Module merge 

IO Merging

This module provides functionality to merge multiple sequential IO requests into a single larger request.

Overview

Merging IO requests can significantly improve performance by:

• Reducing the number of system calls (io_uring_enter or io_submit).
• Allowing larger sequential transfers which are often more efficient for storage devices.
• Reducing per-request overhead in the driver and completion handling.

Mechanism

The core component is MergeSubmitter, which buffers incoming IOEvents.

• Buffering: Events are added to MergeBuffer. They are merged if they are:

  • Sequential (contiguous offsets).
  • Same IO action (Read/Write).
  • Same file descriptor.
  • Total size does not exceed merge_size_limit.

• Flushing: When the buffer is full, the limit is reached, or flush() is called, the merged request is submitted.

• Sub-tasks:

  • If events are merged, a new “master” IOEvent is created covering the entire range.
  • The original events are attached as sub_tasks (a linked list) to this master event.
  • Write: The data from individual buffers is copied into a single large aligned buffer.
  • Read: A large buffer is allocated for the master event. Upon completion, data is copied back to the individual event buffers.
  • Completion: When the master event completes, it iterates over sub-tasks, sets their results (copying data for reads), and triggers their individual callbacks.

Components

• MergeBuffer: Internal buffer logic.
• MergeSubmitter: Wraps a sender channel and manages the merge logic before sending.

Structs

MergeBuffer

Buffers sequential IO events for merging.

MergeSubmitter

Manages the submission of IO events, attempting to merge sequential events before sending them to the IO driver.