Re: [PATCH RFC 5/5] f2fs: support age threshold based garbage collection

[Chao Yu]

Jaegeuk,

This is a commercialized feature in huawei products for years, I think
it's time to try to upstream it, could you please take a look at this
idea, to see whether it could be a formal feature of f2fs?

On 2020/6/30 18:04, Chao Yu wrote:
> There are several issues in current background GC algorithm:
> - valid blocks is one of key factors during cost overhead calculation,
> so if segment has less valid block, however even its age is young or
> it locates hot segment, CB algorithm will still choose the segment as
> victim, it's not appropriate.
> - GCed data/node will go to existing logs, no matter in-there datas'
> update frequency is the same or not, it may mix hot and cold data
> again.
> - GC alloctor mainly use LFS type segment, it will cost free segment
> more quickly.
> 
> This patch introduces a new algorithm named age threshold based
> garbage collection to solve above issues, there are three steps
> mainly:
> 
> 1. select a source victim:
> - set an age threshold, and select candidates beased threshold:
> e.g.
>  0 means youngest, 100 means oldest, if we set age threshold to 80
>  then select dirty segments which has age in range of [80, 100] as
>  candiddates;
> - set candidate_ratio threshold, and select candidates based the
> ratio, so that we can shrink candidates to those oldest segments;
> - select target segment with fewest valid blocks in order to
> migrate blocks with minimum cost;
> 
> 2. select a target victim:
> - select candidates beased age threshold;
> - set candidate_radius threshold, search candidates whose age is
> around source victims, searching radius should less than the
> radius threshold.
> - select target segment with most valid blocks in order to avoid
> migrating current target segment.
> 
> 3. merge valid blocks from source victim into target victim with
> SSR alloctor.
> 
> Test steps:
> - create 160 dirty segments:
>  * half of them have 128 valid blocks per segment
>  * left of them have 384 valid blocks per segment
> - run background GC
> 
> Benefit: GC count and block movement count both decrease obviously:
> 
> - Before:
>   - Valid: 86
>   - Dirty: 1
>   - Prefree: 11
>   - Free: 6001 (6001)
> 
> GC calls: 162 (BG: 220)
>   - data segments : 160 (160)
>   - node segments : 2 (2)
> Try to move 41454 blocks (BG: 41454)
>   - data blocks : 40960 (40960)
>   - node blocks : 494 (494)
> 
> IPU: 0 blocks
> SSR: 0 blocks in 0 segments
> LFS: 41364 blocks in 81 segments
> 
> - After:
> 
>   - Valid: 87
>   - Dirty: 0
>   - Prefree: 4
>   - Free: 6008 (6008)
> 
> GC calls: 75 (BG: 76)
>   - data segments : 74 (74)
>   - node segments : 1 (1)
> Try to move 12813 blocks (BG: 12813)
>   - data blocks : 12544 (12544)
>   - node blocks : 269 (269)
> 
> IPU: 0 blocks
> SSR: 12032 blocks in 77 segments
> LFS: 855 blocks in 2 segments