Re: high resolution timers, scheduling & sleep granularity

[Josef Bacik <jbacik@redhat.com>]

On Fri, Aug 01, 2008 at 08:05:37AM -0400, Ric Wheeler wrote:
>
> Hi Thomas & Ingo,
>
> Josef has been working on some patches to try and get ext3/4 to dynamically 
> detect the latency of a storage device and use that base latency to tune 
> the amount of time we sleep waiting for others to join in a transaction. 
> The logic in question lives in jbd/transaction.c (transaction_stop).
>
> The code was originally developed to try and allow multiple threads to join 
> in a big, slow transaction. For example, transacations that write to a slow 
> ATA or S-ATA drive take in the neighborhood of 10 to 20 ms.
>
> Faster devices, for example a disk array,  can complete the transaction in 
> 1.3 ms. Even higher speed SSD devices boast of a latency of 0.1ms, not to 
> mention RAM disks ;-)
>
> The current logic makes us wait way too long, especially with a 250HZ 
> kernel since we sleep many times longer than it takes to complete the IO 
> ;-)
>
> Do either of you have any thoughts on how to get a better, fine grained 
> sleep capability that we could use that would allow us to sleep in 
> sub-jiffie chunks?
>

Hello,

This is the most recent iteration of my patch using hrtimers.  It works really
well for ramdisks, so anything with low latency writes is going to be really
fast, but I'm still trying to come up with a smart way to sleep long enough to
not hurt SATA performance.  As it stands now I'm getting a 5% decrease in speed
on SATA.  So I think I've got the sleep as little as possible part down right,
just can't quite get it to sleep long enough if the disk is slow.  Thanks,

Josef


Index: linux-2.6/fs/jbd/commit.c
===================================================================
--- linux-2.6.orig/fs/jbd/commit.c
+++ linux-2.6/fs/jbd/commit.c
@@ -288,6 +288,7 @@ void journal_commit_transaction(journal_
 	int flags;
 	int err;
 	unsigned long blocknr;
+	unsigned long long commit_time, start_time;
 	char *tagp = NULL;
 	journal_header_t *header;
 	journal_block_tag_t *tag = NULL;
@@ -400,6 +401,7 @@ void journal_commit_transaction(journal_
 	commit_transaction->t_state = T_FLUSH;
 	journal->j_committing_transaction = commit_transaction;
 	journal->j_running_transaction = NULL;
+	start_time = jiffies;
 	commit_transaction->t_log_start = journal->j_head;
 	wake_up(&journal->j_wait_transaction_locked);
 	spin_unlock(&journal->j_state_lock);
@@ -873,6 +875,19 @@ restart_loop:
 	J_ASSERT(commit_transaction == journal->j_committing_transaction);
 	journal->j_commit_sequence = commit_transaction->t_tid;
 	journal->j_committing_transaction = NULL;
+	commit_time = elapsed_jiffies(start_time, jiffies);
+	if (unlikely(!journal->j_average_commit_time))
+		journal->j_average_commit_time = commit_time;
+	else
+		journal->j_average_commit_time = (commit_time +
+					journal->j_average_commit_time) / 2;
+
+	if (journal->print_count < 100) {
+		journal->print_count++;
+		printk(KERN_ERR "avg commit time = %lu\n",
+		       journal->j_average_commit_time);
+	}
+
 	spin_unlock(&journal->j_state_lock);
 
 	if (commit_transaction->t_checkpoint_list == NULL &&
Index: linux-2.6/fs/jbd/transaction.c
===================================================================
--- linux-2.6.orig/fs/jbd/transaction.c
+++ linux-2.6/fs/jbd/transaction.c
@@ -25,6 +25,7 @@
 #include <linux/timer.h>
 #include <linux/mm.h>
 #include <linux/highmem.h>
+#include <linux/hrtimer.h>
 
 static void __journal_temp_unlink_buffer(struct journal_head *jh);
 
@@ -49,6 +50,7 @@ get_transaction(journal_t *journal, tran
 {
 	transaction->t_journal = journal;
 	transaction->t_state = T_RUNNING;
+	transaction->t_start_time = jiffies;
 	transaction->t_tid = journal->j_transaction_sequence++;
 	transaction->t_expires = jiffies + journal->j_commit_interval;
 	spin_lock_init(&transaction->t_handle_lock);
@@ -63,6 +65,32 @@ get_transaction(journal_t *journal, tran
 	return transaction;
 }
 
