MPS Environment Variables

Created On: Jun 11, 2025 | Last Updated On: Jun 11, 2025

PyTorch Environment Variables

Variable	Description
PYTORCH_DEBUG_MPS_ALLOCATOR	If set to 1, set allocator logging level to verbose.
PYTORCH_MPS_LOG_PROFILE_INFO	Set log options bitmask to MPSProfiler. See LogOptions enum in aten/src/ATen/mps/MPSProfiler.h.
PYTORCH_MPS_TRACE_SIGNPOSTS	Set profile and signpost bitmasks to MPSProfiler. See ProfileOptions and SignpostTypes.
PYTORCH_MPS_HIGH_WATERMARK_RATIO	High watermark ratio for MPS allocator. Default is 1.7.
PYTORCH_MPS_LOW_WATERMARK_RATIO	Low watermark ratio for MPS allocator. Default is 1.4 (unified) or 1.0 (discrete).
PYTORCH_MPS_FAST_MATH	If 1, enables fast math for MPS kernels. See section 1.6.3 in the Metal Shading Language Spec.
PYTORCH_MPS_PREFER_METAL	If 1, uses metal kernels instead of MPS Graph APIs. Used for matmul.
PYTORCH_ENABLE_MPS_FALLBACK	If 1, falls back to CPU when MPS ops aren’t supported.

Note

high watermark ratio is a hard limit for the total allowed allocations

• 0.0 : disables high watermark limit (may cause system failure if system-wide OOM occurs)

• 1.0 : recommended maximum allocation size (i.e., device.recommendedMaxWorkingSetSize)

• >1.0: allows limits beyond the device.recommendedMaxWorkingSetSize

e.g., value 0.95 means we allocate up to 95% of recommended maximum allocation size; beyond that, the allocations would fail with OOM error.

low watermark ratio is a soft limit to attempt limiting memory allocations up to the lower watermark level by garbage collection or committing command buffers more frequently (a.k.a, adaptive commit). Value between 0 to m_high_watermark_ratio (setting 0.0 disables adaptive commit and garbage collection) e.g., value 0.9 means we ‘attempt’ to limit allocations up to 90% of recommended maximum allocation size.