W2K max CPU usage 50% on Pentium 2.8GHz

[Steve Sutton]

I just installed Windows2000 on a new Pentium 2.8 GHz 800FSB and noticed
that a single, CPU-intensive process can never use more than 50% of the CPU.
(Viewed with Task Mgr.) It will grab up to exactly 50%, but no more. Two
such processes can each use 50% thereby using 100% overall, but when only
one is running the CPU remains 50% idle. I notice that the System Idle
Process can use effectively 100%, so it would seem an aspect of W2K--rather
than the CPU. I know that on my older 500 MHz Pentium any CPU-intensive
process would grab as much of the processor as it could.

Anybody know what's going on here?

 

[Steve Sutton]

I fixed the problem by disabling the P4 Hyperthreading in the BIOS. I guess
W2K (and XP, on which I also observed the problem) impose this 50%
limitation under Hyperthreading.

Anybody know why?

 

[Rob Stow]

I fixed the problem by disabling the P4 Hyperthreading in the BIOS. I guess
W2K (and XP, on which I also observed the problem) impose this 50%
limitation under Hyperthreading.

Anybody know why?

HT is *not* limiting your cpu to 50%.

W2K predates HT by a few years and simply doesn't know
how to deal with it properly. HT makes W2K think your
single physical processor is actually two physical
processors, whereas an OS that is truly HT-aware will
properly recognize that one physical processor as being
two /logical/ processors.

By disabling HT, you simply hide one of those two logical
processors from W2K.

When you have a process that shows 50% cpu usage when
you think it should be at 100%, that just means that
one of the two logical processors is runnning at 100%
while the other is running at 0% and W2K is simply
reporting the average to you. Next time, tell task
manager to display the cpu usage to you separately
for each processor - and you will see a graph with
one line for each processor: one at 100% and one at
0%.