David Marchand

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Debugging vHost user TX contention in Open vSwitch

Debugging vHost user TX contention in Open vSwitch

It isn’t always easy to understand how Open vSwitch (OVS) cycles are spent, especially because various parameters and configuration options can affect how OVS behaves. Members of the Open vSwitch community are actively working to understand what causes packets drops in Open vSwitch. Efforts so far have included adding a custom statistic for vHost TX retries, tracking vHost TX contention, and adding a coverage counter to count vHost IRQs. We are particularly interested in the user space datapath that uses the Data Plane Development Kit (DPDK) for fast I/O.

Adding these statistics is an ongoing effort, and we won’t cover all of the corners. In some cases, the statistics leave doubts about what is causing a behavior.

In this article, I will introduce a new counter we’ve added to learn more about contention in the vHost transmission path. I’ll also show you how to use the new counter with perf, and I’ll discuss what’s next for our ongoing efforts.

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