Autodesk Revit 2011 optimized file loading by using multiple CPU threads to transfer model data to RAM; maximizing the use of computational resources required to open a model. Since the slowest performing hardware in a computer is often static data storage, usually a hard drive, Revit employs an “in memory” data model, taking advantage of much faster RAM to manage the model in an editing session. Due to the constricted performance represented by hard drive access, it is recommended, that whenever possible, to prevent the underlying Windows operating system (OS) from caching active model data to the hard drive.
To understand how much RAM a given model will use in a typical editing session, check the size of the RVT file on disk. Because an RVT file on disk is highly compressed, loading the model into memory requires an expansion process. As a result, Revit uses roughly 6 times as much RAM as the size of the RVT file on disk. As users open views, add and change elements, additional expansion takes place, typically topping out at a memory use of approximately 20 times the RVT’s size on disk. These factors can vary with the complexity of the model, but often they provide good guidelines to understand how a Revit model will reside in RAM. Looking at a common workstation configuration with 8GB of RAM, we can calculate the approximate size of a model that can reside in memory:
8GB – 1GB (OS) – 1GB (video and other drivers) = 6GB / 20 = ~300 MB model on disk
Different modeling techniques can vary the division factor of 20 somewhat, but in general this formula provides a good guideline to understand when you’d expect a model to perform optimally through complete memory residency.
One exception to this rule occurs on Revit model upgrade, say from Revit 2011 to Revit 2012. Where a typical editing session will only require the expansion of some elements of the model, an upgrade requires the expansion of all model elements, resulting in higher memory use or increased swapping to hard drive by the OS. For this reason, the one-time upgrade operation requires more processing time than subsequent file opens.
Considering the inevitable trade-offs when specifying a new workstation, maximum performance can be achieved by including enough RAM to fully load a typical model into memory, enabling Revit to take the best advantage of the Windows platform.
Author: Anthony A. Hauck, Revit Senior Product Line Manager, Autodesk AEC Solutions Division