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Subject[PATCH 12/16] readahead: events accounting
A debugfs file named `readahead/events' is created according to advices from
J?rn Engel, Andrew Morton and Ingo Oeser. It yields to much better
readability than the preious /proc/vmstat interface :)

It reveals various read-ahead activities/events, and is vital to the testing.

-------------------------------------------------------------------------
If you are experiencing performance problems, or want to help improve the
read-ahead logic, please send me the debug data. Thanks.

- Preparations

## Compile with CONFIG_DEBUG_FS('Kernel hacking ---> Debug Filesystem')
mkdir /debug
mount -t debug none /debug

- For each session with distinct access pattern

echo > /debug/readahead/events # reset the counters
# echo > /var/log/kern.log # you may want to backup it first
# echo 5 > /debug/readahead/debug_level # show verbose printk traces
## do one benchmark/task
# echo 0 > /debug/readahead/debug_level # revert to normal value
cp /debug/readahead/events readahead-events-`date +'%F_%R'`
# bzip2 -c /var/log/kern.log > kern.log-`date +'%F_%R'`.bz2

The commented out commands can uncover the very detailed file accesses,
which are useful sometimes. But the log file will be rather huge.

-------------------------------------------------------------------------
This is a trimmed down output on my PC:
# cat /debug/readahead/events
[table requests] total newfile state context none
cache_miss 403 56 12 69 263
read_random 260 37 5 17 201
io_congestion 0 0 0 0 0
io_cache_hit 85 0 24 46 0
io_block 9796 5613 822 143 3203
readahead 5956 5418 383 139 0
lookahead 961 650 212 98 0
lookahead_hit 449 181 164 58 41
lookahead_ignore 0 0 0 0 0
readahead_eof 4981 4768 171 28 0
readahead_shrink 0 0 0 0 0
readahead_thrash 0 0 0 0 0
readahead_mutilt 0 0 0 0 0
readahead_rescue 45 0 0 0 45

[table pages] total newfile state context none
cache_miss 5590 72 2506 181 2826
read_random 265 37 5 17 206
io_congestion 0 0 0 0 0
io_cache_hit 2440 0 1054 1366 0
io_block 165848 11117 147794 3668 3203
readahead 43080 11360 28949 2621 0
readahead_hit 38251 10716 25669 1818 9
lookahead 24013 1718 21641 647 0
lookahead_hit 20161 770 18679 712 0
lookahead_ignore 0 0 0 0 0
readahead_eof 15961 7924 7440 461 0
readahead_shrink 0 0 0 0 0
readahead_thrash 0 0 0 0 0
readahead_mutilt 0 0 0 0 0
readahead_rescue 240 0 0 0 240

[table summary] total newfile state context none
random_rate 4% 0% 1% 10% 99%
ra_hit_rate 88% 94% 88% 69% 900%
la_hit_rate 46% 27% 76% 58% 4100%
avg_ra_size 7 2 75 19 0
avg_la_size 25 3 102 7 0

Signed-off-by: Wu Fengguang <wfg@mail.ustc.edu.cn>
---

mm/readahead.c | 202 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
1 files changed, 201 insertions(+), 1 deletion(-)

--- linux.orig/mm/readahead.c
+++ linux/mm/readahead.c
@@ -112,6 +112,9 @@ enum ra_event {
u32 readahead_debug_level = 0;
u32 debug_disable_stateful_method = 0;

+static struct dentry *readahead_events_dentry;
+extern struct file_operations ra_debug_fops;
+
static int __init readahead_init(void)
{
struct dentry *root;
@@ -122,6 +125,9 @@ static int __init readahead_init(void)
debugfs_create_bool("disable_stateful_method", 0644, root,
&debug_disable_stateful_method);

+ readahead_events_dentry = debugfs_create_file("events",
+ 0644, root, NULL, &ra_debug_fops);
+
return 0;
}

