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

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Date:   Tue, 20 Sep 2016 11:05:46 +0800
From:   Chao Yu <yuchao0@...wei.com>
To:     Jaegeuk Kim <jaegeuk@...nel.org>
CC:     <linux-f2fs-devel@...ts.sourceforge.net>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] f2fs: fix to avoid slowing down background gc

On 2016/9/20 10:54, Jaegeuk Kim wrote:
> 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.

Alright, let's think about this.

Thanks,

> 
> 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
>>>
>>> .
>>>
> 
> .
>