[PATCH] revert lazy readahead

From: Trond Myklebust <trond.myklebust@fys.uio.no>

The following reversion is what fixes my regression.  That puts the
sequential read numbers back to the 2.6.0 values of ~140MB/sec (from the
current 2.6.1 values of 14MB/second)...

We were triggering I/O of the `ahead' when we hit the last page in the
`current' window.  That's bad because it gives no pipelining at all.

So go back to full pipelining.

It's not at all clear why this change made a 10x difference in NFS
throughput.

diff --git a/mm/readahead.c b/mm/readahead.c
index 46a6ff8e1f65c2eacb88900e3b7227cf24958014..042376caeec6267d53dae3788323f694119afd57 100644 (file)
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -449,12 +449,8 @@ do_io:
                          * accessed in the current window, there
                          * is a high probability that around 'n' pages
                          * shall be used in the next current window.
-                         *
-                         * To minimize lazy-readahead triggered
-                         * in the next current window, read in
-                         * an extra page.
                          */
-                       ra->next_size = preoffset - ra->start + 2;
+                       ra->next_size = preoffset - ra->start + 1;
                }
                ra->start = offset;
                ra->size = ra->next_size;
@@ -474,13 +470,9 @@ do_io:
                /*
                 * This read request is within the current window.  It is time
                 * to submit I/O for the ahead window while the application is
-                * about to step into the ahead window.
-                * Heuristic: Defer reading the ahead window till we hit
-                * the last page in the current window. (lazy readahead)
-                * If we read in earlier we run the risk of wasting
-                * the ahead window.
+                * crunching through the current window.
                 */
-               if (ra->ahead_start == 0 && offset == (ra->start + ra->size -1)) {
+               if (ra->ahead_start == 0) {
                        ra->ahead_start = ra->start + ra->size;
                        ra->ahead_size = ra->next_size;
                        actual = do_page_cache_readahead(mapping, filp,