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

[Chao Yu <yuchao0@huawei.com>]

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?

Thanks,

> 
> Thanks,
> 
>>
>> Signed-off-by: Chao Yu <yuchao0@huawei.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
> 
> .
>