A debugfs file named `readahead/events' is created according to advises from
J?rn Engel, Andrew Morton and Ingo Oeser.
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
## First compile kernel with CONFIG_DEBUG_READAHEAD
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 3 > /debug/readahead/debug_level # show verbose printk traces
## do one benchmark/task
# echo 1 > /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 more detailed file accesses,
which are useful sometimes. Note that the log file can grow huge!
Signed-off-by: Wu Fengguang <[email protected]>
---
mm/readahead.c | 293 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
1 files changed, 292 insertions(+), 1 deletion(-)
--- linux-2.6.17-rc4-mm3.orig/mm/readahead.c
+++ linux-2.6.17-rc4-mm3/mm/readahead.c
@@ -25,6 +25,69 @@
#define MAX_RA_PAGES (VM_MAX_READAHEAD*1024 / PAGE_CACHE_SIZE)
#define MIN_RA_PAGES DIV_ROUND_UP(VM_MIN_READAHEAD*1024, PAGE_CACHE_SIZE)
+/*
+ * Detailed classification of read-ahead behaviors.
+ */
+#define RA_CLASS_SHIFT 4
+#define RA_CLASS_MASK ((1 << RA_CLASS_SHIFT) - 1)
+enum ra_class {
+ RA_CLASS_ALL,
+ RA_CLASS_INITIAL,
+ RA_CLASS_STATE,
+ RA_CLASS_CONTEXT,
+ RA_CLASS_CONTEXT_AGGRESSIVE,
+ RA_CLASS_BACKWARD,
+ RA_CLASS_THRASHING,
+ RA_CLASS_SEEK,
+ RA_CLASS_NONE,
+ RA_CLASS_COUNT
+};
+
+/* Read-ahead events to be accounted. */
+enum ra_event {
+ RA_EVENT_CACHE_MISS, /* read cache misses */
+ RA_EVENT_RANDOM_READ, /* random reads */
+ RA_EVENT_IO_CONGESTION, /* i/o congestion */
+ RA_EVENT_IO_CACHE_HIT, /* canceled i/o due to cache hit */
+ RA_EVENT_IO_BLOCK, /* wait for i/o completion */
+
+ RA_EVENT_READAHEAD, /* read-ahead issued */
+ RA_EVENT_READAHEAD_HIT, /* read-ahead page hit */
+ RA_EVENT_LOOKAHEAD, /* look-ahead issued */
+ RA_EVENT_LOOKAHEAD_HIT, /* look-ahead mark hit */
+ RA_EVENT_LOOKAHEAD_NOACTION, /* look-ahead mark ignored */
+ RA_EVENT_READAHEAD_MMAP, /* read-ahead for mmap access */
+ RA_EVENT_READAHEAD_EOF, /* read-ahead reaches EOF */
+ RA_EVENT_READAHEAD_SHRINK, /* ra_size falls under previous la_size */
+ RA_EVENT_READAHEAD_THRASHING, /* read-ahead thrashing happened */
+ RA_EVENT_READAHEAD_MUTILATE, /* read-ahead mutilated by imbalanced aging */
+ RA_EVENT_READAHEAD_RESCUE, /* read-ahead rescued */
+
+ RA_EVENT_READAHEAD_CUBE,
+ RA_EVENT_COUNT
+};
+
+#ifdef CONFIG_DEBUG_READAHEAD
+u32 initial_ra_hit;
+u32 initial_ra_miss;
+u32 debug_level = 1;
+u32 disable_stateful_method = 0;
+static const char * const ra_class_name[];
+static void ra_account(struct file_ra_state *ra, enum ra_event e, int pages);
+# define debug_inc(var) do { var++; } while (0)
+# define debug_option(o) (o)
+#else
+# define ra_account(ra, e, pages) do { } while (0)
+# define debug_inc(var) do { } while (0)
+# define debug_option(o) (0)
+# define debug_level (0)
+#endif /* CONFIG_DEBUG_READAHEAD */
+
+#define dprintk(args...) \
+ do { if (debug_level >= 2) printk(KERN_DEBUG args); } while(0)
+#define ddprintk(args...) \
+ do { if (debug_level >= 3) printk(KERN_DEBUG args); } while(0)
+
