• Andrew Morton's avatar
    [PATCH] readahead · 8fa49846
    Andrew Morton authored
    I'd like to be able to claim amazing speedups, but
    the best benchmark I could find was diffing two
    256 megabyte files, which is about 10% quicker.  And
    that is probably due to the window size being effectively
    50% larger.
    
    Fact is, any disk worth owning nowadays has a segmented
    2-megabyte cache, and OS-level readahead mainly seems
    to save on CPU cycles rather than overall throughput.
    Once you start reading more streams than there are segments
    in the disk cache we start to win.
    
    Still.  The main motivation for this work is to
    clean the code up, and to create a central point at
    which many pages are marshalled together so that
    they can all be encapsulated into the smallest possible
    number of BIOs, and injected into the request layer.
    
    A number of filesystems were poking around inside the
    readahead state variables.  I'm not really sure what they
    were up to, but I took all that out.  The readahead
    code manages its own state autonomously and should not
    need any hints.
    
    - Unifies the current three readahead functions (mmap reads, read(2)
      and sys_readhead) into a single implementation.
    
    - More aggressive in building up the readahead windows.
    
    - More conservative in tearing them down.
    
    - Special start-of-file heuristics.
    
    - Preallocates the readahead pages, to avoid the (never demonstrated,
      but potentially catastrophic) scenario where allocation of readahead
      pages causes the allocator to perform VM writeout.
    
    - Gets all the readahead pages gathered together in
      one spot, so they can be marshalled into big BIOs.
    
    - reinstates the readahead ioctls, so hdparm(8) and blockdev(8)
      are working again.  The readahead settings are now per-request-queue,
      and the drivers never have to know about it.  I use blockdev(8).
      It works in units of 512 bytes.
    
    - Identifies readahead thrashing.
    
      Also attempts to handle it.  Certainly the changes here
      delay the onset of catastrophic readahead thrashing by
      quite a lot, and decrease it seriousness as we get more
      deeply into it, but it's still pretty bad.
    8fa49846
ll_rw_blk.c 44.8 KB