DRS has an advanced option 'MaxMemMBHeadroomPerHost' which can be used to configure the headroom that DRS needs to set aside to handle dynamic changes in ESXi resource usage.
You can reserve up to a maximum of 5% of host memory and capped at the DRS advanced option. By default DRS reserves 128 MB as headroom reservation in MB for each ESXi host in the cluster. This headroom option is used in vCenter Server, the host doesn't have any knowledge of this headroom. So if a VM or any host system process requires memory, then the memory from headroom can still be utilized.
Additionally growth in ESXi resource usage can eat into the headroom. DRS will do its best to again grow this headroom by updating how it divvies out resources and its VM placement. The resource pool status and the DRS faults indicate when DRS is unable to do so.
To set the DRS advanced option perform the following steps from the vSphere Webclient:
1. Right-click the DRS cluster and click Settings > vSphere DRS > Edit > Advanced Options > Add
2. In the Option column enter:
MaxMemMBHeadroomPerHost
(Note: Do not change the case of MaxMemMBHeadroomPerHost)
3. Click in the Value column to enter the desired headroom value in MB, then click OK for this setting to take effect.