void default_unplug_io_fn(struct backing_dev_info *bdi, struct page *page)
{
}
@@ -365,6 +428,9 @@ int force_page_cache_readahead(struct ad
offset += this_chunk;
nr_to_read -= this_chunk;
}
+
+ ra_account(NULL, RA_EVENT_READAHEAD, ret);
+
return ret;
}
@@ -400,10 +466,16 @@ static inline int check_ra_success(struc
int do_page_cache_readahead(struct address_space *mapping, struct file *filp,
pgoff_t offset, unsigned long nr_to_read)
{
+ unsigned long ret;
+
if (bdi_read_congested(mapping->backing_dev_info))
return -1;
- return __do_page_cache_readahead(mapping, filp, offset, nr_to_read, 0);
+ ret = __do_page_cache_readahead(mapping, filp, offset, nr_to_read, 0);
+
+ ra_account(NULL, RA_EVENT_READAHEAD, ret);
+
+ return ret;
}
/*
@@ -425,6 +497,10 @@ blockable_page_cache_readahead(struct ad
actual = __do_page_cache_readahead(mapping, filp, offset, nr_to_read, 0);
+ ra_account(NULL, RA_EVENT_READAHEAD, actual);
+ dprintk("blockable-readahead(ino=%lu, ra=%lu+%lu) = %d\n",
+ mapping->host->i_ino, offset, nr_to_read, actual);
+
return check_ra_success(ra, nr_to_read, actual);
}
@@ -604,3 +680,217 @@ unsigned long max_sane_readahead(unsigne
__get_zone_counts(&active, &inactive, &free, NODE_DATA(numa_node_id()));
return min(nr, (inactive + free) / 2);
}
+
+/*
+ * Read-ahead events accounting.
+ */
+#ifdef CONFIG_DEBUG_READAHEAD
+
+#include <linux/init.h>
+#include <linux/jiffies.h>
+#include <linux/debugfs.h>
+#include <linux/seq_file.h>
+
+static const char * const ra_class_name[] = {
+ "total",
+ "initial",
+ "state",
+ "context",
+ "contexta",
+ "backward",
+ "onthrash",
+ "onseek",
+ "none"
+};
+
+static const char * const ra_event_name[] = {
+ "cache_miss",
+ "random_read",
+ "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_events[RA_CLASS_COUNT][RA_EVENT_COUNT][2];
+
+static void ra_account(struct file_ra_state *ra, enum ra_event e, int pages)
+{
+ enum ra_class c;
+
+ if (!debug_level)
+ return;
+
+ 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_NONE;
+
+ if (!c)
+ c = RA_CLASS_NONE;
+
+ ra_events[c][e][0] += 1;
+ ra_events[c][e][1] += pages;
+
+ if (e == RA_EVENT_READAHEAD)
+ ra_events[c][RA_EVENT_READAHEAD_CUBE][1] += pages * pages;
+}
+
+static int ra_events_show(struct seq_file *s, void *_)
+{
+ int i;
+ int c;
+ int e;
+ static const char event_fmt[] = "%-16s";
+ static const char class_fmt[] = "%10s";
+ static const char item_fmt[] = "%10lu";
+ static const char percent_format[] = "%9lu%%";
+ static const char * const table_name[] = {
+ "[table requests]",
+ "[table pages]",
+ "[table summary]"};
+
+ for (i = 0; i <= 1; i++) {
+ for (e = 0; e < RA_EVENT_COUNT; e++) {
+ ra_events[RA_CLASS_ALL][e][i] = 0;
+ for (c = RA_CLASS_INITIAL; c < RA_CLASS_NONE; c++)
+ ra_events[RA_CLASS_ALL][e][i] += ra_events[c][e][i];
+ }
+
+ seq_printf(s, event_fmt, table_name[i]);
+ for (c = 0; c < RA_CLASS_COUNT; c++)
+ seq_printf(s, class_fmt, ra_class_name[c]);
+ seq_puts(s, "\n");
+
+ for (e = 0; e < RA_EVENT_COUNT; e++) {
+ if (e == RA_EVENT_READAHEAD_CUBE)
+ continue;
+ if (e == RA_EVENT_READAHEAD_HIT && i == 0)
