• Andrew Morton's avatar
    [PATCH] use the congestion APIs in pdflush · c9b22619
    Andrew Morton authored
    The key concept here is that pdflush does not block on request queues
    any more.  Instead, it circulates across the queues, keeping any
    non-congested queues full of write data.  When all queues are full,
    pdflush takes a nap, to be woken when *any* queue exits write
    congestion.
    
    This code can keep sixty spindles saturated - we've never been able to
    do that before.
    
     - Add the `nonblocking' flag to struct writeback_control, and teach
       the writeback paths to honour it.
    
     - Add the `encountered_congestion' flag to struct writeback_control
       and teach the writeback paths to set it.
    
    So as soon as a mapping's backing_dev_info indicates that it is getting
    congested, bale out of writeback.  And don't even start writeback
    against filesystems whose queues are congested.
    
     - Convert pdflush's background_writeback() function to use
       nonblocking writeback.
    
    This way, a single pdflush thread will circulate around all the
    dirty queues, keeping them filled.
    
     - Convert the pdlfush `kupdate' function to do the same thing.
    
    This solves the problem of pdflush thread pool exhaustion.
    
    It solves the problem of pdflush startup latency.
    
    It solves the (minor) problem wherein `kupdate' writeback only writes
    back a single disk at a time (it was getting blocked on each queue in
    turn).
    
    It probably means that we only ever need a single pdflush thread.
    c9b22619
fs-writeback.c 15.8 KB