Classify the 4 cases into 2 classes, and assign proper ra_min for them.
Also update comments correspondly.
Signed-off-by: Fengguang Wu <[EMAIL PROTECTED]>
---
mm/readahead.c | 49 +++++++++++++++++++++++++++++++----------------
1 file changed, 33 insertions(+), 16 deletions(-)
--- linux-2.6.20-rc3-mm1.orig/mm/readahead.c
+++ linux-2.6.20-rc3-mm1/mm/readahead.c
@@ -1285,17 +1285,26 @@ static int
try_context_based_readahead(struct address_space *mapping,
struct file_ra_state *ra,
struct page *page, pgoff_t offset,
- unsigned long ra_min, unsigned long ra_max)
+ unsigned long req_size, unsigned long ra_max)
{
pgoff_t start;
+ unsigned long ra_min;
unsigned long ra_size;
unsigned long la_size;
- /* Check if there is a segment of history pages, and its end index.
+ /*
+ * Check if there is a segment of history pages, and its end index.
* Based on which we decide whether and where to start read-ahead.
- *
- * Case 1: we have a current page.
- * Search forward for a nearby hole.
+ */
+
+ /*
+ * Select a reasonable large initial size for sequential reads.
+ */
+ ra_min = min(req_size * 4, mapping->backing_dev_info->ra_pages0);
+
+ /*
+ * Case s1: we have a current page.
+ * =======> Search forward for a nearby hole.
*/
read_lock_irq(&mapping->tree_lock);
if (page) {
@@ -1308,8 +1317,9 @@ try_context_based_readahead(struct addre
return -1;
}
- /* Case 2: current page is missing; previous page is present.
- * Just do read-ahead from the current index on.
+ /*
+ * Case s2: current page is missing; previous page is present.
+ * =======> Just do read-ahead from the current index on.
* There's clear sign of sequential reading. It can be
* a) seek => read => this read
* b) cache hit read(s) => this read
@@ -1321,26 +1331,33 @@ try_context_based_readahead(struct addre
goto has_history_pages;
}
- /* Case 2x: the same context info as 2.
- * It can be the early stage of semi-sequential reads(interleaved/nfsd),
- * or an ugly random one. So be conservative.
+ /*
+ * Not an obvious sequential read:
+ * select a conservative initial size, plus user prefered agressiveness.
+ */
+ ra_min = min(req_size, MIN_RA_PAGES) +
+ readahead_hit_rate * 8192 / PAGE_CACHE_SIZE;
+
+ /*
+ * Case r1: the same context info as s2, but not that obvious.
+ * =======> The same action as s2, but be conservative.
+ * It can be the early stage of intermixed sequential reads,
+ * or an ugly random one.
*/
if (readahead_hit_rate && __probe_page(mapping, offset - 1)) {
start = offset;
- if (ra_min > 2 * readahead_hit_rate)
- ra_min = 2 * readahead_hit_rate;
goto has_history_pages;
}
- /* Case 3: no current/previous pages;
- * sparse read-ahead is enabled: ok, be aggressive.
- * Check if there's any adjecent history pages.
+ /*
+ * Case r2: no current/previous pages; sparse read-ahead is enabled.
+ * =======> Do sparse read-ahead if there are adjecent history pages.
*/
if (readahead_hit_rate > 1) {
start = radix_tree_scan_data_backward(&mapping->page_tree,
offset, ra_min);
if (start != ULONG_MAX && offset - start < ra_min) {
- ra_min += offset - start;
+ ra_min *= 2;
offset = ++start; /* pretend the request starts here */
goto has_history_pages;
}
--
-
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