Megatron Sequence Parallel requires Tensor Parallel size > 1

Quick answer

Megatron Sequence Parallel requires Tensor Parallel size > 1 means Sequence parallelism was enabled while tensor parallel size remained 1. Megatron partitions sequence work across the tensor-parallel group, so there is no group to partition across in this configuration. Preserve the first preceding error, then run the targeted control below.

Symptom: Megatron Sequence Parallel requires Tensor Parallel size > 1
Root cause: Sequence parallelism was enabled while tensor parallel size remained 1. Megatron partitions sequence work across the tensor-parallel group, so there is no group to partition across in this configuration. The decisive evidence is the first log line that precedes "Megatron Sequence Parallel requires Tensor Parallel size > 1" and differs from a healthy run.
Recommended fix: either set tensor-model-parallel-size to at least 2 or disable sequence parallelism.
How Denpex helps: Denpex investigates Megatron Sequence Parallel requires Tensor Parallel size > 1 using the evidence you provide or your connected workload collects. Earlier rank, host or application evidence is needed to distinguish an initiating failure from a downstream report.

Distributed Training#megatron#sequence#parallel#assertion#required

What this failure is

The literal signature is "Megatron Sequence Parallel requires Tensor Parallel size > 1". It is a distributed training failure associated with Megatron Core tensor and pipeline parallelism. The line identifies the failing operation or subsystem, while the surrounding evidence decides whether it is the initiating fault or a downstream symptom.

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Why it happens (the mechanism)

Sequence parallelism was enabled while tensor parallel size remained 1. Megatron partitions sequence work across the tensor-parallel group, so there is no group to partition across in this configuration. The failure becomes visible at this call site because the operation first requires the missing resource, valid state, healthy peer, or correct result. Earlier log lines and a known-good control carry more causal value than the final wrapper exception.

What you'll observe

The workload stops or loses forward progress after emitting "Megatron Sequence Parallel requires Tensor Parallel size > 1".
A retry on the same configuration reproduces the failure because the causal state has not changed.
The outer framework exception can hide the rank, node, allocation, or dependency that failed first.
Increasing timeouts or reducing workload size can suppress the symptom without correcting the cause.

Common symptoms and what they mean

Symptom	Why it happens
Megatron Sequence Parallel requires Tensor Parallel size > 1	Sequence parallelism was enabled while tensor parallel size remained 1. Megatron partitions sequence work across the tensor-parallel group, so there is no group to partition across in this configuration.
The same operation fails at a consistent stage of Megatron Core tensor and pipeline parallelism.	The decisive evidence is the first log line that precedes "Megatron Sequence Parallel requires Tensor Parallel size > 1" and differs from a healthy run.
The first related warning appears before the final exception and names the causal subsystem.	A nearby failure remains a competing hypothesis until a control separates configuration, capacity, transport, and hardware causes.
A known-good control changes one variable and either reproduces or clears the failure.	Sequence parallelism was enabled while tensor parallel size remained 1. Megatron partitions sequence work across the tensor-parallel group, so there is no group to partition across in this configuration.

Which systems are affected

Megatron Core tensor and pipeline parallelism
production-shaped multi-accelerator workloads
containerized and bare-metal deployments of the same stack

How to confirm this is the problem

Use this checklist to test the hypothesis against a small reproduction. No single line proves the root cause, so preserve the preceding events and compare one variable at a time.

✓Find the first occurrence of "Megatron Sequence Parallel requires Tensor Parallel size > 1" and preserve at least 100 lines before it.
✓Identify which rank, node, device, or process emitted the first related warning.
✓print the resolved Megatron arguments on every rank. Launcher defaults or a stale checkpoint configuration can restore sequence_parallel after a command-line change.
✓Repeat the same input after the targeted change and require the signature to disappear.
✓Resume from job start after correcting parallelism settings only after the control passes.

