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

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Message-ID: <20180409180241.GC17558@jaegeuk-macbookpro.roam.corp.google.com>
Date:   Mon, 9 Apr 2018 11:02:41 -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 04/08, 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.
> 
> 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?

> 
> 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
> > 
> > .
> >