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SubjectRe: Re: Re: blk-throttle.c : When limit is changed, must start anewslice
On Mon, Mar 14, 2011 at 11:52:36PM +0800, Lina Lu wrote:
> 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?

Yes. When limits are very high, you will never enqueue a bio hence a
group will never be enqueued hence throtl_tg_on_rr=0.

>
> >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.

Here is Jens's block tree. It is separate from linus's tree.

http://git.kernel.org/?p=linux/kernel/git/axboe/linux-2.6-block.git;a=summary

Thanks
Vivek

>
> >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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