What Happens When You Pipe a Build Process to less: Processor Impact

When piping a build process (e.g., make, gcc, or npm run build) to less, the processor handles the workload as follows:

1. CPU Utilization & Process Scheduling

  • The build process and less run concurrently, connected via a pipe (|).

  • The CPU splits time between:

    • The build process (generating output).

    • less (displaying/outputting the data).

  • If the build is CPU-intensive (e.g., compiling code), the system prioritizes it, while less mostly waits for input (unless scrolling/searching).

2. Buffering & I/O Impact

  • Stdout is line-buffered when printing to a terminal but fully buffered when piped.

    • Build tools (e.g., gcc, make) may buffer output in chunks, causing slight delays in display.

    • Output appears slower, but actual build speed remains unchanged.

  • If less reads slowly (e.g., manual scrolling), the build process may block temporarily when the pipe buffer fills.

3. Memory Usage

  • The pipe uses a fixed-size buffer (typically 64KB on Linux).

    • If less doesn’t read fast enough, the build process pauses until buffer space frees.

    • No significant extra memory is used unless less loads an extremely large log (rare for builds).

4. Multi-core Systems

  • On multi-core CPUs:

    • The build process and less can run on separate cores, minimizing interference.

    • Parallel builds (e.g., make -j4) still utilize multiple cores; less merges their output.

5. Performance Overhead

  • Piping to less adds negligible CPU overhead (only copying data between processes).

  • The main cost is latency: output appears in chunks due to buffering.

Key Takeaways

  • The processor runs the build at full speed, but output delivery may feel delayed.

  • For long builds, piping to less has almost no performance penalty.

  • For short builds, starting less may introduce minor overhead.

Example: Parallel Build with Paging

make -j8 2>&1 | less -R # Uses 8 cores, pipes output to less

  • The -j8 flag ensures parallel compilation across 8 cores.

  • less displays output as it arrives, with minimal CPU impact.