celeborn/docs/developers/trafficcontrol.md

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Traffic Control

This article describes the detailed design of Celeborn Worker's traffic control.

Design Goal

The design goal of Traffic Control is to prevent Worker OOM without harming performance. At the same time, Celeborn tries to achieve fairness without harming performance.

Celeborn reaches the goal through Back Pressure and Congestion Control.

Data Flow

From the Worker's perspective, the income data flow comes from two sources:

• ShuffleClient that pushes primary data to the primary Worker
• Primary Worker that sends data replication to the replica Worker

The buffered memory can be released when the following conditions are satisfied:

• Data is flushed to file
• If replication is on, after primary data is written to wire

The basic idea is that, when Worker is under high memory pressure, slow down or stop income data, and at same time force flush to release memory.

Back Pressure

Back Pressure defines three watermarks:

• Pause Receive watermark (defaults to 0.85). If used direct memory ratio exceeds this, Worker will pause receiving data from ShuffleClient, and force flush buffered data into file.
• Pause Replicate watermark (defaults to 0.95). If used direct memory ratio exceeds this, Worker will pause receiving both data from ShuffleClient and replica data from primary Worker, and force flush buffered data into file.
• Resume watermark (defaults to 0.5). When either Pause Receive or Pause Replicate is triggered, to resume receiving data from ShuffleClient, the used direct memory ratio should decrease under this watermark.

Worker high-frequently checks used direct memory ratio, and triggers Pause Receive, Pause Replicate and Resume accordingly. The state machine is as follows:

Back Pressure is the basic traffic control and can't be disabled. Users can tune the three watermarks through the following configuration.

• celeborn.worker.directMemoryRatio*

Congestion Control

Congestion Control is an optional mechanism for traffic control, the purpose is to slow down the push rate from ShuffleClient when memory is under pressure, and suppress those who occupied the most resources in the last time window. It defines two watermarks:

• Low Watermark, under which everything goes OK
• High Watermark, when exceeds this, top users will be Congestion Controlled

Celeborn uses UserIdentifier to identify users. Worker collects bytes pushed from each user in the last time window. When used direct memory exceeds High Watermark, users who occupied more resources than the average occupation will receive Congestion Control message.

ShuffleClient controls the push ratio in a fashion that is very like TCP Congestion Control. Initially, it's in Slow Start phase, with a low push rate but increases very fast. When threshold is reached, it transfers to Congestion Avoidance phase, which slowly increases push rate. Upon receiving Congestion Control, it goes back to Slow Start phase.

Congestion Control can be enabled and tuned by the following configurations:

• celeborn.worker.congestionControl.*