EDM01-36v10 DAG_9.2X2_Card_User_Guide - Guidance for stream buffer sizing
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©2010 - 2012 Endace Technology Ltd. Confidential - Version 10 - May 2012
Latency
The latency is the time delay between:
•
the packet being written to the stream buffer and
•
the application reading the packet from the stream buffer.
The maximum latency is the time needed to empty an almost full stream buffer. To calculate the
maximum latency, use the following formula:
Where the multipliers:
•
1048576 converts MiB to bytes.
•
125000 converts Megabits per second to bytes per second.
Multiple streams
With multi-stream DAG cards, the incoming traffic can be spread across multiple streams for processing.
This allows you to take advantage of multi-core processors when processing large amounts of incoming
traffic.
This changes how you calculate the stream buffer size - you need to calculate the required stream buffer
size on a per stream basis. For example, in an 8 stream situation (assuming equal distribution of
incoming traffic over all streams) the effective incoming traffic rate to each stream is 1/8 of the original
rate. Use this value when calculating the values for the Network graph similar to the one above.
Note:
This works best when the incoming data is spread evenly across all streams. Where incoming traffic
cannot be spread evenly across stream you will need to adjust the stream buffer sizes for each stream.
Calculating the stream buffer size
Using the above two formulas you can calculate the Maximum Burst Duration and Maximum Latency for
any network and create a Network graph similar to the following. If plotted, you get a graph similar to
the following. See example 1 for the parameters for this graph.
Note:
We recommend that the stream buffers are a multiple of 4 MiB in size.
