The issue is related to the changes introduced in Kernel 3.10.0-957.21.3.el7 and 4.14.123-86.109.amzn1.x86_64. However, same issue may also occur in the newer kernels.
To work around this issue, run the command below on the Deep Security Manager server to increase TCP socket buffer to 16K for the incoming heartbeats:
/opt/dsm/dsm_c -action changesetting -name settings.configuration.heartbeatSocketBufferSize -value 16384
By default, the value for TCP socket buffer is 2048.
Increasing the value may cause extra memory consumed and the memory usage relies on the number of incoming heartbeats from Deep Security Agents. The value can be lowered if excessive memory consumption is observed, however, the issue may reoccur if the value is too low.
Fix is planned to be implemented in the future update release.