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Date: Mon, 14 Mar 2011 23:52:36 +0800
From: "Lina Lu" <lulina_nuaa@...mail.com>
To: "Vivek Goyal" <vgoyal@...hat.com>
Cc: "linux kernel mailing list" <linux-kernel@...r.kernel.org>
Subject: Re: Re: Re: blk-throttle.c : When limit is changed, must start anewslice
On 2011-03-14 23:18:31, Vivek Goyal wrote:
>On Sat, Mar 12, 2011 at 07:33:07PM +0800, Lina Lu wrote:
>> On 2011-03-11 03:55:55, Vivek Goyal wrote:
>> >On Fri, Mar 11, 2011 at 12:38:18AM +0800, Lina Lu wrote:
>> >> [..]
>> >> 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.
>> >
>> >lina,
>> >
>> >Trim slice should not even matter now. Upon limit change, this patch
>> >should reset the slice and start a new one irrespective of the fact
>> >where are.
>> >
>> >In your traces, do you see limit change message and do you see a new
>> >slice starting.
>> >
>> >I did similar test yesterday on my box and this patch worked. Can you
>> >capture some block traces and I can have a look at those. Key thing
>> >to look for is limit change message and whether it started a new
>> >slice or not.
>> >
>> >Thanks
>> >Vivek
>> >
>>
>> Hi Vivek,
>>
>> Here is the blktrace and iostat results when I change the limit from 1024000000000000
>> to 1024000. When the limit changed, there is about 3 seconds lantency.
>>
>> blktrace:
>> 253,1 0 0 4.177733270 0 m N throtl / [R] trim slice nr=1 bytes=102400000000000 io=429496729 start=4297788991 end=4297789100 jiffies=4297788992
>> 253,1 0 0 4.187393582 0 m N throtl / [R] extend slice start=4297788991 end=4297789200 jiffies=4297789002
>> 253,1 0 0 4.276120505 0 m N throtl / [R] trim slice nr=1 bytes=102400000000000 io=429496729 start=4297789091 end=4297789200 jiffies=4297789091
>> 253,1 0 0 4.285934091 0 m N throtl / [R] extend slice start=4297789091 end=4297789300 jiffies=4297789101
>> 253,1 1 0 4.348552814 0 m N throtl schedule work. delay=0 jiffies=4297789163
>> 253,1 1 0 4.348571560 0 m N throtl limit changed =1
>> 253,1 0 0 4.349839104 0 m N throtl / [R] extend slice start=4297789091 end=4297793000 jiffies=4297789164
>> 253,1 0 0 4.349844118 0 m N throtl / [R] bio. bdisp=3928064 sz=4096 bps=1024000 iodisp=959 iops=4294967295 queued=0/0
>
>Lina,
>
>Thanks for the traces.
>
>I think we did call process_limit_change() but we did not start the new
>slice. I guess this happened because, we seem to be starting slice only
>if group on run tree. Because before limit udpates, most likely group
>is not on run tree as limits are very high, hence we missed resetting
>the slice.
>
> hlist_for_each_entry_safe(tg, pos, n, &td->tg_list, tg_node) {
> if (throtl_tg_on_rr(tg) && tg->limits_changed) {
> throtl_log_tg(td, tg, "limit change rbps=%llu wbps=%llu"
> " riops=%u wiops=%u", tg->bps[READ],
> tg->bps[WRITE], tg->iops[READ],
> tg->iops[WRITE]);
>
Do you mean that throtl_tg_on_rr() function returns 0 when the limits are very
high?
>Actually many races have been fixed in Jens's block tree. Is it possible to
>test origin/for-2.6.39/core branch of Jens's tree with following patch applied
>and see if it fixes the issue for you?
I only find 2.6.38 core in gitweb. Do you mean origin/for-2.6.38/core branch?
I'll test it as soon as possible and keep you know the result.
>Thanks
>Vivek
>
>---
> block/blk-throttle.c | 25 ++++++++++++++++++++++++-
> 1 file changed, 24 insertions(+), 1 deletion(-)
>
>Index: linux-2.6-block/block/blk-throttle.c
>===================================================================
>--- linux-2.6-block.orig/block/blk-throttle.c 2011-03-14 10:27:57.000000000 -0400
>+++ linux-2.6-block/block/blk-throttle.c 2011-03-14 10:30:47.267170956 -0400
>@@ -756,6 +756,15 @@ 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);
>+
> if (throtl_tg_on_rr(tg))
> tg_update_disptime(td, tg);
> }
>@@ -821,7 +830,8 @@ 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 || td->limits_changed) {
> /*
> * We might have a work scheduled to be executed in future.
> * Cancel that and schedule a new one.
>@@ -1002,6 +1012,19 @@ 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;
> }
t
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