More CPU Cores Can Be Slower

My contacts at Adobe asked me in January 2011 whether anything had changed with respect to my "12 cores are slower than 6" findings that I first reported back in April 2010.  Regrettably, nothing has changed.

The results shown below are consistent and repeatable over the past 9 months. Watching the two Mac Pro models go at it simultaneously makes it self-evident just how much slower the 12-core model runs. To my knowledge, Adobe has yet to formally acknowledge this issue.

This puzzling performance hit is a Photoshop-specific  issue. It does not apply in general, certainly not to programs that are written to use multiple CPU cores efficiently.

Ironically, disabling half the CPU cores with a developer tool so that Photoshop sees half as many cores, speeds up Photoshop on the 12-core machine!

There is a prima facie assumption that a 12-core machine should be as fast or faster than a 6-core machine, especially when configured with 48GB memory (vs 24GB for the 6-core), yet it is far slower.

In the real world, more cores means that code must be written with skill to avoid the pitfalls that await multi-threaded code.  Code that ran well on 2 or 4 cores can show its limitations quickly at 6 cores and beyond (as your author well knows, having been a professional software architect for over two decades).

Photoshop CS5 performance of two 3.33GHz Mac Pros, one with 6 cores, and one with 12 cores