@@ -139,6 +145,195 @@ static inline int disable_stateful_metho
return 0;
}

+#endif
+
+/*
+ * Read-ahead events accounting.
+ */
+#ifdef DEBUG_READAHEAD
+
+static char *ra_class_name[] = {
+ "total",
+ "newfile",
+ "state",
+ "context",
+ "contexta",
+ "around",
+ "backward",
+ "onthrash",
+ "onraseek",
+ "none",
+};
+
+static char *ra_event_name[] = {
+ "cache_miss",
+ "read_random",
+ "io_congestion",
+ "io_cache_hit",
+ "io_block",
+ "readahead",
+ "readahead_hit",
+ "lookahead",
+ "lookahead_hit",
+ "lookahead_ignore",
+ "readahead_mmap",
+ "readahead_eof",
+ "readahead_shrink",
+ "readahead_thrash",
+ "readahead_mutilt",
+ "readahead_rescue",
+};
+
+static unsigned long ra_event_count[RA_CLASS_END+1][RA_EVENT_END+1][2];
+
+static inline void ra_account(struct file_ra_state *ra,
+ enum ra_event e, int pages)
+{
+ enum ra_class c;
+
+ if (e == RA_EVENT_READAHEAD_HIT && pages < 0) {
+ c = (ra->flags >> RA_CLASS_SHIFT) & RA_CLASS_MASK;
+ pages = -pages;
+ } else if (ra)
+ c = ra->flags & RA_CLASS_MASK;
+ else
+ c = RA_CLASS_END;
+
+ if (!c)
+ c = RA_CLASS_END;
+
+ ra_event_count[c][e][0] += 1;
+ ra_event_count[c][e][1] += pages;
+
+ if (e == RA_EVENT_READAHEAD)
+ ra_event_count[c][RA_EVENT_END][1] += pages * pages;
+}
+
+static int ra_account_show(struct seq_file *s, void *_)
+{
+ int i;
+ int c;
+ int e;
+ static char event_fmt[] = "%-16s";
+ static char class_fmt[] = "%10s";
+ static char item_fmt[] = "%10lu";
+ static char percent_format[] = "%9lu%%";
+ static char *table_name[] = {
+ "[table requests]",
+ "[table pages]",
+ "[table summary]"};
+
+ for (i = 0; i <= 1; i++) {
+ for (e = 0; e <= RA_EVENT_END; e++) {
+ ra_event_count[0][e][i] = 0;
+ for (c = 1; c <= RA_CLASS_END; c++)
+ ra_event_count[0][e][i] +=
+ ra_event_count[c][e][i];
+ }
+
+ seq_printf(s, event_fmt, table_name[i]);
+ for (c = 0; c <= RA_CLASS_END; c++)
+ seq_printf(s, class_fmt, ra_class_name[c]);
+ seq_puts(s, "\n");
+
+ for (e = 0; e < RA_EVENT_END; e++) {
+ if (e == RA_EVENT_READAHEAD_HIT && i == 0)
+ continue;
+
+ seq_printf(s, event_fmt, ra_event_name[e]);
+ for (c = 0; c <= RA_CLASS_END; c++)
+ seq_printf(s, item_fmt,
+ ra_event_count[c][e][i]);
+ seq_puts(s, "\n");
+ }
+ seq_puts(s, "\n");
+ }
+
+ seq_printf(s, event_fmt, table_name[2]);
+ for (c = 0; c <= RA_CLASS_END; c++)
+ seq_printf(s, class_fmt, ra_class_name[c]);
+ seq_puts(s, "\n");
+
+ seq_printf(s, event_fmt, "random_rate");
+ for (c = 0; c <= RA_CLASS_END; c++)
+ seq_printf(s, percent_format,
+ (ra_event_count[c][RA_EVENT_READRANDOM][0] * 100) /
+ (ra_event_count[c][RA_EVENT_READRANDOM][0] +
+ ra_event_count[c][RA_EVENT_READAHEAD][0] + 1));
+ seq_puts(s, "\n");
+
+ seq_printf(s, event_fmt, "ra_hit_rate");
+ for (c = 0; c <= RA_CLASS_END; c++)
+ seq_printf(s, percent_format,