+static void precision_sleep(unsigned long time)
+{
+	struct hrtimer_sleeper t;
+
+	hrtimer_init_on_stack(&t.timer, CLOCK_REALTIME, HRTIMER_MODE_ABS);
+	hrtimer_init_sleeper(&t, current);
+	t.timer.expires = ktime_add_ns(ktime_get_real(), time);
+
+	do {
+		set_current_state(TASK_UNINTERRUPTIBLE);
+
+		hrtimer_start(&t.timer, t.timer.expires, HRTIMER_MODE_ABS);
+		if (!hrtimer_active(&t.timer))
+			t.task = NULL;
+
+		if (likely(t.task))
+			schedule();
+
+		hrtimer_cancel(&t.timer);
+	} while (t.task);
+
+	set_current_state(TASK_RUNNING);
+
+	destroy_hrtimer_on_stack(&t.timer);
+}
+
 /*
  * Handle management.
  *
@@ -1361,7 +1389,7 @@ int journal_stop(handle_t *handle)
 {
 	transaction_t *transaction = handle->h_transaction;
 	journal_t *journal = transaction->t_journal;
-	int old_handle_count, err;
+	int err;
 	pid_t pid;
 
 	J_ASSERT(journal_current_handle() == handle);
@@ -1397,11 +1425,31 @@ int journal_stop(handle_t *handle)
 	 */
 	pid = current->pid;
 	if (handle->h_sync && journal->j_last_sync_writer != pid) {
+		unsigned long commit_time;
+		int print_count;
+		unsigned long sleep_time;
+
 		journal->j_last_sync_writer = pid;
-		do {
-			old_handle_count = transaction->t_handle_count;
-			schedule_timeout_uninterruptible(1);
-		} while (old_handle_count != transaction->t_handle_count);
+
+		spin_lock(&journal->j_state_lock);
+		commit_time = journal->j_average_commit_time;
+		spin_unlock(&journal->j_state_lock);
+
+		sleep_time = elapsed_jiffies(transaction->t_start_time, jiffies);
+		if (!sleep_time)
+			sleep_time = 1;
+		sleep_time = (commit_time / sleep_time) * commit_time;
+
+		/*
+		if (print_count >= 100 && print_count < 200) {
+			journal->print_count++;
+			printk(KERN_ERR "sleep_time=%lu\n", (unsigned long)sleep_time);
+		}
+
+		sleep_time = (long)(commit_time - sleep_time);
+		*/
+
+		precision_sleep((unsigned long)sleep_time);
 	}
 
 	current->journal_info = NULL;
Index: linux-2.6/include/linux/jbd.h
===================================================================
--- linux-2.6.orig/include/linux/jbd.h
+++ linux-2.6/include/linux/jbd.h
@@ -31,6 +31,7 @@
 #include <linux/mutex.h>
 #include <linux/timer.h>
 #include <linux/lockdep.h>
+#include <linux/jiffies.h>
 
 #define journal_oom_retry 1
 
@@ -79,6 +80,15 @@ static inline void jbd_free(void *ptr, s
 	free_pages((unsigned long)ptr, get_order(size));
 };
 
+static inline unsigned long elapsed_jiffies(unsigned long start,
+					    unsigned long end)
+{
+	if (end >= start)
+		return end - start;
+
+	return end + (MAX_JIFFY_OFFSET - start) + 1;
+}
+
 #define JFS_MIN_JOURNAL_BLOCKS 1024
 
 
@@ -543,6 +553,11 @@ struct transaction_s
 	unsigned long		t_expires;
 
 	/*
+	 * When this transaction started, in jiffies [no locking]
+	 */
+	unsigned long		t_start_time;
+
+	/*
 	 * How many handles used this transaction? [t_handle_lock]
 	 */
 	int t_handle_count;
@@ -800,6 +815,10 @@ struct journal_s
 
 	pid_t			j_last_sync_writer;
 
+	unsigned long		j_average_commit_time;
+
+	int print_count;
+
 	/*
 	 * An opaque pointer to fs-private information.  ext3 puts its
 	 * superblock pointer here
Index: linux-2.6/kernel/hrtimer.c
===================================================================
--- linux-2.6.orig/kernel/hrtimer.c
+++ linux-2.6/kernel/hrtimer.c
@@ -1458,6 +1458,7 @@ void hrtimer_init_sleeper(struct hrtimer
 	sl->timer.cb_mode = HRTIMER_CB_IRQSAFE_NO_SOFTIRQ;
 #endif
 }
+EXPORT_SYMBOL_GPL(hrtimer_init_sleeper);
 
 static int __sched do_nanosleep(struct hrtimer_sleeper *t, enum hrtimer_mode mode)
 {