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Date: Fri, 01 Aug 2008 09:57:33 -0400
From: Ric Wheeler <rwheeler@...hat.com>
To: Josef Bacik <jbacik@...hat.com>
CC: Ric Wheeler <rwheeler@...hat.com>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...e.hu>, linux-fsdevel@...r.kernel.org,
Chris Mason <chris.mason@...cle.com>,
linux-kernel@...r.kernel.org
Subject: Re: high resolution timers, scheduling & sleep granularity
Josef Bacik wrote:
> 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
>
I think that this (or similar) kind of precision_sleep() should be
generically useful.
One question on the code, would it be better to measure the average
transaction time in the same units as your precision sleep uses - aren't
jiffies are still too coarse?
It also might be interesting to export the measured average commit time
via some non-printk method (/sys/fs ??)
ric
>
> 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)
> {
> --
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