On Sun, 16 Jul 2017, Coly Li wrote:
> On 2017/7/1 上午4:43, bcache@xxxxxxxxxxxxxxxxxx wrote:
> > From: Tang Junhui <tang.junhui@xxxxxxxxxx>
> >
> > When there is not enough dirty data in writeback cache,
> > writeback rate is at minimum 1 key per second
> > util all dirty data to be cleaned, it is inefficiency,
> > and also causes waste of energy;
>
> Hi Junhui and Eric,
>
> What: /sys/block/<disk>/bcache/writeback_percent
> Description:
> For backing devices: If nonzero, writeback from cache to
> backing device only takes place when more than this percentage
> of the cache is used, allowing more write coalescing to take
> place and reducing total number of writes sent to the backing
> device. Integer between 0 and 40.
>
> I see above text from Documentation/ABI/testing/sysfs-block-bcache (I
> know this document is quite old), it seems if "not enough" means dirty
> data percentage is less then writback_percent, bcache should not
> performance writeback I/O. But in __update_writeback_rate(),
> writeback_rate.rate is clamped in [1, NSEC_PER_MSEC]. It seems in PD
> controller code of __update_writeback_rate(), writeback_percent is only
> used to calculate dirty target number, its another functionality as
> writeback threshold is not handled here.
>
> >
> > in this patch, When there is not enough dirty data,
> > let the writeback rate to be 0, and writeback re-schedule
> > in bch_writeback_thread() periodically with schedule_timeout(),
> > the behaviors are as follows :
> >
> > 1) If no dirty data have been read into dc->writeback_keys,
> > goto step 2), otherwise keep writing these dirty data to
> > back-end device at 1 key per second, until all these dirty data
> > write over, then goto step 2).
> >
> > 2) Loop in bch_writeback_thread() to check if there is enough
> > dirty data for writeback. if there is not enough diry data for
> > writing, then sleep 10 seconds, otherwise, write dirty data to
> > back-end device.
>
> Bcache uses a Proportion-Differentiation Controller to control writeback
> rate. When dirty data is far from target, writeback rate is higher; when
> dirty data is close to target, writeback rate is slower. The advantage
> of PD controller here is, when regular I/O and writeback I/O happens in
> same time,
> - When there are a lot of dirty data, writeback I/O can have more chance
> to write them back to cached device, which in turns has positive impact
> to regular I/O.
> - When dirty data is decreased and close to target dirty number, less
> writeback I/O can help regular I/O has better throughput and latency.
>
> The root cause of 1 key per second is, the PD controller is designed for
> better I/O performance, not less energy consumption. When the existing
> dirty data gets closed to target dirty number, the PD controller chooses
> to use longer writeback time to make a better regular I/O performance.
> If it is designed for less energy consumption, it should keep the
> writeback rate in a high level and finish writing back all dirty data as
> soon as possible.
>
> This patch may introduce an unexpected behavior of dirty data writeback
> throughput, when regular write I/O and writeback I/O happen in same
> time. In this case, dirty data number may shake up and down around
> target dirty number, it is possible that change (the variable in
> __update_writeback_rate()) is a minus value, and the result of
> dc->writeback_rate.rate + change happens to be 0. This patch changes the
> clamp range of writeback_rate.rate to [0, NSEC_PER_MSEC], so
> writeback_rate.rate can be possible to be 0. And in bch_next_delay() if
> d->rate is zero, the write back I/O will be delayed to now +
> NSEC_PER_SEC. When there is no regular I/O it works well, but when there
> is regular I/O, this longer delay may cause more dirty data piled in
> cache device, and PD controller cannot generage a stable writeback rate.
> This is not an expected behavior for the writeback rate PD controller.
>
> Another method to fix might be,
> 1) define a sysfs to define writeback_rate with max/dynamic option.
> 2) dynamic writeback_rate as default
> 3) when max is set, in __update_writeback_rate() assign NSEC_PER_MSEC to
> writeback_rate.rate
> 4) in bch_writeback_thread(), if no writeback I/O on fly, and dirty data
> does not reach dc->writeback_percent, schedule out.
