[PATCH 9/9] readahead: basic support for backwards prefetching
From: Wu Fengguang
Date: Sat Feb 11 2012 - 04:52:53 EST
Add the backwards prefetching feature. It's pretty simple if we don't
support async prefetching and interleaved reads.
tail and tac are observed to have the reverse read pattern:
tail-3501 [006] 111.881191: readahead: readahead-random(bdi=0:16, ino=1548450, req=750+1, ra=750+1-0, async=0) = 1
tail-3501 [006] 111.881506: readahead: readahead-backwards(bdi=0:16, ino=1548450, req=748+2, ra=746+5-0, async=0) = 4
tail-3501 [006] 111.882021: readahead: readahead-backwards(bdi=0:16, ino=1548450, req=744+2, ra=726+25-0, async=0) = 20
tail-3501 [006] 111.883713: readahead: readahead-backwards(bdi=0:16, ino=1548450, req=724+2, ra=626+125-0, async=0) = 100
tac-3528 [001] 118.671924: readahead: readahead-random(bdi=0:16, ino=1548445, req=750+1, ra=750+1-0, async=0) = 1
tac-3528 [001] 118.672371: readahead: readahead-backwards(bdi=0:16, ino=1548445, req=748+2, ra=746+5-0, async=0) = 4
tac-3528 [001] 118.673039: readahead: readahead-backwards(bdi=0:16, ino=1548445, req=744+2, ra=726+25-0, async=0) = 20
Here is the behavior with an 8-page read sequence from 10000 down to 0.
(The readahead size is a bit large since it's an NFS mount.)
readahead-random(dev=0:16, ino=3948605, req=10000+8, ra=10000+8-0, async=0) = 8
readahead-backwards(dev=0:16, ino=3948605, req=9992+8, ra=9968+32-0, async=0) = 32
readahead-backwards(dev=0:16, ino=3948605, req=9960+8, ra=9840+128-0, async=0) = 128
readahead-backwards(dev=0:16, ino=3948605, req=9832+8, ra=9584+256-0, async=0) = 256
readahead-backwards(dev=0:16, ino=3948605, req=9576+8, ra=9072+512-0, async=0) = 512
readahead-backwards(dev=0:16, ino=3948605, req=9064+8, ra=8048+1024-0, async=0) = 1024
readahead-backwards(dev=0:16, ino=3948605, req=8040+8, ra=6128+1920-0, async=0) = 1920
readahead-backwards(dev=0:16, ino=3948605, req=6120+8, ra=4208+1920-0, async=0) = 1920
readahead-backwards(dev=0:16, ino=3948605, req=4200+8, ra=2288+1920-0, async=0) = 1920
readahead-backwards(dev=0:16, ino=3948605, req=2280+8, ra=368+1920-0, async=0) = 1920
readahead-backwards(dev=0:16, ino=3948605, req=360+8, ra=0+368-0, async=0) = 368
And a simple 1-page read sequence from 10000 down to 0.
readahead-random(dev=0:16, ino=3948605, req=10000+1, ra=10000+1-0, async=0) = 1
readahead-backwards(dev=0:16, ino=3948605, req=9999+1, ra=9996+4-0, async=0) = 4
readahead-backwards(dev=0:16, ino=3948605, req=9995+1, ra=9980+16-0, async=0) = 16
readahead-backwards(dev=0:16, ino=3948605, req=9979+1, ra=9916+64-0, async=0) = 64
readahead-backwards(dev=0:16, ino=3948605, req=9915+1, ra=9660+256-0, async=0) = 256
readahead-backwards(dev=0:16, ino=3948605, req=9659+1, ra=9148+512-0, async=0) = 512
readahead-backwards(dev=0:16, ino=3948605, req=9147+1, ra=8124+1024-0, async=0) = 1024
readahead-backwards(dev=0:16, ino=3948605, req=8123+1, ra=6204+1920-0, async=0) = 1920
readahead-backwards(dev=0:16, ino=3948605, req=6203+1, ra=4284+1920-0, async=0) = 1920
readahead-backwards(dev=0:16, ino=3948605, req=4283+1, ra=2364+1920-0, async=0) = 1920
readahead-backwards(dev=0:16, ino=3948605, req=2363+1, ra=444+1920-0, async=0) = 1920
readahead-backwards(dev=0:16, ino=3948605, req=443+1, ra=0+444-0, async=0) = 444
CC: Andi Kleen <andi@xxxxxxxxxxxxxx>
CC: Li Shaohua <shaohua.li@xxxxxxxxx>
Acked-by: Jan Kara <jack@xxxxxxx>
Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
---
include/linux/fs.h | 2 ++
include/trace/events/vfs.h | 1 +
mm/readahead.c | 20 ++++++++++++++++++++
3 files changed, 23 insertions(+)
--- linux-next.orig/include/linux/fs.h 2012-02-11 12:31:05.000000000 +0800
+++ linux-next/include/linux/fs.h 2012-02-11 12:31:11.000000000 +0800
@@ -976,6 +976,7 @@ struct file_ra_state {
* streams.
* RA_PATTERN_MMAP_AROUND read-around on mmap page faults
* (w/o any sequential/random hints)
+ * RA_PATTERN_BACKWARDS reverse reading detected
* RA_PATTERN_FADVISE triggered by POSIX_FADV_WILLNEED or FMODE_RANDOM
* RA_PATTERN_OVERSIZE a random read larger than max readahead size,
* do max readahead to break down the read size
@@ -986,6 +987,7 @@ enum readahead_pattern {
RA_PATTERN_SUBSEQUENT,
RA_PATTERN_CONTEXT,
RA_PATTERN_MMAP_AROUND,
+ RA_PATTERN_BACKWARDS,
RA_PATTERN_FADVISE,
RA_PATTERN_OVERSIZE,
RA_PATTERN_RANDOM,
--- linux-next.orig/mm/readahead.c 2012-02-11 12:31:09.000000000 +0800
+++ linux-next/mm/readahead.c 2012-02-11 12:31:11.000000000 +0800
@@ -719,6 +719,26 @@ ondemand_readahead(struct address_space
}
/*
+ * backwards reading
+ */
+ if (offset < ra->start && offset + req_size >= ra->start) {
+ ra->pattern = RA_PATTERN_BACKWARDS;
+ ra->size = get_next_ra_size(ra, max);
+ if (ra->size > ra->start) {
+ /*
+ * ra->start may be concurrently set to some huge
+ * value, the min() at least avoids submitting huge IO
+ * in this race condition
+ */
+ ra->size = min(ra->start, max);
+ ra->start = 0;
+ } else
+ ra->start -= ra->size;
+ ra->async_size = 0;
+ goto readit;
+ }
+
+ /*
* Query the page cache and look for the traces(cached history pages)
* that a sequential stream would leave behind.
*/
--- linux-next.orig/include/trace/events/vfs.h 2012-02-11 12:30:59.000000000 +0800
+++ linux-next/include/trace/events/vfs.h 2012-02-11 12:31:11.000000000 +0800
@@ -14,6 +14,7 @@
{ RA_PATTERN_SUBSEQUENT, "subsequent" }, \
{ RA_PATTERN_CONTEXT, "context" }, \
{ RA_PATTERN_MMAP_AROUND, "around" }, \
+ { RA_PATTERN_BACKWARDS, "backwards" }, \
{ RA_PATTERN_FADVISE, "fadvise" }, \
{ RA_PATTERN_OVERSIZE, "oversize" }, \
{ RA_PATTERN_RANDOM, "random" }, \
--
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