Adaptive read-ahead V8

Changelog
=========

V8 2005-11-25

- balance zone aging only in page relaim paths and do it right
- do the aging of slabs in the same way as zones
- add debug code to dump the detailed page reclaim steps
- undo exposing of struct radix_tree_node and uninline related functions
- work better with nfsd
- generalize accelerated context based read-ahead
- account smooth read-ahead aging based on page referenced/activate bits
- avoid divide error in compute_thrashing_threshold()
- more low lantency efforts
- update some comments
- rebase debug actions on debugfs entries instead of magic readahead_ratio values

V7 2005-11-09

- new tunable parameters: readahead_hit_rate/readahead_live_chunk
- support sparse sequential accesses
- delay look-ahead if drive is spinned down in laptop mode
- disable look-ahead for loopback file
- make mandatory thrashing protection more simple and robust
- attempt to improve responsiveness on large read-ahead size

V6 2005-11-01

- cancel look-ahead in laptop mode
- increase read-ahead limit to 0xFFFF pages

V5 2005-10-28

- rewrite context based method to make it clean and robust
- improved accuracy of stateful thrashing threshold estimation
- make page aging equal to the number of code pages scanned
- sort out the thrashing protection logic
- enhanced debug/accounting facilities

V4 2005-10-15

- detect and save live chunks on page reclaim
- support database workload
- support reading backward
- radix tree lookup look-aside cache

V3 2005-10-06

- major code reorganization and documention
- stateful estimation of thrashing-threshold
- context method with accelerated grow up phase
- adaptive look-ahead
- early detection and rescue of pages in danger
- statitics data collection
- synchronized page aging between zones

V2 2005-09-15

- delayed page activation
- look-ahead: towards pipelined read-ahead

V1 2005-09-13

Initial release which features:
o stateless (for now)
o adapts to available memory / read speed
o free of thrashing (in theory)

And handles:
o large number of slow streams (FTP server)
o open/read/close access patterns (NFS server)
o multiple interleaved, sequential streams in one file (multithread / multimedia / database)

Overview
========

The current read-ahead logic uses an inflexible algorithm with 128KB VM_MAX_READAHEAD. Less memory leads to thrashing, more memory helps no throughput. The new logic is simply safer and faster. It makes sure every single read-ahead request is safe for the current load. Memory tight systems are expected to benefit a lot: no thrashing any more. It can also help boost I/O throughput for large memory systems, for VM_MAX_READAHEAD now defaults to 1MB. The value is no longer tightly coupled with the thrashing problem, and therefore constrainted by it.

Thanks,
Wu Fengguang
--
Dept. Automation University of Science and Technology of China