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More CPU Cores Can Be Slower

Last updated January 27, 2011 - Send Feedback

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 (see dgly.org/12tooslow).  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 CPU core performance optimization
Photoshop CS5 performance of two 3.33GHz Mac Pros, one with 6 cores, and one with 12 cores

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