On 2017/6/8 下午5:34, tang.junhui@xxxxxxxxxx wrote:
> From: Tang Junhui <tang.junhui@xxxxxxxxxx>
>
> When there is not enough data in writeback cache, let the writeback rate
> to be 0, and delay 30 seconds in read_dirty().
>
I feel uncomfortable to the above idea. Is the purpose of this change is
to make writeback more efficient with more dirty data ? Or some other
reason that you want to make this change?
When data writing back to the backing device, most of the writes are
random. If the backing device is a hard disk, the write back rate will
be very slow. Such a longer delay only makes more data accumulated in
writeback cache, does not help the writeback rate indeed.
Do you observe performance improvement by this patch ? If yes, I'd like
to see the performance number.
Thanks.
Coly
> Signed-off-by: Tang Junhui <tang.junhui@xxxxxxxxxx>
> ---
> drivers/md/bcache/util.c | 17 ++++++++++++-----
> drivers/md/bcache/writeback.c | 2 +-
> 2 files changed, 13 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/md/bcache/util.c b/drivers/md/bcache/util.c
> index dde6172..cf750df 100644
> --- a/drivers/md/bcache/util.c
> +++ b/drivers/md/bcache/util.c
> @@ -15,6 +15,8 @@
>
> #include "util.h"
>
> +#define ZERO_RATE_DELAY_NS 30*NSEC_PER_SEC
> +
> #define simple_strtoint(c, end, base) simple_strtol(c, end, base)
> #define simple_strtouint(c, end, base) simple_strtoul(c, end, base)
>
> @@ -209,13 +211,18 @@ 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) {
> + d->next = now + ZERO_RATE_DELAY_NS;
> + }
> + 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;
> + if (time_before64(now + NSEC_PER_SEC, d->next))
> + d->next = now + NSEC_PER_SEC;
>
> - if (time_after64(now - NSEC_PER_SEC * 2, d->next))
> - d->next = now - NSEC_PER_SEC * 2;
> + if (time_after64(now - NSEC_PER_SEC * 2, d->next))
> + d->next = now - NSEC_PER_SEC * 2;
> + }
>
> return time_after64(d->next, now)
> ? div_u64(d->next - now, NSEC_PER_SEC / HZ)
> diff --git a/drivers/md/bcache/writeback.c b/drivers/md/bcache/writeback.c
> index 69e1ae5..8fac280 100644
> --- a/drivers/md/bcache/writeback.c
> +++ b/drivers/md/bcache/writeback.c
> @@ -60,7 +60,7 @@ static void __update_writeback_rate(struct cached_dev *dc)
>
> 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;
>
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