Re: Re: blk-throttle.c : When limit is changed, must start a new slice

[Lina Lu]

On 2011-03-09 04:54:43, Vivek Goyal wrote:
>
>On Tue, Mar 08, 2011 at 11:03:59PM +0800, lina wrote:

>[..]
>> >>  Unfortunately, the following patch still has 5~10 seconds latency. I have no
>> >>  idea to resolve this problem, it seens hard to find a more suitable func to
>> >>  call throtl_start_new_slice().
>> >
>> >So are you saying that following patch did not solve the latnecy issue?
>> >Resetting slice upon limit change did not work for you?
>> >
>>   
>>  Yes, the following patch did not solve the latency issue. There is still 5~10
>>  seconds latency when I change the limit from a very high value to low. From
>>  blktrace, I find that the throtl_process_limit_change() is called after work 
>>  queue delay.
>>   
>>  Thanks
>>  Lina
>
>Ok,
>
>Can you try the attached patch. I think what was happening that after
>changing limits, work was not being scheduled as there were no queued
>bios hence no slice reset was taking place immediately.
>
>[..]
>
>Thanks
>Vivek
>

Hi Vivek,
I have test the following patch, but the latency still there.

I try to find why there are 5~10 seconds latency today. After collect the blktrace, I 
think the reason is that throtl_trim_slice() don't aways update the tg->slice_start[rw], 
although we call it once dispatch a bio.

Suppose that if the limits change now from 102400000000 to 1024000, the 
tg->slice_start[rw] and tg->slice_end[rw] just like in the following chart. There is two 
throtl_slice in the chart. Here my HZ is 250, so the throtl_slice is 25.

                  jiffies
                  |
   |------------------|------------------|
   |                         |
start                    end

As the jiffies - start < 25(throtl_slice), throtl_trim_slice() will not update the 
tg->slice_start[rw] and tg->bytes_disp[rw]. If the tg->bytes_disp[rw] now is 8M, then
there will be about 7 seconds from jiffies 0 bps as I have set the limits at 1M/s, in 
these seconds no bio can be dispatched.

As the tg->slice_start[rw] must less than or equal to jiffies, and we can not know the 
reason of tg->bytes_disp[rw] > the theoretical value with limits 1M/s, So can not just 
set the tg->slice_start[rw] to jiffies here. If set the start to jiffies, throtl will not work.

I think if we can start a new slice in the next throtl_slice when the limits changed from 
high to low and the tg->bytes_disp[rw] is critical greater than the theoretical value with
now limits, this problem can be solved.

Thanks
Lina

>---
> block/blk-throttle.c |   24 +++++++++++++++++++++++-
> 1 file changed, 23 insertions(+), 1 deletion(-)
>
>Index: linux-2.6/block/blk-throttle.c
>===================================================================
>--- linux-2.6.orig/block/blk-throttle.c	2011-03-04 13:59:45.000000000 -0500
>+++ linux-2.6/block/blk-throttle.c	2011-03-08 15:41:19.384654732 -0500
>@@ -757,6 +757,14 @@ static void throtl_process_limit_change(
> 				" riops=%u wiops=%u", tg->bps[READ],
> 				tg->bps[WRITE], tg->iops[READ],
> 				tg->iops[WRITE]);
>+			/*
>+			 * Restart the slices for both READ and WRITES. It
>+			 * might happen that a group's limit are dropped
>+			 * suddenly and we don't want to account recently
>+			 * dispatched IO with new low rate
>+			 */
>+			throtl_start_new_slice(td, tg, 0);
>+			throtl_start_new_slice(td, tg, 1);
> 			tg_update_disptime(td, tg);
> 			tg->limits_changed = false;
> 		}
>@@ -825,7 +833,8 @@ throtl_schedule_delayed_work(struct thro
> 
> 	struct delayed_work *dwork = &td->throtl_work;
> 
>-	if (total_nr_queued(td) > 0) {
>+	/* schedule work if limits changed even if no bio is queued */
>+	if (total_nr_queued(td) > 0 || atomic_read(&td->limits_changed)) {
> 		/*
> 		 * We might have a work scheduled to be executed in future.
> 		 * Cancel that and schedule a new one.
>@@ -1023,6 +1032,19 @@ int blk_throtl_bio(struct request_queue 
> 	/* Bio is with-in rate limit of group */
> 	if (tg_may_dispatch(td, tg, bio, NULL)) {
> 		throtl_charge_bio(tg, bio);
>+
>+		/*
>+		 * We need to trim slice even when bios are not being queued
>+		 * otherwise it might happen that a bio is not queued for
>+		 * a long time and slice keeps on extending and trim is not
>+		 * called for a long time. Now if limits are reduced suddenly
>+		 * we take into account all the IO dispatched so far at new
>+		 * low rate and * newly queued IO gets a really long dispatch
>+		 * time.
>+		 *
>+		 * So keep on trimming slice even if bio is not queued.
>+		 */
>+		throtl_trim_slice(td, tg, rw);
> 		goto out;
> 	}
 


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