{"record":{"id":"16166bab49a8fe69","repo":"vllm-project/vllm","slug":"data-parallel-size-exceeds-the-two-byte-engin","errorCode":null,"errorMessage":"data parallel size ({}) exceeds the two-byte engine identity limit","messagePattern":"data parallel size \\((.+?)\\) exceeds the two-byte engine identity limit","errorType":"validation","errorClass":null,"httpStatus":null,"severity":"error","filePath":"rust/src/server/src/config.rs","lineNumber":246,"sourceCode":"    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,\n                engine_count,\n                ..\n            } => {","sourceCodeStart":228,"sourceCodeEnd":264,"githubUrl":"https://github.com/vllm-project/vllm/blob/c794754062d49a8fdb63ab3c5215b488b865030c/rust/src/server/src/config.rs#L228-L264","documentation":"Thrown by Config::validate() when --data-parallel-size exceeds usize::from(u16::MAX) + 1, i.e. 65536. Engine identities in the Rust transport are packed into two bytes, so more than 65536 engines cannot be addressed. The offending value is included in the message.","triggerScenarios":"Passing --data-parallel-size 70000 or any value > 65536. Realistically reached only by scripting errors (e.g. passing a byte count, PID, or memory value instead of an engine count).","commonSituations":"Automation bugs where a large integer (GPU memory in MB, total shard count from another cluster) is fed into the DP size flag; copy-paste from a generated config.","solutions":["Pass the actual number of engine workers you intend to run (typically the number of GPUs).","Audit the script that produces the value — anything above 65536 is almost certainly the wrong variable.","If you genuinely need >65536 engines, that is not supported by the current two-byte engine identity scheme; file an issue instead of working around it."],"exampleFix":"# before\n--data-parallel-size 480000   # accidentally passed VRAM MB\n\n# after\n--data-parallel-size 8          # 8 engines / GPUs","handlingStrategy":"validation","validationCode":"const MAX_ENGINES: usize = usize::from(u16::MAX) + 1; // 65536\nassert!(dp <= MAX_ENGINES, \"too many engines\");","typeGuard":"fn is_valid_dp_size(dp: usize) -> bool {\n    (1..=usize::from(u16::MAX) + 1).contains(&dp)\n}","tryCatchPattern":null,"preventionTips":["Sanity-check DP size against physical GPU/engine counts before launch.","Name variables explicitly (engine_count, not 'size') to avoid unit confusion.","Log the final value right before exec to catch scripting bugs."],"tags":["configuration","data-parallel","limits","rust","vllm","startup"],"backgroundTag":null,"analyzedSha":"c794754062d49a8fdb63ab3c5215b488b865030c","analyzedAt":"2026-08-14T21:17:39.825Z","schemaVersion":2},"datasetVersion":"2026-08-15T17:31:12.345Z"}