Maximum number of instances: n+1?

MichaelJenkins

I'm seeking clarification on setting the maximum number of instances for a service in ArcGIS Server 11.5

Prior versions of Esri Wikis, arctitecture recommendations, etc have stated the maximum should be no more than n+1, where n is the number fo CPU cores.   So on an 8 core machine, the recommendation would be no more than 9 instances for a service.

Is that still the recommendation for current version of ArcGIS Server, 11.5 and up?  The current documentation provides no guidance or warnings about setting this too high.   For example, setting it to 14 on an 8 core server.  

Anticipate and accommodate users—ArcGIS Server | Documentation for ArcGIS Enterprise

I've had conflicting recommendations from Esri support, with one analyst recommending we set the max instances of a service to equal the max number of editors we expect (14 in that case on a 8 core server), and another (different support ticket) recommending the n+1 limit for a heavily used service.

JoshuaBixby

Part of the reason Esri has moved away from documenting single-service recommendations like the old "n+1" limit is that any such recommendation is so generic that it isn't really applicable, or at least isn't really helpful.  Unless your ArcGIS Server site only has a single service published, which I doubt very few do, how a single service is tuned really depends on all the services on the machine.  A rule like "n+1" may be a reasonable suggestion in a vacuum, but what if you have published many heavily-used services and tuned them all the same way?  It won't be pretty for any of the services when the machine is maxing out its CPU (and likely memory as well) because many heavily used services all have "n+1" max instances.

Tuning an ArcGIS Server site has so many factors to consider that it is hard for single, simple suggestion to be made.  Unless you are trying to maximize or optimize computing resources too eek every last penny out of some capital expense or monthly hosting bill, at the end of the day the most basic metric of whether a service is tuned well enough is whether you are getting complaints from users.  Start with the defaults Esri provides and see what feedback you get from users.