Root cause

Sequence parallelism was enabled while tensor parallel size remained 1. Megatron partitions sequence work across the tensor-parallel group, so there is no group to partition across in this configuration.
The decisive evidence is the first log line that precedes "Megatron Sequence Parallel requires Tensor Parallel size > 1" and differs from a healthy run.
A nearby failure remains a competing hypothesis until a control separates configuration, capacity, transport, and hardware causes.

The fix and how to prevent it

Searchable error signature

search key

Megatron Sequence Parallel requires Tensor Parallel size > 1

Use this text as a lookup key in logs and upstream issue trackers. It is not presented as a captured customer log. Confirm the cause from your own preceding events, versions, configuration and the cited references.

The fix and the prevention pattern

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Why the recommended fix works

either set tensor-model-parallel-size to at least 2 or disable sequence parallelism. This changes the condition in the causal diagnosis instead of hiding the outer exception. The repeated control proves ownership before recovery from job start after correcting parallelism settings.

Code examples

snippet

# Preserve evidence before restarting
rg -n -i 'error|exception|timeout|failed' <log-file>
nvidia-smi
python -m torch.utils.collect_env

# Find the exact signature in the complete log
rg -n -F -- "Megatron Sequence Parallel requires Tensor Parallel size > 1" <log-file>

Adapt the snippet to your framework. The same pattern holds for PyTorch Lightning, Hugging Face Trainer, DeepSpeed, Megatron-LM, and vLLM training wrappers. Where the wrapper exposes a config flag (for examplelr_scheduler_type in Trainer), prefer the flag over the imperative API to keep the schedule declarative and reproducible.

Best practices by model family

Model / Stack	Recommendation	Notes
First response	Preserve the first failure	Keep the context before "Megatron Sequence Parallel requires Tensor Parallel size > 1" so aggregation does not erase causality.
Confirmation	Change one variable	Use a known-good node, rank, input, or configuration as the control.
Recovery	Resume from job start after correcting parallelism settings	Resume only after the literal signature no longer appears in the same control.

With the fix vs without the fix

Dimension	With the fix	Without the fix
Evidence	First preceding error and one controlled comparison	Only the final aggregated exception
Fix	either set tensor-model-parallel-size to at least 2 or disable sequence parallelism.	Retrying the unchanged workload
Exit criterion	"Megatron Sequence Parallel requires Tensor Parallel size > 1" is absent in the repeated control	The job happened to run once

Diagnostic note

“Treat "Megatron Sequence Parallel requires Tensor Parallel size > 1" as a search key and an investigation checkpoint, not as proof of every cause associated with the phrase. The high-value evidence is what changed immediately before it and whether the failure follows the workload, node, or configuration.”

Visual fingerprint

Decision path for Megatron Sequence Parallel requires Tensor Parallel size > 1

literal error captured
        |
        v
find first preceding failure
        |
        v
run one known-good control
        |
        +-- follows workload --> inspect input or configuration
        +-- follows node ------> inspect hardware or platform
        +-- disappears --------> validate the targeted fix

The control separates workload, configuration, and node ownership before recovery from job start after correcting parallelism settings.

Diagnose this failure in VS Code

Select the traceback or open the failed terminal, then run Denpex locally to see the initiating rank, collateral failures, exact fix, and verification command without uploading the log.

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Related failures to investigate next

Frequently asked questions

Questions engineers and on-call staff commonly ask about this failure.

What does "Megatron Sequence Parallel requires Tensor Parallel size > 1" mean?

Is this line always the root cause?

No. It can be the direct failure or the point where an earlier failure becomes visible. The first preceding error and a controlled comparison decide which.

What should I collect before restarting?

Collect complete log context, the emitting rank or node, component versions, resolved configuration, and the diagnostic output shown above.

What is the fastest confirmation?

print the resolved Megatron arguments on every rank. Launcher defaults or a stale checkpoint configuration can restore sequence_parallel after a command-line change.

How do I prevent it from recurring?

validate sequence_parallel implies tensor_model_parallel_size > 1 before process-group initialization, so the job fails before allocating model memory.

References

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