Bdbuf improvements

Mentors: Sebastian Huber

Students:

Status: Uninitiated.

Introduction: The Bdbuf is the block device buffer management in RTEMS. It suffers from a number of inefficiencies that should be addressed and fixed.

Goal: To improve the Bdbuf implementation in terms of its replacement strategy, disk api, driver api, and block prefetching.

Requirements: Advanced C programming. Algorithmic evaluation and development.

Replacement Strategy

Bdbuf uses currently the LRU replacement strategy. This was easy to implement but has several severe drawbacks. Due to patent issues we should use LIRS instead of ARC. Steps to move from LRU to LIRS:

# Define a set of benchmarks # Add test cases for LIRS # Implement LIRS # Run the benchmark with Bdbuf based on LRU and LIRS (optionally also ARC)

Further information:

• ​Cache algorithms
• ​Page replacement algorithms
• ​Advanced page replacement
• ​IO performance
• ​ARC
• ​LIRS
• ​Clock PRO

Disk API

The access by device identifier should be replaced with the disk device structure. This avoids the look up overhead and ensures that the disks have a proper usage count.

Driver API

The driver API should be changed to use the buffer chains directly. Currently a table is allocated on the stack to hold the request structure.

Prefetch Strategy

Implement a proper block prefetch strategy. The current read ahead approach is virtually useless.