Rollup fields were introduced with Microsoft Dynamics CRM 2015. A Rollup field contains aggregated value calculated from child records for a parent record. For example: Active Products for a Company or Open Opportunities for an Account.
To understand how Rollup calculation works and how frequent this calculation happens, we need to understand what happens when you create a Rollup field for an entity.
Select Actions menu in top navigation and Click on “Resume option”. This option will initiate the job right away instead of waiting for 12 hours.
In case you want to calculate Rollup field immediately whenever child record gets created, updated or deleted, you can write custom C# plugin or custom workflow activity and use SDK message “CalculateRollupFieldRequest”. For more information refer blog.
You should have write access on parent entity where rollup field is created. You do not need to have write access on child entity. For ex: In case of Open Opportunities for an Account rollup field, you need write permissions on account entity only.