H.265 - Codec / Performance

Wavefront Parallel Processing

The encoder may begin encoding a row as soon as the row above it is at least two CTUs ahead in the encode process.
This gives a 3-5x gain in parallelism for about 1% overhead in compression efficiency.
This feature is implicitly disabled when no thread pool is present.

Parallel mode decision

Parallel mode decision, or distributed mode analysis. When enabled the encoder will distribute the analysis work of each CU (merge, inter, intra) across multiple worker threads. Only recommended if x265 is not already saturating the CPU cores. In RD levels 3 and 4 it will be most effective if –rect is enabled. At RD levels 5 and 6 there is generally always enough work to distribute to warrant the overhead, assuming your CPUs are not already saturated.

Parallel mode decision will increase utilization without reducing compression efficiency. In fact, since the modes are all measured in parallel it makes certain early-outs impractical and thus you usually get slightly better compression when it is enabled (at the expense of not skipping improbable modes). This bypassing of early-outs can cause pmode to slow down encodes, especially at faster presets. This feature is implicitly disabled when no thread pool is present.

Parallel motion estimation

When enabled the encoder will distribute motion estimation across multiple worker threads when more than two references require motion searches for a given CU. Only recommended if x265 is not already saturating CPU cores. Parallel mode decision is much more effective than this option, since the amount of work it distributes is substantially higher. With Parallel motion estimation it is not unusual for the overhead of distributing the work to outweigh the parallelism benefits.

This feature is implicitly disabled when no thread pool is present.

Parallel motion estimation will increase utilization on many core systems with no effect on the output bitstream.