librav1e Behavior Under RAM Starvation

This article explores how the Rust-based AV1 encoder library, librav1e, behaves when it runs out of system memory (RAM) during resource-intensive encoding tasks. We examine the immediate impacts on encoding performance, the role of operating system swapping, and how Rust’s memory management handles allocation failures under extreme resource constraints.

Operating System Intervention and OOM Termination

When librav1e is subjected to a heavy encoding process—such as 4K video encoding with high lookahead settings—its memory footprint expands significantly. If the system completely runs out of physical RAM and has no swap space configured, the operating system’s Out-Of-Memory (OOM) killer will intervene.

Because librav1e is executing resource-heavy operations, the OS will likely flag the parent process (such as FFmpeg or the rav1e CLI) as the primary target for termination. The process will be abruptly terminated with a SIGKILL signal (or the Windows equivalent), resulting in an immediate crash without saving the progress of the current encode.

Rust Allocation Failures and Aborts

Because librav1e is written in Rust, it benefits from strict memory safety guarantees, but it is still subject to physical hardware limitations. When the library requests more memory than the system can allocate, the behavior depends on how the memory allocator is configured:

• Default Abort Behavior: By default, Rust’s standard library treats allocation failures as unrecoverable errors. If librav1e attempts to allocate memory for a new frame buffer or reference frame and the system refuses, the runtime will trigger a panic or an immediate abort.
• No Clean Exit: Unlike software that might gracefully pause or throw a caught exception to save configuration state, a RAM-starved Rust abort will instantly halt the execution pipeline.

Severe Performance Degradation (Disk Thrashing)

If the host operating system has swap space (virtual memory) configured on a hard drive or SSD, the system will not immediately crash. Instead, the OS will begin paging inactive memory blocks to the disk to free up physical RAM.

However, because video encoding requires continuous, high-frequency access to multiple reference frames, motion vectors, and lookahead buffers, these memory pages cannot remain inactive. The system will enter a state of severe disk thrashing—constantly moving data between physical RAM and the storage drive. During thrashing, librav1e’s encoding speed will plummet to near-zero frames per second (FPS), rendering the system highly unresponsive.

Key Factors Driving rav1e Memory Consumption

The severity of memory starvation when using librav1e is directly tied to several encoding parameters:

• Lookahead Limit: The lookahead_bits or lookahead frame count determines how many future frames the encoder analyzes to optimize rate control. High lookahead values exponentially increase RAM usage.
• Resolution and Bit Depth: 10-bit 4K video requires substantially larger frame buffers than 8-bit 1080p video.
• Threading and Tiles: Utilizing a high number of tile rows, tile columns, and worker threads forces librav1e to allocate concurrent memory buffers for parallel processing.

To prevent starvation issues, users running librav1e on low-memory systems should manually restrict the number of encoding threads, reduce the lookahead frame count, and ensure sufficient virtual memory is allocated.