The patch titled
readahead: backward prefetching method
has been added to the -mm tree. Its filename is
readahead-backward-prefetching-method.patch
See http://www.zip.com.au/~akpm/linux/patches/stuff/added-to-mm.txt to find
out what to do about this
------------------------------------------------------
Subject: readahead: backward prefetching method
From: Wu Fengguang <wfg@xxxxxxxxxxxxxxxx>
Readahead policy for reading backward.
Signed-off-by: Wu Fengguang <wfg@xxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxx>
---
mm/readahead.c | 46 ++++++++++++++++++++++++++++++++++++++++++++++
1 files changed, 46 insertions(+)
diff -puN mm/readahead.c~readahead-backward-prefetching-method mm/readahead.c
--- a/mm/readahead.c~readahead-backward-prefetching-method
+++ a/mm/readahead.c
@@ -1441,6 +1441,52 @@ initial_readahead(struct address_space *
}
/*
+ * Backward prefetching.
+ *
+ * No look-ahead and thrashing safety guard: should be unnecessary.
+ *
+ * Important for certain scientific arenas(i.e. structural analysis).
+ */
+static int
+try_backward_prefetching(struct file_ra_state *ra, pgoff_t offset,
+ unsigned long size, unsigned long ra_max)
+{
+ pgoff_t prev = ra->prev_page;
+
+ /* Reading backward? */
+ if (offset >= prev)
+ return 0;
+
+ /* Close enough? */
+ size += readahead_hit_rate;
+ if (offset + 2 * size <= prev)
+ return 0;
+
+ if (ra_class_new(ra) == RA_CLASS_BACKWARD && ra_has_index(ra, prev)) {
+ prev = ra->la_index;
+ size += 2 * ra_readahead_size(ra);
+ } else
+ size *= 2;
+
+ if (size > ra_max)
+ size = ra_max;
+ if (size > prev)
+ size = prev;
+
+ /* The readahead-request covers the read-request? */
+ if (offset < prev - size)
+ return 0;
+
+ offset = prev - size;
+
+ ra_set_class(ra, RA_CLASS_BACKWARD);
+ ra_set_index(ra, offset, offset);
+ ra_set_size(ra, size, 0);
+
+ return 1;
+}
+
+/*
* ra_min is mainly determined by the size of cache memory. Reasonable?
*
* Table of concrete numbers for 4KB page size:
_
Patches currently in -mm which might be from wfg@xxxxxxxxxxxxxxxx are
readahead-kconfig-options.patch
radixtree-introduce-scan-hole-data-functions.patch
mm-introduce-probe_page.patch
mm-introduce-pg_readahead.patch
readahead-add-look-ahead-support-to-__do_page_cache_readahead.patch
readahead-insert-cond_resched-calls.patch
readahead-minmax_ra_pages.patch
readahead-events-accounting.patch
readahead-rescue_pages.patch
readahead-sysctl-parameters.patch
readahead-min-max-sizes.patch
readahead-state-based-method-aging-accounting.patch
readahead-state-based-method-routines.patch
readahead-state-based-method.patch
readahead-context-based-method.patch
readahead-initial-method-guiding-sizes.patch
readahead-initial-method-thrashing-guard-size.patch
readahead-initial-method-user-recommended-size.patch
readahead-initial-method.patch
readahead-backward-prefetching-method.patch
readahead-thrashing-recovery-method.patch
readahead-call-scheme.patch
readahead-laptop-mode.patch
readahead-loop-case.patch
readahead-nfsd-case.patch
readahead-turn-on-by-default.patch
readahead-remove-size-limit-on-read_ahead_kb.patch
readahead-remove-size-limit-of-max_sectors_kb-on-read_ahead_kb.patch
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