With the device-to-host copy gone, the remaining host-to-device copy is the
stage's cost, and from pageable memory it is staged through a bounce buffer at
roughly 2.6 GB/s instead of full PCIe speed. The host copies are made once, so
pinning them costs nothing per request.
It does cost the module's size in non-pageable host memory (10.4 GB for the
H3 video VAE, 0.6 GB for the audio VAE, per rank) for as long as the pipeline
is alive. It follows --pin-cpu-memory, which is on by default; --no-pin-cpu-
memory keeps the copies pageable and only loses the bandwidth. Note that this
flag did not already reach the VAE: its other use is FSDP's own CPUOffloadPolicy
and the VAE does not go through FSDP.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ATw7g6cxq9N2LtnMratsru
