linux-kernel - Re: [PATCH] f2fs: enlarge block plug coverage

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Message-ID: <20180413040702.GC59368@jaegeuk-macbookpro.roam.corp.google.com>
Date:   Thu, 12 Apr 2018 21:07:02 -0700
From:   Jaegeuk Kim <jaegeuk@...nel.org>
To:     Chao Yu <yuchao0@...wei.com>
Cc:     linux-f2fs-devel@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org, chao@...nel.org
Subject: Re: [PATCH] f2fs: enlarge block plug coverage

On 04/13, Chao Yu wrote:
> On 2018/4/13 9:06, Jaegeuk Kim wrote:
> > On 04/10, Chao Yu wrote:
> >> On 2018/4/10 12:10, Jaegeuk Kim wrote:
> >>> On 04/10, Chao Yu wrote:
> >>>> On 2018/4/10 2:02, Jaegeuk Kim wrote:
> >>>>> On 04/08, Chao Yu wrote:
> >>>>>> On 2018/4/5 11:51, Jaegeuk Kim wrote:
> >>>>>>> On 04/04, Chao Yu wrote:
> >>>>>>>> This patch enlarges block plug coverage in __issue_discard_cmd, in
> >>>>>>>> order to collect more pending bios before issuing them, to avoid
> >>>>>>>> being disturbed by previous discard I/O in IO aware discard mode.
> >>>>>>>
> >>>>>>> Hmm, then we need to wait for huge discard IO for over 10 secs, which
> >>>>>>
> >>>>>> We found that total discard latency is rely on total discard number we issued
> >>>>>> last time instead of range or length discard covered. IMO, if we don't change
> >>>>>> .max_requests value, we will not suffer longer latency.
> >>>>>>
> >>>>>>> will affect following read/write IOs accordingly. In order to avoid that,
> >>>>>>> we actually need to limit the discard size.
> >>>>
> >>>> Do you mean limit discard count or discard length?
> >>>
> >>> Both of them.
> >>>
> >>>>
> >>>>>>
> >>>>>> If you are worry about I/O interference in between discard and rw, I suggest to
> >>>>>> decrease .max_requests value.
> >>>>>
> >>>>> What do you mean? This will produce more pending requests in the queue?
> >>>>
> >>>> I mean after applying this patch, we can queue more discard IOs in plug inside
> >>>> task, otherwise, previous issued discard in block layer can make is_idle() be false,
> >>>> then it can stop IO awared user to issue pending discard command.
> >>>
> >>> Then, unplug will issue lots of discard commands, which affects the following rw
> >>> latencies. My preference would be issuing discard commands one by one as much as
> >>> possible.
> >>
> >> Hmm.. for you concern, we can turn down IO priority of discard from background?
> > 
> > That makes much more sense to me. :P
> 
> Then, this patch which enlarge plug coverage will not still a problem, right? ;)

This is different one.

> 
> Thanks,
> 
> > 
> >>
> >> Thanks,
> >>
> >>>
> >>>>
> >>>> Thanks,
> >>>>
> >>>>>
> >>>>>>
> >>>>>> Thanks,
> >>>>>>
> >>>>>>>
> >>>>>>> Thanks,
> >>>>>>>
> >>>>>>>>
> >>>>>>>> Signed-off-by: Chao Yu <yuchao0@...wei.com>
> >>>>>>>> ---
> >>>>>>>>  fs/f2fs/segment.c | 7 +++++--
> >>>>>>>>  1 file changed, 5 insertions(+), 2 deletions(-)
> >>>>>>>>
> >>>>>>>> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> >>>>>>>> index 8f0b5ba46315..4287e208c040 100644
> >>>>>>>> --- a/fs/f2fs/segment.c
> >>>>>>>> +++ b/fs/f2fs/segment.c
> >>>>>>>> @@ -1208,10 +1208,12 @@ static int __issue_discard_cmd(struct f2fs_sb_info *sbi,
> >>>>>>>>  		pend_list = &dcc->pend_list[i];
> >>>>>>>>  
> >>>>>>>>  		mutex_lock(&dcc->cmd_lock);
> >>>>>>>> +
> >>>>>>>> +		blk_start_plug(&plug);
> >>>>>>>> +
> >>>>>>>>  		if (list_empty(pend_list))
> >>>>>>>>  			goto next;
> >>>>>>>>  		f2fs_bug_on(sbi, !__check_rb_tree_consistence(sbi, &dcc->root));
> >>>>>>>> -		blk_start_plug(&plug);
> >>>>>>>>  		list_for_each_entry_safe(dc, tmp, pend_list, list) {
> >>>>>>>>  			f2fs_bug_on(sbi, dc->state != D_PREP);
> >>>>>>>>  
> >>>>>>>> @@ -1227,8 +1229,9 @@ static int __issue_discard_cmd(struct f2fs_sb_info *sbi,
> >>>>>>>>  			if (++iter >= dpolicy->max_requests)
> >>>>>>>>  				break;
> >>>>>>>>  		}
> >>>>>>>> -		blk_finish_plug(&plug);
> >>>>>>>>  next:
> >>>>>>>> +		blk_finish_plug(&plug);
> >>>>>>>> +
> >>>>>>>>  		mutex_unlock(&dcc->cmd_lock);
> >>>>>>>>  
> >>>>>>>>  		if (iter >= dpolicy->max_requests)
> >>>>>>>> -- 
> >>>>>>>> 2.15.0.55.gc2ece9dc4de6
> >>>>>>>
> >>>>>>> .
> >>>>>>>
> >>>>>
> >>>>> .
> >>>>>
> >>>
> >>> .
> >>>
> > 
> > .
> >