+ (ra_event_count[c][RA_EVENT_READAHEAD_HIT][1] * 100) /
+ (ra_event_count[c][RA_EVENT_READAHEAD][1] | 1));
+ seq_puts(s, "\n");
+
+ seq_printf(s, event_fmt, "la_hit_rate");
+ for (c = 0; c <= RA_CLASS_END; c++)
+ seq_printf(s, percent_format,
+ (ra_event_count[c][RA_EVENT_LOOKAHEAD_HIT][0] * 100) /
+ (ra_event_count[c][RA_EVENT_LOOKAHEAD][0] | 1));
+ seq_puts(s, "\n");
+
+ seq_printf(s, event_fmt, "var_ra_size");
+ for (c = 0; c <= RA_CLASS_END; c++)
+ seq_printf(s, item_fmt,
+ (ra_event_count[c][RA_EVENT_END][1] -
+ ra_event_count[c][RA_EVENT_READAHEAD][1] *
+ (ra_event_count[c][RA_EVENT_READAHEAD][1] /
+ (ra_event_count[c][RA_EVENT_READAHEAD][0] | 1))) /
+ (ra_event_count[c][RA_EVENT_READAHEAD][0] | 1));
+ seq_puts(s, "\n");
+
+ seq_printf(s, event_fmt, "avg_ra_size");
+ for (c = 0; c <= RA_CLASS_END; c++)
+ seq_printf(s, item_fmt,
+ (ra_event_count[c][RA_EVENT_READAHEAD][1] +
+ ra_event_count[c][RA_EVENT_READAHEAD][0] / 2) /
+ (ra_event_count[c][RA_EVENT_READAHEAD][0] | 1));
+ seq_puts(s, "\n");
+
+ seq_printf(s, event_fmt, "avg_la_size");
+ for (c = 0; c <= RA_CLASS_END; c++)
+ seq_printf(s, item_fmt,
+ (ra_event_count[c][RA_EVENT_LOOKAHEAD][1] +
+ ra_event_count[c][RA_EVENT_LOOKAHEAD][0] / 2) /
+ (ra_event_count[c][RA_EVENT_LOOKAHEAD][0] | 1));
+ seq_puts(s, "\n");
+
+ return 0;
+}
+
+static int ra_debug_open(struct inode *inode, struct file *file)
+{
+ return single_open(file, ra_account_show, NULL);
+}
+
+static ssize_t ra_debug_write(struct file *file, const char __user *buf,
+ size_t size, loff_t *offset)
+{
+ if (file->f_dentry == readahead_events_dentry)
+ memset(ra_event_count, 0, sizeof(ra_event_count));
+ return 1;
+}
+
+static struct file_operations ra_debug_fops = {
+ .owner = THIS_MODULE,
+ .open = ra_debug_open,
+ .write = ra_debug_write,
+ .read = seq_read,
+ .llseek = seq_lseek,
+ .release = single_release,
+};
+
+#else /* !DEBUG_READAHEAD */
+
+static inline void ra_account(struct file_ra_state *ra,
+ enum ra_event e, int pages)
+{
+}
+
#endif /* DEBUG_READAHEAD */


@@ -1033,6 +1228,8 @@ static int ra_dispatch(struct file_ra_st
ra_account(ra, RA_EVENT_READAHEAD_EOF, actual);
if (la_size)
ra_account(ra, RA_EVENT_LOOKAHEAD, la_size);
+ if (ra_size > actual)
+ ra_account(ra, RA_EVENT_IO_CACHE_HIT, ra_size - actual);
ra_account(ra, RA_EVENT_READAHEAD, actual);

dprintk("readahead-%s(ino=%lu, index=%lu, ra=%lu+%lu-%lu) = %d\n",
@@ -1728,8 +1925,11 @@ page_cache_readahead_adaptive(struct add
if (page) {
if(!TestClearPageReadahead(page))
return 0;
- if (bdi_read_congested(mapping->backing_dev_info))
+ if (bdi_read_congested(mapping->backing_dev_info)) {
+ ra_account(ra, RA_EVENT_IO_CONGESTION,
+ end_index - index);
return 0;
+ }
}

if (page)
--
-
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