The `multi_accept` directive in NGINX controls how worker processes accept new incoming connections. Its settings, "on" or "off," have different performance implications for handling client connections at the server level. Below is a detailed explanation of the implications of setting `multi_accept` to "on" or "off," covering its behavior, performance effects, and practical considerations.
Definition and Default Behavior
The `multi_accept` directive is configured in the `events` context of the NGINX configuration file. It determines whether a worker process accepts one new connection at a time (`multi_accept off`) or accepts all new connections available in the listen queue at once (`multi_accept on`). The default setting of this directive is `off`.When `multi_accept` is set to `off`, a worker process will accept new connections one by one as they come. When it is set to `on`, a worker will accept all available new connections at once when it gets notified about new connection events. This notification happens based on the underlying event processing mechanism used by NGINX, which varies by operating system but generally involves epoll, kqueue, or similar scalable event notification systems.
Implications of multi_accept set to OFF
- Single Connection Acceptance: The worker accepts just one connection per notification event. This approach simplifies the processing because each accepted connection can be processed in order without additional load spikes.- CPU Load Distribution: Since workers accept connections one at a time, the CPU load tends to spread more evenly because workers are not overwhelmed by multiple connection acceptances at once.
- Reduced Risk of Thundering Herd: This setting tends to avoid the "thundering herd" problem, where multiple workers wake up simultaneously but only one handles the connection, causing CPU cycles to be wasted on context switching.
- Latency Control: Accepting one connection at a time can slightly increase the latency under heavy load since the listen queue is drained more slowly.
- Potential Throughput Limitation: Under very high connection rates, accepting connections one by one can lead to lower throughput because the worker may not process incoming connections as quickly as they arrive.
- Resource Efficiency: This setting tends to use system resources more conservatively, which is beneficial for servers that do not experience high concurrent connection loads.
Implications of multi_accept set to ON
- Batch Connection Acceptance: The worker accepts all incoming connections waiting in the listen queue immediately upon notification. This can drastically increase the number of connections the worker processes per cycle.- Higher Throughput: This setting can improve throughput under heavy load conditions where many connections arrive in short bursts, allowing the worker to handle more connections quickly.
- Risk of Overloading Workers: Accepting all queued connections at once can cause a worker to become overloaded if the incoming connections exceed the worker's processing capacity, potentially leading to performance degradation.
- Increased CPU Wakes and Load Spikes: Workers may experience spikes in CPU usage because all connections are accepted at once, which can lead to bursty CPU consumption patterns.
- Potential for Dropped Connections or Delays: If the number of connections accepted exceeds the maximum concurrent connections the worker can efficiently handle (based on `worker_connections` or system limits), some connections may be delayed or dropped.
- Useful in Consistent High Load Environments: When a server consistently handles a high volume of simultaneous connections, this setting helps to reduce the overhead of accepting connections individually and enhances response speed.
Interaction with Other Settings
- accept_mutex: This directive, often enabled by default, controls how worker processes take turns accepting connections to avoid the "thundering herd" problem. With `accept_mutex` on, workers accept connections one at a time in turn, which complements a `multi_accept off` setting for more orderly connection handling.- When `accept_mutex` is turned off, all workers wake on new connections but only one handles them, potentially causing inefficient CPU usage, especially if `multi_accept` is also off.
- worker_connections and worker_processes: These determine how many concurrent connections each worker and the server overall can handle, affecting the impact of `multi_accept`. If `multi_accept` is on and connection inflow exceeds the combined capacities, it might overload workers.
- Event Polling Mechanism: The multi_accept directive is ignored if kqueue is used on some systems because kqueue reports the exact number of new connections, allowing better control without batching.
Performance Considerations
- Enabling `multi_accept` (`on`) is beneficial where the server experiences a constant stream of numerous incoming connections, as it enables a worker to accept and process many connections quickly.- Disabling it (`off`) is better for environments with less connection churn, where it prevents workers from being overwhelmed and reduces wasted CPU cycles.
- Using `multi_accept` with high connection rates without adequate tuning of `worker_connections` and CPU resources may lead to resource exhaustion and increased latency.
- Workloads that involve blocking operations for each connection, such as SSL handshakes, can benefit from multiple workers combined with appropriate `multi_accept` settings to avoid bottlenecks.
Use Cases and Recommendations
- High Traffic APIs or Web Servers: Turning `multi_accept` on allows better utilization of server resources by accepting all pending connections as episodes of activity arise, reducing latency in high concurrency scenarios.- Low to Moderate Traffic or Development Environments: Keeping `multi_accept` off helps maintain stable CPU usage and simpler load handling.
- Tuning for Optimal Performance: It is important to consider application context, worker count, system limits, and the expected connection volume. Combining `multi_accept on` with optimized configurations for worker processes and connection handling leads to improved throughput.
- Hybrid Approaches: Some administrators might experiment with different combinations of `multi_accept` and `accept_mutex` to achieve the desired balance between CPU efficiency and connection handling speed.
Summary
In summary, setting `multi_accept` to "on" enables a worker to accept all pending connections at once, improving throughput in high load scenarios but risking worker overload and CPU spikes. Leaving it "off" restricts workers to accepting one connection at a time, leading to steadier resource use and potentially higher latency under heavy connection loads. The choice depends on traffic patterns, workload characteristics, and system tuning to maximize server efficiency and responsiveness.This explanation covers the technical behavior of `multi_accept`, its performance impact, interaction with other NGINX worker settings, and practical usage scenarios. The directive is a critical part of fine-tuning NGINX server performance for different environments and traffic conditions.