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    Subject[PATCH 01/33] 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 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
    1 files changed, 57 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.
    --
    -
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