With the default paging policy for SSAS Tabular set to 1, paging is allowed. This means that if the memory required for the VertiPaq storage engine exceeds the configured VertiPaq memory limit, data can be paged to the Windows page file. Similarly, a setting of 2 allows utilizing a memory-mapped file. While these files may take advantage of operating system file cache, it is still less efficient than storing the data as nonpaged VertiPaq memory. Increasing the VertiPaq Memory Limit setting in your Analysis Server properties is recommended if this issue persists.
To determine a proper limit, detect the largest consumers of memory, then allow another 2 - 3x for processing overhead, etc. Be sure to consider the other SSAS memory limit settings, which apply to both Tabular and Multidimentional modes.
For more details, see the Microsoft technical article at : Performance Tuning of Tabular Models
Note this conditons supports SSAS 2014, other conditions are available for support of other versions of SSAS.
