It's harmless. Jetsam is a system that monitors memory use in OSX and iOS. It keeps a list of processes on the device, to monitor for situations where a device is running out of free RAM and look for things to kill to free up RAM. It also watches for processes that are using too much RAM; breaching a "high water mark" level.
This error message is because the code is shared between OSX and iOS but the _DirtyJetsamMemoryLimit key is only available on iOS.
You can read more about Jetsam at the following link.
And if you are really interested you can read some of the Jetsam code in the following two files.
Long story short, I don't think this error message necessarily indicates a performance problem. The memory system on iOS is a lot more strict because iOS doesn't have things like swapfiles that allow the system to exceed the physical amount of RAM. OSX has a lot more room to play in.
Anyone can verify that it has no impact on performance by using Activity Monitor and showing all processes in the CPU view. Look at the
syslogd line and make sure the CPU time is reasonable compared to other tasks you wish to perform with your Mac.