[gvfs] Fix readahead behaviour

[Alexander Larsson <alexl src gnome org>]

commit 4efd634afdb564b890e75404aefea5198e3b6487
Author: Alexander Larsson <alexl redhat com>
Date:   Fri Apr 5 14:06:37 2013 +0200

    Fix readahead behaviour
    
    We were constantly adding extra readahead operations that were not really
    needed. A single readahead is necessary to get the read operations
    pipelined (see comment). Also, avoid readahead for the first read
    to handle random access i/o better.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=697289

 daemon/gvfsreadchannel.c |   17 +++++++++++++++--
 1 files changed, 15 insertions(+), 2 deletions(-)
---
diff --git a/daemon/gvfsreadchannel.c b/daemon/gvfsreadchannel.c
index 125708d..092f728 100644
--- a/daemon/gvfsreadchannel.c
+++ b/daemon/gvfsreadchannel.c
@@ -209,7 +209,20 @@ read_channel_readahead (GVfsChannel  *channel,
       read_job = G_VFS_JOB_READ (job);
       read_channel = G_VFS_READ_CHANNEL (channel);
 
-      if (read_job->data_count != 0)
+      /* If the last operation was a read and it succeeded then we
+        might want to start a readahead. We don't do this for the
+        first read op as we're not sure we're streaming larger
+        parts of the file yet. However, after the second read we
+        queue a readahead read. After this the reading side will
+        constantly be one read operation behind, such that by
+        the time the second read operation is done and a third
+        read() is done we will send a read request but start
+        reading the readahead data, and after that is done
+        send a new request but start reading the result of the
+        previous read request. This way the reading will be
+        fully pipelined. */
+      if (read_job->data_count != 0 &&
+         read_channel->read_count == 2)
        {
          read_channel->read_count++;
          readahead_job = g_vfs_job_read_new (read_channel,
@@ -218,7 +231,7 @@ read_channel_readahead (GVfsChannel  *channel,
                                              g_vfs_channel_get_backend (channel));
        }
     }
-  
+
   return readahead_job;
 }