On 2017/9/25 10:52, Chao Yu wrote:
> On 2017/9/23 17:02, Yunlong Song wrote:
>> This reverts commit b9cd20619e359d199b755543474c3d853c8e3415.
>>
>> That patch causes much fewer node segments (which can be used for SSR)
>> than before, and in the corner case (e.g. create and delete *.txt files in
>> one same directory, there will be very few node segments but many data
>> segments), if the reserved free segments are all used up during gc, then
>> the write_checkpoint can still flush dentry pages to data ssr segments,
>> but will probably fail to flush node pages to node ssr segments, since
>> there are not enough node ssr segments left (the left ones are all
>> full).
>
> IMO, greedy algorithm wants to minimize price of moving one dirty segment, our
> behavior is accord with semantics of our algorithm to select victim with least
> valid blocks. Pengyang's patch tries to adjust greedy algorithm to consider
> minimizing total number of valid blocks in all selected victim segments during
> whole FGGC cycle, but its algorithm is corrupted, since if all valid data
> blocks
> in current victim segment is not belong to different dnode block, our
> selection
> may be incorrect.
>
> Anyway, I agree to revert Pengyang's patch first before we got a entire
> scheme.
Please replace old commit message with above reason. :)
Thanks,
>
> BTW, for SSR or LFS selection, there is a trade-off in between: a) SSR-write
> costs less free segment and move less data/node blocks, but it triggers random
> write which results in bad performance. b) LFS-write costs more free segment
> and
> move more data/node blocks, but it triggers sequential write which results in
> good performance. So I don't think more SSR we trigger, lower latency our FGGC
> faces.
>
> Thanks,
>
>>
>> So revert this patch to give a fair chance to let node segments remain
>> for SSR, which provides more robustness for corner cases.
>>
>> Conflicts:
>> fs/f2fs/gc.c
>> ---
>> fs/f2fs/gc.c | 12 +-----------
>> 1 file changed, 1 insertion(+), 11 deletions(-)
>>
>> diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
>> index bfe6a8c..f777e07 100644
>> --- a/fs/f2fs/gc.c
>> +++ b/fs/f2fs/gc.c
>> @@ -267,16 +267,6 @@ static unsigned int get_cb_cost(struct f2fs_sb_info
>> *sbi, unsigned int segno)
>> return UINT_MAX - ((100 * (100 - u) * age) / (100 + u));
>> }
>>
>> -static unsigned int get_greedy_cost(struct f2fs_sb_info *sbi,
>> - unsigned int segno)
>> -{
>> - unsigned int valid_blocks =
>> - get_valid_blocks(sbi, segno, true);
>> -
>> - return IS_DATASEG(get_seg_entry(sbi, segno)->type) ?
>> - valid_blocks * 2 : valid_blocks;
>> -}
>> -
>> static inline unsigned int get_gc_cost(struct f2fs_sb_info *sbi,
>> unsigned int segno, struct victim_sel_policy *p)
>> {
>> @@ -285,7 +275,7 @@ static inline unsigned int get_gc_cost(struct
>> f2fs_sb_info *sbi,
>>
>> /* alloc_mode == LFS */
>> if (p->gc_mode == GC_GREEDY)
>> - return get_greedy_cost(sbi, segno);
>> + return get_valid_blocks(sbi, segno, true);
>> else
>> return get_cb_cost(sbi, segno);
>> }
>>
