vllm-project/vllm · error

data parallel size must be at least 1

Error message

data parallel size must be at least 1

What it means

Thrown by Config::validate() when --data-parallel-size is 0. Data parallelism of zero engines is meaningless; the frontend requires at least one. This check runs after the max_logprobs check and before the two-byte engine identity limit check.

Source

Thrown at rust/src/server/src/config.rs:243

impl Config {
    /// Validate frontend configuration that can be checked before engine
    /// startup.
    pub fn validate(&self) -> Result<()> {
        vllm_chat::validate_parser_overrides(&self.tool_call_parser, &self.reasoning_parser)?;
        self.cors.validate()?;
        if let Some(tls) = &self.tls {
            tls.validate()?;
        }
        if let Some(max_logprobs) = self.max_logprobs
            && max_logprobs < -1
        {
            bail!(
                "max_logprobs must be non-negative or -1, got {}",
                max_logprobs
            );
        }
        if self.data_parallel_size == 0 {
            bail!("data parallel size must be at least 1");
        }
        if self.data_parallel_size > usize::from(u16::MAX) + 1 {
            bail!(
                "data parallel size ({}) exceeds the two-byte engine identity limit",
                self.data_parallel_size
            );
        }
        match &self.transport_mode {
            TransportMode::HandshakeOwner { engine_count, .. } => {
                if *engine_count != self.data_parallel_size {
                    bail!(
                        "managed frontend engine count ({engine_count}) must equal data parallel size ({})",
                        self.data_parallel_size
                    );
                }
            }
            TransportMode::Bootstrapped {
                engine_start_index,

View on GitHub (pinned to c794754062)

Solutions

1. Set --data-parallel-size to at least 1 (single engine, no data parallelism).
2. If computing from hardware, use max(1, computed_value) in your launcher.
3. Leave the flag at its default rather than passing 0.

Example fix

# before
--data-parallel-size 0

# after
--data-parallel-size 1

let dp = args.data_parallel_size;
assert!(dp >= 1, "data parallel size must be >= 1");

fn is_valid_dp_size(dp: usize) -> bool {
    dp >= 1
}

When it happens

Trigger: Passing --data-parallel-size 0 explicitly, or a launcher/CI script computing the value (e.g. dividing GPU count by a batch factor that evaluates to 0).

Common situations: Templated deployment scripts deriving DP size from an unset variable that defaults to 0; misreading DP size as a ratio (0 = 'auto') instead of an engine count.

Related errors

• max_logprobs must be non-negative or -1, got {}
• data parallel size ({}) exceeds the two-byte engine identity
• managed frontend engine count ({engine_count}) must equal da
• engine count must be at least 1
• engine start index does not fit usize

AI-assisted analysis of vllm-project/vllm@c794754062 (2026-08-14). Data as JSON: /api/errors/9d212c22120de5f9. Report an issue: GitHub.