linux-kernel - Re: [PATCH] f2fs: fix to avoid slowing down background gc

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Date:   Mon, 19 Sep 2016 19:54:32 -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
Subject: Re: [PATCH] f2fs: fix to avoid slowing down background gc

On Tue, Sep 20, 2016 at 10:22:22AM +0800, Chao Yu wrote:
> Hi Jaegeuk,
> 
> On 2016/9/20 6:12, Jaegeuk Kim wrote:
> > Hi Chao,
> > 
> > On Sun, Sep 18, 2016 at 07:52:27PM +0800, Chao Yu wrote:
> >> Previously, we will choose to speed up background gc when the below
> >> conditions are both satisfied:
> >> a. There are a number of invalid blocks
> >> b. There is not enough free space
> >>
> >> But, when space utilization is high (utilization > 60%), there will be
> >> not enough invalid blocks, result in slowing down background gc, after
> >> then there are more opportunities that triggering foreground gc due to
> >> high fragmented free space in fs.
> >>
> >> Remove condition a) in order to avoid slow down background gc speed in
> >> a high utilization fs.
> > 
> > There exists a trade-off here: wear-out vs. eager gc for future speed-up.
> > How about using a kind of f2fs's dirty level (e.g., BDF)?
> 
> Yep, I think that f2fs can implement a mechanism which can provide more
> dynamically adjustable GC speed in the specified scenario of user, by this, user
> can choose the strategy which is more beneficial to aspect
> (wear-out/performance) they care. Let me think a while, anyway I agree that BDF
> is a good reference value here.
> 
> And Before we can provide above ability, how about treat this patch as a fixing
> patch, since it fixes to not adjust speed of GC according to utilization watermark?

Well, this is not a bug fix, but a very conservative policy. So, please let's
make a better policy, if possible.

Thanks,

> 
> Thanks,
> 
> > 
> > Thanks,
> > 
> >>
> >> Signed-off-by: Chao Yu <yuchao0@...wei.com>
> >> ---
> >>  fs/f2fs/gc.h | 18 +++---------------
> >>  1 file changed, 3 insertions(+), 15 deletions(-)
> >>
> >> diff --git a/fs/f2fs/gc.h b/fs/f2fs/gc.h
> >> index a993967..5d0a19c 100644
> >> --- a/fs/f2fs/gc.h
> >> +++ b/fs/f2fs/gc.h
> >> @@ -16,7 +16,6 @@
> >>  #define DEF_GC_THREAD_MIN_SLEEP_TIME	30000	/* milliseconds */
> >>  #define DEF_GC_THREAD_MAX_SLEEP_TIME	60000
> >>  #define DEF_GC_THREAD_NOGC_SLEEP_TIME	300000	/* wait 5 min */
> >> -#define LIMIT_INVALID_BLOCK	40 /* percentage over total user space */
> >>  #define LIMIT_FREE_BLOCK	40 /* percentage over invalid + free space */
> >>  
> >>  /* Search max. number of dirty segments to select a victim segment */
> >> @@ -52,11 +51,6 @@ static inline block_t free_user_blocks(struct f2fs_sb_info *sbi)
> >>  			<< sbi->log_blocks_per_seg;
> >>  }
> >>  
> >> -static inline block_t limit_invalid_user_blocks(struct f2fs_sb_info *sbi)
> >> -{
> >> -	return (long)(sbi->user_block_count * LIMIT_INVALID_BLOCK) / 100;
> >> -}
> >> -
> >>  static inline block_t limit_free_user_blocks(struct f2fs_sb_info *sbi)
> >>  {
> >>  	block_t reclaimable_user_blocks = sbi->user_block_count -
> >> @@ -88,15 +82,9 @@ static inline void decrease_sleep_time(struct f2fs_gc_kthread *gc_th,
> >>  
> >>  static inline bool has_enough_invalid_blocks(struct f2fs_sb_info *sbi)
> >>  {
> >> -	block_t invalid_user_blocks = sbi->user_block_count -
> >> -					written_block_count(sbi);
> >>  	/*
> >> -	 * Background GC is triggered with the following conditions.
> >> -	 * 1. There are a number of invalid blocks.
> >> -	 * 2. There is not enough free space.
> >> +	 * Background GC should speed up when there is not enough free blocks
> >> +	 * in total unused (free + invalid) blocks.
> >>  	 */
> >> -	if (invalid_user_blocks > limit_invalid_user_blocks(sbi) &&
> >> -			free_user_blocks(sbi) < limit_free_user_blocks(sbi))
> >> -		return true;
> >> -	return false;
> >> +	return free_user_blocks(sbi) < limit_free_user_blocks(sbi);
> >>  }
> >> -- 
> >> 2.8.2.311.gee88674
> > 
> > .
> >