patch-2_6_7-vs1_9_1_12

[linux-2.6.git] / mm / readahead.c

diff --git a/mm/readahead.c b/mm/readahead.c
index 71bf246..9723956 100644 (file)
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -15,7 +15,7 @@
 #include <linux/backing-dev.h>
 #include <linux/pagevec.h>
 
-void default_unplug_io_fn(struct backing_dev_info *bdi)
+void default_unplug_io_fn(struct backing_dev_info *bdi, struct page *page)
 {
 }
 EXPORT_SYMBOL(default_unplug_io_fn);
@@ -353,7 +353,7 @@ page_cache_readahead(struct address_space *mapping, struct file_ra_state *ra,
        unsigned orig_next_size;
        unsigned actual;
        int first_access=0;
-       unsigned long preoffset=0;
+       unsigned long average;
 
        /*
         * Here we detect the case where the application is performing
@@ -394,10 +394,17 @@ page_cache_readahead(struct address_space *mapping, struct file_ra_state *ra,
                if (ra->serial_cnt <= (max * 2))
                        ra->serial_cnt++;
        } else {
-               ra->average = (ra->average + ra->serial_cnt) / 2;
+               /*
+                * to avoid rounding errors, ensure that 'average'
+                * tends towards the value of ra->serial_cnt.
+                */
+               average = ra->average;
+               if (average < ra->serial_cnt) {
+                       average++;
+               }
+               ra->average = (average + ra->serial_cnt) / 2;
                ra->serial_cnt = 1;
        }
-       preoffset = ra->prev_page;
        ra->prev_page = offset;
 
        if (offset >= ra->start && offset <= (ra->start + ra->size)) {
@@ -457,18 +464,13 @@ do_io:
                 * ahead window and get some I/O underway for the new
                 * current window.
                 */
-               if (!first_access && preoffset >= ra->start &&
-                               preoffset < (ra->start + ra->size)) {
-                        /* Heuristic:  If 'n' pages were
-                         * 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.
+               if (!first_access) {
+                        /* Heuristic: there is a high probability
+                         * that around  ra->average number of
+                         * pages shall be accessed in the next
+                         * current window.
                          */
-                       ra->next_size = preoffset - ra->start + 2;
+                       ra->next_size = min(ra->average , (unsigned long)max);
                }
                ra->start = offset;
                ra->size = ra->next_size;
@@ -492,21 +494,19 @@ do_io:
                 */
                if (ra->ahead_start == 0) {
                        /*
-                        * if the average io-size is less than maximum
+                        * If the average io-size is more than maximum
                         * readahead size of the file the io pattern is
                         * sequential. Hence  bring in the readahead window
                         * immediately.
-                        * Else the i/o pattern is random. Bring
-                        * in the readahead window only if the last page of
-                        * the current window is accessed (lazy readahead).
+                        * If the average io-size is less than maximum
+                        * readahead size of the file the io pattern is
+                        * random. Hence don't bother to readahead.
                         */
-                       unsigned long average = ra->average;
-
+                       average = ra->average;
                        if (ra->serial_cnt > average)
-                               average = (ra->serial_cnt + ra->average) / 2;
+                               average = (ra->serial_cnt + ra->average + 1) / 2;
 
-                       if ((average >= max) || (offset == (ra->start +
-                                                       ra->size - 1))) {
+                       if (average > max) {
                                ra->ahead_start = ra->start + ra->size;
                                ra->ahead_size = ra->next_size;
                                actual = do_page_cache_readahead(mapping, filp,