Messages in this thread | | | Date | Mon, 14 Mar 2011 11:51:32 -0400 | From | Vivek Goyal <> | Subject | Re: Re: Re: blk-throttle.c : When limit is changed, must start anewslice |
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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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