[RFC PATCH] blk-iocost: Optimize the ioc_refreash_vrate() function

[RFC PATCH] blk-iocost: Optimize the ioc_refreash_vrate() function

[Baolin Wang]

The ioc_refreash_vrate() will only be called in ioc_timer_fn() after
starting a new period or stopping the period.

So when starting a new period, the variable 'pleft' in ioc_refreash_vrate()
is always the period's time, which means if the abs(ioc->vtime_err)
is less than the period's time, the vcomp is 0, and we do not need
compensate the vtime_rate in this case, just set it as the base vrate
and return.

When stopping the period, the ioc->vtime_err will be cleared to 0,
and we also do not need to compensate the vtime_rate, just set it as
the base vrate and return.

Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
---
 block/blk-iocost.c | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/block/blk-iocost.c b/block/blk-iocost.c
index 8348db4..58c9533 100644
--- a/block/blk-iocost.c
+++ b/block/blk-iocost.c
@@ -943,30 +943,29 @@ static bool ioc_refresh_params(struct ioc *ioc, bool force)
  */
 static void ioc_refresh_vrate(struct ioc *ioc, struct ioc_now *now)
 {
-	s64 pleft = ioc->period_at + ioc->period_us - now->now;
 	s64 vperiod = ioc->period_us * ioc->vtime_base_rate;
 	s64 vcomp, vcomp_min, vcomp_max;
 
 	lockdep_assert_held(&ioc->lock);
 
-	/* we need some time left in this period */
-	if (pleft <= 0)
-		goto done;
+	if (abs(ioc->vtime_err) < ioc->period_us) {
+		atomic64_set(&ioc->vtime_rate, ioc->vtime_base_rate);
+		return;
+	}
 
 	/*
 	 * Calculate how much vrate should be adjusted to offset the error.
 	 * Limit the amount of adjustment and deduct the adjusted amount from
 	 * the error.
 	 */
-	vcomp = -div64_s64(ioc->vtime_err, pleft);
+	vcomp = -div64_s64(ioc->vtime_err, ioc->period_us);
 	vcomp_min = -(ioc->vtime_base_rate >> 1);
 	vcomp_max = ioc->vtime_base_rate;
 	vcomp = clamp(vcomp, vcomp_min, vcomp_max);
 
-	ioc->vtime_err += vcomp * pleft;
+	ioc->vtime_err += vcomp * ioc->period_us;
 
 	atomic64_set(&ioc->vtime_rate, ioc->vtime_base_rate + vcomp);
-done:
 	/* bound how much error can accumulate */
 	ioc->vtime_err = clamp(ioc->vtime_err, -vperiod, vperiod);
 }
-- 
1.8.3.1

Re: [RFC PATCH] blk-iocost: Optimize the ioc_refreash_vrate() function

[Tejun Heo]

On Sun, Nov 29, 2020 at 10:37:18AM +0800, Baolin Wang wrote:
> The ioc_refreash_vrate() will only be called in ioc_timer_fn() after
> starting a new period or stopping the period.
> 
> So when starting a new period, the variable 'pleft' in ioc_refreash_vrate()
> is always the period's time, which means if the abs(ioc->vtime_err)
> is less than the period's time, the vcomp is 0, and we do not need
> compensate the vtime_rate in this case, just set it as the base vrate
> and return.
> 
> When stopping the period, the ioc->vtime_err will be cleared to 0,
> and we also do not need to compensate the vtime_rate, just set it as
> the base vrate and return.

Before, the function did something which is conceptually discrete and
describable. After, its operation is intertwined with when it's called. I
don't think this sort of micro optimizations are beneficial in cold paths.

Thanks.

-- 
tejun

Re: [RFC PATCH] blk-iocost: Optimize the ioc_refreash_vrate() function

[Baolin Wang]

在 2020/12/3 4:32, Tejun Heo 写道:
> On Sun, Nov 29, 2020 at 10:37:18AM +0800, Baolin Wang wrote:
>> The ioc_refreash_vrate() will only be called in ioc_timer_fn() after
>> starting a new period or stopping the period.
>>
>> So when starting a new period, the variable 'pleft' in ioc_refreash_vrate()
>> is always the period's time, which means if the abs(ioc->vtime_err)
>> is less than the period's time, the vcomp is 0, and we do not need
>> compensate the vtime_rate in this case, just set it as the base vrate
>> and return.
>>
>> When stopping the period, the ioc->vtime_err will be cleared to 0,
>> and we also do not need to compensate the vtime_rate, just set it as
>> the base vrate and return.
> 
> Before, the function did something which is conceptually discrete and
> describable. After, its operation is intertwined with when it's called. I
> don't think this sort of micro optimizations are beneficial in cold paths.

OK. I understood your concern. Thanks for your input.