+ continue;
+ if (e == RA_EVENT_IO_BLOCK && i == 1)
+ continue;
+
+ seq_printf(s, event_fmt, ra_event_name[e]);
+ for (c = 0; c < RA_CLASS_COUNT; c++)
+ seq_printf(s, item_fmt, ra_events[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_COUNT; 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_COUNT; c++)
+ seq_printf(s, percent_format,
+ (ra_events[c][RA_EVENT_RANDOM_READ][0] * 100) /
+ ((ra_events[c][RA_EVENT_RANDOM_READ][0] +
+ ra_events[c][RA_EVENT_READAHEAD][0]) | 1));
+ seq_puts(s, "\n");
+
+ seq_printf(s, event_fmt, "ra_hit_rate");
+ for (c = 0; c < RA_CLASS_COUNT; c++)
+ seq_printf(s, percent_format,
+ (ra_events[c][RA_EVENT_READAHEAD_HIT][1] * 100) /
+ (ra_events[c][RA_EVENT_READAHEAD][1] | 1));
+ seq_puts(s, "\n");
+
+ seq_printf(s, event_fmt, "la_hit_rate");
+ for (c = 0; c < RA_CLASS_COUNT; c++)
+ seq_printf(s, percent_format,
+ (ra_events[c][RA_EVENT_LOOKAHEAD_HIT][0] * 100) /
+ (ra_events[c][RA_EVENT_LOOKAHEAD][0] | 1));
+ seq_puts(s, "\n");
+
+ seq_printf(s, event_fmt, "var_ra_size");
+ for (c = 0; c < RA_CLASS_COUNT; c++)
+ seq_printf(s, item_fmt,
+ (ra_events[c][RA_EVENT_READAHEAD_CUBE][1] -
+ ra_events[c][RA_EVENT_READAHEAD][1] *
+ (ra_events[c][RA_EVENT_READAHEAD][1] /
+ (ra_events[c][RA_EVENT_READAHEAD][0] | 1))) /
+ (ra_events[c][RA_EVENT_READAHEAD][0] | 1));
+ seq_puts(s, "\n");
+
+ seq_printf(s, event_fmt, "avg_ra_size");
+ for (c = 0; c < RA_CLASS_COUNT; c++)
+ seq_printf(s, item_fmt,
+ (ra_events[c][RA_EVENT_READAHEAD][1] +
+ ra_events[c][RA_EVENT_READAHEAD][0] / 2) /
+ (ra_events[c][RA_EVENT_READAHEAD][0] | 1));
+ seq_puts(s, "\n");
+
+ seq_printf(s, event_fmt, "avg_la_size");
+ for (c = 0; c < RA_CLASS_COUNT; c++)
+ seq_printf(s, item_fmt,
+ (ra_events[c][RA_EVENT_LOOKAHEAD][1] +
+ ra_events[c][RA_EVENT_LOOKAHEAD][0] / 2) /
+ (ra_events[c][RA_EVENT_LOOKAHEAD][0] | 1));
+ seq_puts(s, "\n");
+
+ return 0;
+}
+
+static int ra_events_open(struct inode *inode, struct file *file)
+{
+ return single_open(file, ra_events_show, NULL);
+}
+
+static ssize_t ra_events_write(struct file *file, const char __user *buf,
+ size_t size, loff_t *offset)
+{
+ memset(ra_events, 0, sizeof(ra_events));
+ return 1;
+}
+
+struct file_operations ra_events_fops = {
+ .owner = THIS_MODULE,
+ .open = ra_events_open,
+ .write = ra_events_write,
+ .read = seq_read,
+ .llseek = seq_lseek,
+ .release = single_release,
+};
+
+#define READAHEAD_DEBUGFS_ENTRY_U32(var) \
+ debugfs_create_u32(__stringify(var), 0644, root, &var)
+
+#define READAHEAD_DEBUGFS_ENTRY_BOOL(var) \
+ debugfs_create_bool(__stringify(var), 0644, root, &var)
+
+static int __init readahead_init(void)
+{
+ struct dentry *root;
+
+ root = debugfs_create_dir("readahead", NULL);
+
+ debugfs_create_file("events", 0644, root, NULL, &ra_events_fops);
+
+ READAHEAD_DEBUGFS_ENTRY_U32(initial_ra_hit);
+ READAHEAD_DEBUGFS_ENTRY_U32(initial_ra_miss);
+
+ READAHEAD_DEBUGFS_ENTRY_U32(debug_level);
+ READAHEAD_DEBUGFS_ENTRY_BOOL(disable_stateful_method);
+
+ return 0;
+}
+
+module_init(readahead_init)
+
+#endif /* CONFIG_DEBUG_READAHEAD */
--