On top of "[PATCH 2/2] readahead: improve sequential read detection".
Signed-off-by: Oleg Nesterov <[EMAIL PROTECTED]>
--- 2.6.11/mm/readahead.c~ Thu Mar 3 21:04:43 2005
+++ 2.6.11/mm/readahead.c Thu Mar 3 22:23:57 2005
@@ -192,18 +192,16 @@ out:
* size: Number of pages in that read
* Together, these form the "current window".
* Together, start and size represent the `readahead window'.
- * next_size: The number of pages to read on the next readahead miss.
- * Has the magical value -1UL if readahead has been disabled.
* prev_page: The page which the readahead algorithm most-recently inspected.
- * prev_page is mainly an optimisation: if page_cache_readahead
- * sees that it is again being called for a page which it just
- * looked at, it can return immediately without making any state
- * changes.
+ * It is mainly used to detect sequential file reading.
+ * If page_cache_readahead sees that it is again being called for
+ * a page which it just looked at, it can return immediately
without
+ * making any state changes.
* ahead_start,
* ahead_size: Together, these form the "ahead window".
* ra_pages: The externally controlled max readahead for this fd.
*
- * When readahead is in the off state (size == -1UL), readahead is disabled.
+ * When readahead is in the off state (size == 0), readahead is disabled.
* In this state, prev_page is used to detect the resumption of sequential I/O.
*
* The readahead code manages two windows - the "current" and the "ahead"
@@ -224,7 +222,7 @@ out:
* ahead window.
*
* A `readahead hit' occurs when a read request is made against a page which is
- * the next sequential page. Ahead windowe calculations are done only when it
+ * the next sequential page. Ahead window calculations are done only when it
* is time to submit a new IO. The code ramps up the size agressively at
first,
* but slow down as it approaches max_readhead.
*
@@ -235,12 +233,9 @@ out:
* read happens to be the first page of the file, it is assumed that a linear
* read is about to happen and the window is immediately set to the initial
size
* based on I/O request size and the max_readahead.
- *
- * A page request at (start + size) is not a miss at all - it's just a part of
- * sequential file reading.
*
* This function is to be called for every read request, rather than when
- * it is time to perform readahead. It is called only oce for the entire I/O
+ * it is time to perform readahead. It is called only once for the entire I/O
* regardless of size unless readahead is unable to start enough I/O to satisfy
* the request (I/O request > max_readahead).
*/
@@ -447,28 +442,28 @@ page_cache_readahead(struct address_spac
int sequential;
/*
- * Here we detect the case where the application is performing
- * sub-page sized reads. We avoid doing extra work and bogusly
- * perturbing the readahead window expansion logic.
+ * We avoid doing extra work and bogusly perturbing the readahead
+ * window expansion logic.
*/
if (offset == ra->prev_page && --req_size)
++offset;
+ /* Note that prev_page == -1 if it is a first read */
sequential = (offset == ra->prev_page + 1);
ra->prev_page = offset;
max = get_max_readahead(ra);
newsize = min(req_size, max);
- /* No readahead or file already in cache or sub-page sized read */
+ /* No readahead or sub-page sized read or file already in cache */
if (newsize == 0 || (ra->flags & RA_FLAG_INCACHE))
goto out;
ra->prev_page += newsize - 1;
/*
- * Special case - first read. We'll assume it's a whole-file read if
- * at start of file, and grow the window fast. Or detect first
+ * Special case - first read at start of file. We'll assume it's
+ * a whole-file read and grow the window fast. Or detect first
* sequential access
*/
if (sequential && ra->size == 0) {
-
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