Using only one CPU core for chess engine (Houdini)

I'm using Tarrasch Chess GUI which comes with the free, older Rybka engine. When performing analysis, Rybka will use only one core (25%) out of four, per the setting. However, when I change the engine to Houdini 1.5a, Houdini uses all of my four CPU cores (100%) regardless of the setting. I only want to use just one core which I think is adequate for my purposes. It also happens with an older Fritz app.

Is this just how Houdini works?

I have an old Fritz too - Fritz 10 and use Houdini 1.5a with that. You can set the number of threads under Engine->Create UCI Engine, then click on the "Parameters..." button. On my system this is defaulted to 4, I'm guessing it will be on your system too. Setting this lower should do what you want.

Just set from the UCI UI the threads parameter to 1.

That's odd. I have a dual core, and I set my Houdini 1.5a to 1 thread, and it's using all my cpu capacity. (No such problems in other GUIs.) Seems Tarrasch may have a problem here.

(Rybka and Shredder work fine on one thread, but there's definitely a problem with the Houdini-Tarrasch combo.)

Might be a bug in the UCI plug of the Tarrasch. It works fine for me using Arena, Chessbase 2009 Premium and ChessX.

When I just set the max cpu cores to 1 in Tarrasch, I had the same problem as the OP. However, see if this works (it worked for me): Add a custom parameter name of "Threads" and a value of "1". (Leave off the quotes.)