Hi Jaegeuk, On 2017/12/20 10:31, Jaegeuk Kim wrote: > On 12/20, Gaoxiang (OS) wrote: >> Hi Jaegeuk, >> >> On 2017/12/20 7:40, Jaegeuk Kim wrote: >>> On 12/17, gaoxiang (P) wrote: >>>> clean up some redundant repeat code in build_sit_entries >>>> and seperate some logic into new functions: >>>> - build_discard_entries >>>> - build_sec_entries >>> >>> This patch makes another big loop redundantly? >>> >> "condition & jump" is the fundamental of "if" and loop. >> In the worst case, both f2fs_discard_en(sbi) and sbi->segs_per_sec > 1 >> are true, and I think, we have: >> - the original approach: >> MAIN_SEGS <-- loop condition >> MAIN_SEGS <-- f2fs_discard_en(sbi) >> MAIN_SEGS <-- sbi->segs_per_sec > 1 >> >> sits_in_cursum(journal) <-- loop_condition >> sits_in_cursum(journal) <-- f2fs_discard_en(sbi) >> sits_in_cursum(journal) <-- sbi->segs_per_sec > 1 >> it takes about 3*(MAIN_SEGS + sits_in_cursum(journal)) jumps effort. >> - this patch: >> MAIN_SEGS <-- loop condition >> sits_in_cursum(journal) <-- loop_condition >> >> MAIN_SEGS <-- f2fs_discard_en(sbi) >> MAIN_SEGS <-- sbi->segs_per_sec > 1 >> it takes abourt 3*MAIN_SEGS + sits_in_cursum(journal) jumps effort. >> >> and in the best case, both f2fs_discard_en(sbi) and sbi->segs_per_sec > >> 1 are false, and we have: >> - this patch >> MAIN_SEGS <-- loop condition >> >> sits_in_cursum(journal) <-- loop_condition >> See >> https://stackoverflow.com/questions/1462710/can-c-compilers-optimize-if-statements-inside-for-loops > > Interesting! > > What I can think of the worst case looks like: > > In current code, > for (N) { > do_something; > if (f2fs_discard_en()) > do _something; > if (sbi->segs_per_sec > 1) > do_something; > } > for (sits_in_cursum()) { > do_something; > if (f2fs_discard_en()) > do _something; > if (sbi->segs_per_sec > 1) > do_something; > } > => O(N + sits_in_cursum()) In the worst case Its 3(1 -- exit condition < MAIN_SEG, 2 -- f2fs_discard_en(), 3 -- sbi->segs_per_sec > 1) *N + 3*sits_in_cursum() if condition & jump instuctions at runtime. If we use Big O notion, yes I think O(N + sits_in_cursum()) > > Once compiler unrolled the loop, we can expect most of jumps could be hit by > branch predictor, since the loop does not have many branches. The current code, the compiler could unroll the loop into, I guess for (N) { do_something; } if (f2fs_discard_en()) { do _something; } if (sbi->segs_per_sec > 1) { do_something; } ... for (sits_in_cursum()) { do_something; } if (f2fs_discard_en()) { do _something; } if (sbi->segs_per_sec > 1) { do_something; } Taking branch predictor into consideration, I think the unrolled one is more easier to predict than the current rolled one. (as you say, the current has more conditions in the loop and a exit condition to predict) > > In the patch, > for (N) > do_something; > for (sits_in_cursum()) > do_something; > > if (f2fs_discard_en()) { > for (N) > do_something; > } > if (sbi->segs_per_sec > 1) { > for (N) > do_something; > } > => O(3*N + sits_in_cursum()) If we use Big O notion, I think O(N + sits_in_cursum()) too. > > BTW, build_discard_entries(struct f2fs_sb_info *sbi) is more likely to split > the loops, as you describe. > > In build_discard_entries(), > for (N) { > if (CP_TRIMMED_FLAG) > do_something; > else > do_semething_else; > } > Thanks, > I think so, yet CP_TRIMMED_FLAG comes from a function argument, so compiler cannot know whether it is constant. I think if we count the runtime condition & jump instruction at runtime. the unrolls above are beneficial, I think. Thanks. >> >> In addtion, this approach is easier to read than the original >> one, so splitting the loop would be beneficial. >> >> Thanks >> >>>> >>>> Note that "if (f2fs_discard_en(sbi))" and "if (sbi->segs_per_sec > 1)" >>>> are all taken out of the loops they are unchangable. >>>> >>>> Signed-off-by: Gao Xiang <gaoxiang25@xxxxxxxxxx> >>>> --- >>>> fs/f2fs/segment.c | 80 +++++++++++++++++++++++++------------------------------ >>>> 1 file changed, 37 insertions(+), 43 deletions(-) >>>> >>>> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c >>>> index 40e1d20..bcd8a40 100644 >>>> --- a/fs/f2fs/segment.c >>>> +++ b/fs/f2fs/segment.c >>>> @@ -3476,12 +3476,39 @@ static int build_curseg(struct f2fs_sb_info *sbi) >>>> return restore_curseg_summaries(sbi); >>>> } >>>> >>>> +static void build_discard_entries(struct f2fs_sb_info *sbi) >>>> +{ >>>> + unsigned segno; >>>> + >>>> + for (segno = 0; segno < MAIN_SEGS(sbi); ++segno) { >>>> + struct seg_entry *se = get_seg_entry(sbi, segno); >>>> + >>>> + if (is_set_ckpt_flags(sbi, CP_TRIMMED_FLAG)) >>>> + memset(se->discard_map, 0xff, SIT_VBLOCK_MAP_SIZE); >>>> + else { >>>> + memcpy(se->discard_map, se->cur_valid_map, >>>> + SIT_VBLOCK_MAP_SIZE); >>>> + >>>> + sbi->discard_blks += sbi->blocks_per_seg - >>>> + se->valid_blocks; >>>> + } >>>> + } >>>> +} >>>> + >>>> +static void build_sec_entries(struct f2fs_sb_info *sbi) >>>> +{ >>>> + unsigned segno; >>>> + >>>> + for (segno = 0; segno < MAIN_SEGS(sbi); ++segno) >>>> + get_sec_entry(sbi, segno)->valid_blocks += >>>> + get_seg_entry(sbi, segno)->valid_blocks; >>>> +} >>>> + >>>> static void build_sit_entries(struct f2fs_sb_info *sbi) >>>> { >>>> struct sit_info *sit_i = SIT_I(sbi); >>>> struct curseg_info *curseg = CURSEG_I(sbi, CURSEG_COLD_DATA); >>>> struct f2fs_journal *journal = curseg->journal; >>>> - struct seg_entry *se; >>>> struct f2fs_sit_entry sit; >>>> int sit_blk_cnt = SIT_BLK_CNT(sbi); >>>> unsigned int i, start, end; >>>> @@ -3498,67 +3525,34 @@ static void build_sit_entries(struct f2fs_sb_info *sbi) >>>> struct f2fs_sit_block *sit_blk; >>>> struct page *page; >>>> >>>> - se = &sit_i->sentries[start]; >>>> page = get_current_sit_page(sbi, start); >>>> sit_blk = (struct f2fs_sit_block *)page_address(page); >>>> sit = sit_blk->entries[SIT_ENTRY_OFFSET(sit_i, start)]; >>>> f2fs_put_page(page, 1); >>>> >>>> check_block_count(sbi, start, &sit); >>>> - seg_info_from_raw_sit(se, &sit); >>>> - >>>> - /* build discard map only one time */ >>>> - if (f2fs_discard_en(sbi)) { >>>> - if (is_set_ckpt_flags(sbi, CP_TRIMMED_FLAG)) { >>>> - memset(se->discard_map, 0xff, >>>> - SIT_VBLOCK_MAP_SIZE); >>>> - } else { >>>> - memcpy(se->discard_map, >>>> - se->cur_valid_map, >>>> - SIT_VBLOCK_MAP_SIZE); >>>> - sbi->discard_blks += >>>> - sbi->blocks_per_seg - >>>> - se->valid_blocks; >>>> - } >>>> - } >>>> >>>> - if (sbi->segs_per_sec > 1) >>>> - get_sec_entry(sbi, start)->valid_blocks += >>>> - se->valid_blocks; >>>> + seg_info_from_raw_sit(&sit_i->sentries[start], &sit); >>>> } >>>> start_blk += readed; >>>> } while (start_blk < sit_blk_cnt); >>>> >>>> down_read(&curseg->journal_rwsem); >>>> for (i = 0; i < sits_in_cursum(journal); i++) { >>>> - unsigned int old_valid_blocks; >>>> - >>>> start = le32_to_cpu(segno_in_journal(journal, i)); >>>> - se = &sit_i->sentries[start]; >>>> sit = sit_in_journal(journal, i); >>>> >>>> - old_valid_blocks = se->valid_blocks; >>>> - >>>> check_block_count(sbi, start, &sit); >>>> - seg_info_from_raw_sit(se, &sit); >>>> - >>>> - if (f2fs_discard_en(sbi)) { >>>> - if (is_set_ckpt_flags(sbi, CP_TRIMMED_FLAG)) { >>>> - memset(se->discard_map, 0xff, >>>> - SIT_VBLOCK_MAP_SIZE); >>>> - } else { >>>> - memcpy(se->discard_map, se->cur_valid_map, >>>> - SIT_VBLOCK_MAP_SIZE); >>>> - sbi->discard_blks += old_valid_blocks - >>>> - se->valid_blocks; >>>> - } >>>> - } >>>> - >>>> - if (sbi->segs_per_sec > 1) >>>> - get_sec_entry(sbi, start)->valid_blocks += >>>> - se->valid_blocks - old_valid_blocks; >>>> + seg_info_from_raw_sit(&sit_i->sentries[start], &sit); >>>> } >>>> up_read(&curseg->journal_rwsem); >>>> + >>>> + /* build discard map only one time */ >>>> + if (f2fs_discard_en(sbi)) >>>> + build_discard_entries(sbi); >>>> + >>>> + if (sbi->segs_per_sec > 1) >>>> + build_sec_entries(sbi); >>>> } >>>> >>>> static void init_free_segmap(struct f2fs_sb_info *sbi) >>>> -- >>>> 2.1.4
