To hyper-thread or not to hyper-thread

[Bill Davidsen]

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