linux-kernel - Re: [f2fs-dev] [PATCH] f2fs: fix unnecessary periodic wakeup of discard thread when dev is busy

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Date:   Sun, 2 Sep 2018 16:04:11 +0530
From:   Sahitya Tummala <stummala@...eaurora.org>
To:     Chao Yu <chao@...nel.org>
Cc:     Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <yuchao0@...wei.com>,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org
Subject: Re: [f2fs-dev] [PATCH] f2fs: fix unnecessary periodic wakeup of
 discard thread when dev is busy

On Sun, Sep 02, 2018 at 04:52:40PM +0800, Chao Yu wrote:
> On 2018/8/31 17:39, Sahitya Tummala wrote:
> > When dev is busy, discard thread wake up timeout can be aligned with the
> > exact time that it needs to wait for dev to come out of busy. This helps
> > to avoid unnecessary periodic wakeups and thus save some power.
> > 
> > Signed-off-by: Sahitya Tummala <stummala@...eaurora.org>
> > ---
> >  fs/f2fs/segment.c | 8 +++++++-
> >  1 file changed, 7 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> > index 8bcbb50..df14030 100644
> > --- a/fs/f2fs/segment.c
> > +++ b/fs/f2fs/segment.c
> > @@ -1379,6 +1379,8 @@ static int issue_discard_thread(void *data)
> >  	struct discard_policy dpolicy;
> >  	unsigned int wait_ms = DEF_MIN_DISCARD_ISSUE_TIME;
> >  	int issued;
> > +	unsigned long interval = sbi->interval_time[REQ_TIME] * HZ;
> > +	long delta;
> >  
> >  	set_freezable();
> >  
> > @@ -1410,7 +1412,11 @@ static int issue_discard_thread(void *data)
> >  			__wait_all_discard_cmd(sbi, &dpolicy);
> >  			wait_ms = dpolicy.min_interval;
> >  		} else if (issued == -1){
> > -			wait_ms = dpolicy.mid_interval;
> > +			delta = (sbi->last_time[REQ_TIME] + interval) - jiffies;
> 
> I agree that we need to consider power consumption. One more consideration is
> that discard thread may need different submission frequency comparing to garbage
> collection thread, maybe a little fast, would it be better to split
> sbi->interval_time[REQ_TIME] according to gc/discard type.
> 
> How do you think?
> 
> Thanks,
> 

Thanks for the review.

You mean when GC type is urgent? I see that for that case, the discard policy is
changed to DPOLICY_FORCE, which sets dpolicy->io_aware as false and hence,
cannot fall into this (issued == -1) case at all.

> > +			if (delta > 0)
> > +				wait_ms = jiffies_to_msecs(delta);
> > +			else
> > +				wait_ms = dpolicy.mid_interval;
> >  		} else {
> >  			wait_ms = dpolicy.max_interval;
> >  		}
> > 

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