6.4.2 Bottleneck analysis at RN-I bridges

The CCN-502 provides events that observe the locations where the nodes or bridges are full, which can cause delays in the rest of the system.

This enables you to monitor the current bottlenecks in the system, and checks multiple events in the RN-Is, HN-Fs, and memory controllers. In the RN-I bridges, the events monitor the following:

Request retry rate at RN-I bridges

TXREQFLITV_RETRIED monitors the efficiency of using dynamic credits in the system.

It does this by measuring the request retry rate:

TXREQFLITV_RETRIEDNumber of retried TXREQ flits dispatched. This event is a measure of the retry rate.

Calculate the request retry rate as follows:

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Read and write delays at RN-I bridges

To monitor the delays for both reads and writes, the CCN-502 enables you to monitor how full the read and write trackers are in the RN-I bridges.

When one of the trackers is full, the bridge cannot accept new requests from the AXI master. This delays the I/O devices that connect to the AXI master.

You can use the measure of how full the trackers are, together with the read and write bandwidth from the RN-I bridge to the interconnect, to help isolate the source of bottlenecks in the system. For example:

  • If the read tracker of a specific RN-I bridge is full but the effective read bandwidth from the bridge is not close to the maximum expected, the interconnect cannot keep up with the read traffic from the specific device.
  • If the bandwidth is close to maximum, the I/O device can send requests to the maximum of its port bandwidth and this is why the tracker is full.

You can also use the measure of how full the trackers are with AXI PMUs to monitor delays to the AXI masters.

The following events monitor the read and write trackers:

RRTFullAll entries in the read request tracker, excluding those reserved for Hi-QPC, are occupied. This is a measure of oversubscription in the read request tracker.
WRTFullAll entries in the write request tracker, excluding those reserved for Hi-QPC, are occupied. This is a measure of oversubscription in the write request tracker.
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