{"record":{"id":"9d212c22120de5f9","repo":"vllm-project/vllm","slug":"data-parallel-size-must-be-at-least-1","errorCode":null,"errorMessage":"data parallel size must be at least 1","messagePattern":"data parallel size must be at least 1","errorType":"validation","errorClass":null,"httpStatus":null,"severity":"error","filePath":"rust/src/server/src/config.rs","lineNumber":243,"sourceCode":"impl Config {\n    /// Validate frontend configuration that can be checked before engine\n    /// startup.\n    pub fn validate(&self) -> Result<()> {\n        vllm_chat::validate_parser_overrides(&self.tool_call_parser, &self.reasoning_parser)?;\n        self.cors.validate()?;\n        if let Some(tls) = &self.tls {\n            tls.validate()?;\n        }\n        if let Some(max_logprobs) = self.max_logprobs\n            && max_logprobs < -1\n        {\n            bail!(\n                \"max_logprobs must be non-negative or -1, got {}\",\n                max_logprobs\n            );\n        }\n        if self.data_parallel_size == 0 {\n            bail!(\"data parallel size must be at least 1\");\n        }\n        if self.data_parallel_size > usize::from(u16::MAX) + 1 {\n            bail!(\n                \"data parallel size ({}) exceeds the two-byte engine identity limit\",\n                self.data_parallel_size\n            );\n        }\n        match &self.transport_mode {\n            TransportMode::HandshakeOwner { engine_count, .. } => {\n                if *engine_count != self.data_parallel_size {\n                    bail!(\n                        \"managed frontend engine count ({engine_count}) must equal data parallel size ({})\",\n                        self.data_parallel_size\n                    );\n                }\n            }\n            TransportMode::Bootstrapped {\n                engine_start_index,","sourceCodeStart":225,"sourceCodeEnd":261,"githubUrl":"https://github.com/vllm-project/vllm/blob/c794754062d49a8fdb63ab3c5215b488b865030c/rust/src/server/src/config.rs#L225-L261","documentation":"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.","triggerScenarios":"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).","commonSituations":"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.","solutions":["Set --data-parallel-size to at least 1 (single engine, no data parallelism).","If computing from hardware, use max(1, computed_value) in your launcher.","Leave the flag at its default rather than passing 0."],"exampleFix":"# before\n--data-parallel-size 0\n\n# after\n--data-parallel-size 1","handlingStrategy":"validation","validationCode":"let dp = args.data_parallel_size;\nassert!(dp >= 1, \"data parallel size must be >= 1\");","typeGuard":"fn is_valid_dp_size(dp: usize) -> bool {\n    dp >= 1\n}","tryCatchPattern":null,"preventionTips":["Clamp computed values: dp = dp.max(1).","Treat 0 as 'auto' nowhere — it is invalid, not a sentinel.","Assert launcher-computed values before exec."],"tags":["configuration","data-parallel","validation","rust","vllm","startup"],"backgroundTag":null,"analyzedSha":"c794754062d49a8fdb63ab3c5215b488b865030c","analyzedAt":"2026-08-14T21:17:39.825Z","schemaVersion":2},"datasetVersion":"2026-08-15T22:17:37.221Z"}