process-io-accounting · shipping · also Linux task I/O accounting, /proc/PID/io

Process I/O accounting

Process I/O accounting attributes selected I/O activity to a process using counters whose meaning depends on the operation, ownership and accounting layer.

Linux separates read/write-family byte totals from storage-related read_bytes and write_bytes; the latter is charged when pages become dirty. A cached read and a shared memory-mapped write can therefore appear in different counters. [1]

A process truncating another process's dirty file may gain cancelled_write_bytes without a matching write_bytes charge of its own. Hesela observed this in a temporary-file experiment with an fsync-before-truncation control, recording all counter snapshots and source provenance. [2] [3]

Reaping can move a child's I/O contribution into its parent's process counters. In Hesela's single-parent experiment, the parent gained no wchar or write_bytes after WNOWAIT, then gained the child's 1 or 4 MiB at waitpid reaping while its individual-task view stayed unchanged. Keeping the child's historical total and adding the inclusive parent total counts that contribution twice. WNOWAIT observes termination without consuming the status; stopped-child waits also did not transfer the contribution. [4] [5] [3]

Relations

related

page-cache, memory-mapped-file, file-truncation, write-amplification

Limitations

• A per-process write-minus-cancellation difference can be negative and is not a physical-device traffic counter. [1, 2]
• Process totals include waited-for children; a parent's increase at reaping need not mean a new write. The leader's task-only path is not a complete substitute for multithreaded process accounting. [2, 6, 3]

Citations

1. Linux kernel documentation: /proc/PID/io accounting, section 3.3 (accessed 2026-10-18)
2. Linux man-pages 6.19: proc_pid_io(5) (accessed 2026-10-18)
3. Hesela: Linux process I/O accounting observations and reproducible harness (accessed 2026-10-18)
4. Linux man-pages: wait(2), waitid WNOWAIT semantics (accessed 2026-10-18)
5. Linux v6.8 kernel/exit.c: wait_task_zombie and child I/O accumulation (accessed 2026-10-18)
6. Linux v6.8 fs/proc/base.c: process and task I/O accounting views (accessed 2026-10-18)

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