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Subject[PATCH 01/32] readahead: kconfig options
This patchset introduces a set of adaptive read-ahead methods.
They enable the kernel to better support many important I/O applications.

MAIN FEATURES
=============
- Adaptive read-ahead buffer management
- aggressive, thrashing safe read-ahead size
- optimal memory utilisation while achieving good I/O throughput
- unnecessary to hand tuning VM_MAX_READAHEAD
- support slow/fast readers at the same time
- support large number of concurrent readers
- aggressive read-ahead on start-of-file
- configurable recommended read-ahead size
- safeguarded by dynamic estimated thrashing threshold
- safeguarded by dynamic estimated expected read size
- good for lots-of-small-files case
- shrinkable look-ahead size
- cut down up to 40% memory consumption on overloaded situation

- Detecting any form of (semi-)sequencial scan
- parallel / interleaved sequential scans on one fd
- sequential reads across file open/close lifetime
- mixed sequential / random accesses
- sparse / skimming sequential read

- Support more access patterns
- backward prefetching
- seeking around reading N pages

- Better special case handling
- nfs daemon support: the raparams cache is no longer required
- laptop mode support: defer look-ahead on drive spinned down
- loopback file support: avoid double look-ahead


DESIGN STRATEGIES
=================
- Dual methods design
- stateful method: the fast and default one
- stateless method: the robust and failsafe one
- if anything abnormal happens, the stateful method bails out, the
stateless method queries the page cache and possibly restart the
read-ahead process

- Robust feedback design
- sense and handle important states so that the logic wont run away
- detect danger of thrashing and prevent it in advance
- extensive accounting and debugging traces


This patch:

Add kconfig options to enable/disable:
- adaptive read-ahead logic
- adaptive read-ahead debug traces and events accounting

The read-ahead introduction text is cited from the well written LWN article
"Adaptive file readahead" <http://lwn.net/Articles/155510/> :)

Signed-off-by: Wu Fengguang <wfg@mail.ustc.edu.cn>
---
mm/Kconfig | 57 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
mm/readahead.c | 2 ++
2 files changed, 59 insertions(+)
--- linux-2.6.17-rc4-mm3.orig/mm/Kconfig
+++ linux-2.6.17-rc4-mm3/mm/Kconfig
@@ -146,3 +146,60 @@ config MIGRATION
while the virtual addresses are not changed. This is useful for
example on NUMA systems to put pages nearer to the processors accessing
the page.
+
+#
+# Adaptive file readahead
+#
+config ADAPTIVE_READAHEAD
+ bool "Adaptive file readahead (EXPERIMENTAL)"
+ default n
+ depends on EXPERIMENTAL
+ help
+ Readahead is a technique employed by the kernel in an attempt
+ to improve file reading performance. If the kernel has reason
+ to believe that a particular file is being read sequentially,
+ it will attempt to read blocks from the file into memory before
+ the application requests them. When readahead works, it speeds
+ up the system's throughput, since the reading application does
+ not have to wait for its requests. When readahead fails, instead,
+ it generates useless I/O and occupies memory pages which are
+ needed for some other purpose.
+
+ The kernel already has a stock readahead logic that is well
+ understood and well tuned. This option enables a more complex and
+ feature rich one. It tries to be smart and memory efficient.
+ However, due to the great diversity of real world applications, it
+ might not fit everyone.
+
+ Please refer to Documentation/sysctl/vm.txt for tunable parameters.
+
+ It is known to work well for many desktops, file servers and
+ postgresql databases. Say Y to try it out for yourself.
+
+config DEBUG_READAHEAD
+ bool "Readahead debug and accounting"
+ default y
+ depends on ADAPTIVE_READAHEAD
+ select DEBUG_FS
+ help
+ This option injects extra code to dump detailed debug traces and do
+ readahead events accounting.
+
+ To actually get the data:
+
+ mkdir /debug
+ mount -t debug none /debug
+
+ After that you can do the following:
+
+ echo > /debug/readahead/events # reset the counters
+ cat /debug/readahead/events # check the counters
+
+ echo 1 > /debug/readahead/debug_level # start events accounting
+ echo 0 > /debug/readahead/debug_level # pause events accounting
+
+ echo 2 > /debug/readahead/debug_level # show printk traces
+ echo 3 > /debug/readahead/debug_level # show printk traces(verbose)
+ echo 1 > /debug/readahead/debug_level # stop filling my kern.log
+
+ Say N for production servers.
--- linux-2.6.17-rc4-mm3.orig/mm/readahead.c
+++ linux-2.6.17-rc4-mm3/mm/readahead.c
@@ -5,6 +5,8 @@
*
* 09Apr2002 akpm@zip.com.au
* Initial version.
+ * 26May2006 Wu Fengguang <wfg@mail.ustc.edu.cn>
+ * Adaptive read-ahead framework.
*/

#include <linux/kernel.h>
--
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