git: kernel - Improve cluster_read()

Matthew Dillon dillon at
Mon Feb 8 10:02:51 PST 2010

commit 6b84c93eb3b1e5815f00edf82f94fa12b79a9cc1
Author: Matthew Dillon <dillon at>
Date:   Mon Feb 8 09:51:05 2010 -0800

    kernel - Improve cluster_read()
    * The cluster_read() code was tripping over itself due to a findblk()
      call which caused it to believe it had found a buffer hole when it
      really found a busy buffer.
      Redo the code to use the FINDBLK_TEST flag to locate the next buffer
      hole.  Also add a shortcut to support efficient coding for larger
      read-ahead values.
    * Change the single-read-ahead in cluster_read() to a multiple-read-ahead
      based on the maxra parameter.  Before we just did a single read-ahead
      and even though this was a cluster read it still created a situation
      where the next cluster_read(0 operation would stall on previous read-ahead
      before issuing the next one.  In otherwords, it wasn't pipelining requests
      as well asit could.
      This change tries to keep at least two read-aheads in progress so when
      the next cluster_read() stalls on the first one the second one is still
      in the pipeline after it unstalls, allowing it to issue the third one
      to keep the pipeline hot.
    * These changes improve SSD swapcache operation as well as normal HD
      cluster_read() pipelining.  In addition the read-ahead is now sufficient
      to keep the pipeline hot across a 2 x Swap (interleaved) setup.

Summary of changes:
 sys/kern/vfs_cluster.c |   88 ++++++++++++++++++++++++++++++------------------
 1 files changed, 55 insertions(+), 33 deletions(-)

DragonFly BSD source repository

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