite is compiling a kernel. With ht on and -j3 I almost always have two threads not blocked for i/o. That's the good news, the clock time to compile the kernel drops by about 30%.

Using older kernels there were cases where the processes run were poorly chosen and there was a small drop in performance on some loads, but current kernels have a smarter scheduler and I would guess that you never see it in normal use and might drop 2% with some specialized test program.

Finally, if you run a threaded program where multiple threads communicate via shared memory, ht on will buy you up to 50% more tps, due to elimination of some context switches (vmstat will show this). Servers like dns, mail, or nntp, which have a lot of small processes running the same code will also show up to 20% more tps using ht.

Overall I would have it on if you can, it will give you some improvement when multiple things are going on.

Just as a caution, I thought only the "extreme edition" had dual core and ht enabled. If yours is something lesser it has the ht bit on, meaning it reports ht status, but it probably doesn't have the extras enabled. There are rumors that a firmware hack can turn it on, but I have never seen a working example.

--
Bill Davidsen <davidsen tmr com>
  "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot


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