Symptoms
Performance degradation of the Broadcom 57414 NIC or Qlogic QL41262HxCU can occur due to an issue with the AMD "iommu" driver in RHEL 7.4AMD systems have 8 IOMMUs. Each IOMMU has a dedicated queue to handle packets. If the operating system has to flush a particular queue, all queues end up getting flushed (whether they all need to be flushed or not). The flushing of all queues takes time and reduces IO throughput.
Cause
Hardware is working fine, issue is industry wide and is not specific to Dell EMC.
Resolution
Solution
Passing the following kernel parameter provides a workaround
"iommu=pt"
Issue will be resolved in Red Hat® Enterprise Linux® 7.5. AMD has published Linux Network Tuning Guide for AMD EPYC based Servers - http://support.amd.com/TechDocs/56224.pdf