> 5) if writeback is necessary then do it in max rate and finish it as
> soon as possible, to save laptop energy.
>
> The above method might be helpful to energy save as well (perform dirty
> dat write back in batch), and does not change default PD controller
> behavior.
>
> Just for your reference. Or if you are too busy to look at it, I can try
> to compose a patch for review.
Hi Coli,
Did this go anywere? I think the 1-key/sec fix is a good idea and your
suggestion will help out mobile users.
--
Eric Wheeler
>
> Coly
>
> >
> > Signed-off-by: Tang Junhui <tang.junhui@xxxxxxxxxx>
> > ---
> > drivers/md/bcache/util.c | 9 ++++++++-
> > drivers/md/bcache/writeback.c | 11 +++++++----
> > 2 files changed, 15 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/md/bcache/util.c b/drivers/md/bcache/util.c
> > index 8c3a938..49dcf09 100644
> > --- a/drivers/md/bcache/util.c
> > +++ b/drivers/md/bcache/util.c
> > @@ -210,7 +210,14 @@ uint64_t bch_next_delay(struct bch_ratelimit *d, uint64_t done)
> > {
> > uint64_t now = local_clock();
> >
> > - d->next += div_u64(done * NSEC_PER_SEC, d->rate);
> > + /*
> > + if d->rate is zero, write the left dirty data
> > + at the speed of one key per second
> > + */
> > + if(!d->rate)
> > + d->next = now + NSEC_PER_SEC;
> > + else
> > + d->next += div_u64(done * NSEC_PER_SEC, d->rate);
> >
> > if (time_before64(now + NSEC_PER_SEC, d->next))
> > d->next = now + NSEC_PER_SEC;
> > diff --git a/drivers/md/bcache/writeback.c b/drivers/md/bcache/writeback.c
> > index 25289e4..4104eaa 100644
> > --- a/drivers/md/bcache/writeback.c
> > +++ b/drivers/md/bcache/writeback.c
> > @@ -16,6 +16,8 @@
> > #include <linux/sched/clock.h>
> > #include <trace/events/bcache.h>
> >
> > +#define WRITE_BACK_WAIT_CYCLE 10 * HZ
> > +
> > /* Rate limiting */
> >
> > static void __update_writeback_rate(struct cached_dev *dc)
> > @@ -55,13 +57,14 @@ static void __update_writeback_rate(struct cached_dev *dc)
> >
> > /* Don't increase writeback rate if the device isn't keeping up */
> > if (change > 0 &&
> > + dc->writeback_rate.rate >0 &&
> > time_after64(local_clock(),
> > dc->writeback_rate.next + NSEC_PER_MSEC))
> > change = 0;
> >
> > dc->writeback_rate.rate =
> > clamp_t(int64_t, (int64_t) dc->writeback_rate.rate + change,
> > - 1, NSEC_PER_MSEC);
> > + 0, NSEC_PER_MSEC);
> >
> > dc->writeback_rate_proportional = proportional;
> > dc->writeback_rate_derivative = derivative;
> > @@ -420,15 +423,15 @@ static int bch_writeback_thread(void *arg)
> > while (!kthread_should_stop()) {
> > down_write(&dc->writeback_lock);
> > if (!atomic_read(&dc->has_dirty) ||
> > - (!test_bit(BCACHE_DEV_DETACHING, &dc->disk.flags) &&
> > - !dc->writeback_running)) {
> > + ((!dc->writeback_rate.rate || !dc->writeback_running) &&
> > + !test_bit(BCACHE_DEV_DETACHING, &dc->disk.flags))) {
> > up_write(&dc->writeback_lock);
> > set_current_state(TASK_INTERRUPTIBLE);
> >
> > if (kthread_should_stop())
> > return 0;
> >
> > - schedule();
> > + schedule_timeout(WRITE_BACK_WAIT_CYCLE);
> > continue;
> > }
> >
> >
>
>
> --
> Coly Li
> --
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