Keysight Technologies U4301 User Manual page 178

11 Viewing Offline Performance Summary
As displayed in the above graph, the UpdateFC_Cpl graph line keeps showing the credit fluctuation
regulating against the corresponding Memory Completion packets coming from root that occurred in
the trace. The drop in credit level shows how many packets are left to be transmitted and helps in
regulating the flow of data packets. When all packets are transmitted and received, the FC level
changes to zero counter value. It remains constant till any further credits are added into account.
Once the FC level is incremented with some value, the same mechanism is repeated till all the
packets are transmitted and received and FC level again comes to a zero value.
Example-3
In the start of the DMA buffer transfer, the link credits are quickly exhausted and reaches to zero
counter value because the buffer transfer runs at a higher rate in comparison to the rest of the data
transfer. The quick drop in credit level is caused by the chain of link buffering. In the start, the data
write or completion capacity remains higher than the sink capacity but lower than the link capacity.
The data transfer at higher rate only happens while the link buffering capacity can sustain it.
178 Keysight U4301 PCIe Gen3 Analyzer User's Guide