On 02/11, Chao Yu wrote: > On 2017/2/9 9:28, Jaegeuk Kim wrote: > > On 02/08, Chao Yu wrote: > >> On 2017/2/7 15:24, Chao Yu wrote: > >>> Hi Jaegeuk, > >>> > >>> Happy Chinese New Year! :) > >>> > >>> On 2017/1/24 12:35, Jaegeuk Kim wrote: > >>>> Hi Chao, > >>>> > >>>> On 01/22, Chao Yu wrote: > >>>>> In scenario of intensively node allocation, free nids will be ran out > >>>>> soon, then it needs to stop to load free nids by traversing NAT blocks, > >>>>> in worse case, if NAT blocks does not be cached in memory, it generates > >>>>> IOs which slows down our foreground operations. > >>>>> > >>>>> In order to speed up node allocation, in this patch we introduce a new > >>>>> option named "nid cache", when turns on this option, it will load all > >>>>> nat entries in NAT blocks when doing mount, and organize all free nids > >>>>> in a bitmap, for any operations related to free nid, we will query and > >>>>> set the new prebuilded bitmap instead of reading and lookuping NAT > >>>>> blocks, so performance of node allocation can be improved. > >>>>> > >>>> > >>>> How does this affect mount time and memory consumption? > >>> > >>> Sorry for the delay. > >>> > >>> Let me figure out some numbers later. > >> > >> a. mount time > >> > >> I choose slow device (Kingston 16GB SD card) to see how this option affect > >> mount > >> time when there is not enough bandwidth in low level, > >> > >> Before the test, I change readahead window size of NAT pages from > >> FREE_NID_PAGES > >> * 8 to sbi->blocks_per_seg for better ra performance, so the result is: > >> > >> time mount -t f2fs -o nid_cache /dev/sde /mnt/f2fs/ > >> > >> before: > >> real 0m0.204s > >> user 0m0.004s > >> sys 0m0.020s > >> > >> after: > >> real 0m3.792s > > > > Oops, we can't accept this even only for 16GB, right? :( > > Pengyang Hou help testing this patch in 64GB UFS, the result of mount time is: > > Before: 110 ms > After: 770 ms > > So these test results shows that we'd better not set nid_cache option by > default > in upstream since anyway it slows down mount procedure obviously, but still > users can decide whether use it or not depending on their requirement. e.g.: > a. For readonly case, this option is complete no needed. > b. For in batch node allocation/deletion case, this option is recommended. > > > > >> user 0m0.000s > >> sys 0m0.140s > >> > >> b. memory consumption > >> > >> For 16GB size image, there is total 34 NAT pages, so memory footprint is: > >> 34 / 2 * 512 * 455 / 8 = 495040 bytes = 483.4 KB > >> > >> Increasing of memory footprint is liner with total user valid blocks in > >> image, > >> and at most it will eat 3900 * 8 * 455 / 8 = 1774500 bytes = 1732.9 KB > > > > How about adding two bitmaps for whole NAT pages and storing the bitmaps in > > checkpoint pack, which needs at most two blocks additionally? > > > > 1. full-assigned NAT bitmap, where 1 means there is no free nids. > > 2. empty NAT bitmap, where 1 means whole there-in nids are free. > > > > With these bitmaps, build_free_nids() can scan from 0'th NAT block by: > > > > if (full-assigned NAT) > > skip; > > else if (empty NAT) > > add_free_nid(all); > > else > > read NAT page and add_free_nid(); > > > > The flush_nat_entries() has to change its bitmaps accordingly. > > > > With this approach, I expect we can reuse nids as much as possible while > > getting cached NAT pages more effectively. > > Good idea! :) > > And there is another approach which do not need to change disk layout is: > > We can allocate free_nid_bitmap[NAT_BLOCKS_COUNT][455] array, each bitmap > indicates usage of free nids in one NAT block, and we introduce another > nat_block_bitmap[NAT_BLOCKS_COUNT] to indicate each NAT block is loaded or > not, > if it is loaded and we can do lookup in free_nid_bitmap correspondingly. So I > expect that we will load one NAT block from disk one time at most, it will: > - not increase mount latency > - after loading NAT blocks from disk, we will build its bitmap inside memory > to > reduce lookup time for second time
Yup, I think both of them are doable together. Meanwhile, I've written patches which support the bitmaps for NAT pages and started to test them. Could you write a patch to introduce free_nid_bitmap[][] as well? Thanks, > > Thoughts? Which one is preferred? > > Thanks, > > > > > Thanks, > > > >> > >> Thanks, > >> > >>> > >>>> IMO, if those do not > >>>> raise huge concerns, we would be able to consider just replacing current > >>>> free > >>>> nid list with this bitmap. > >>> > >>> Yup, I agree with you. > >>> > >>> Thanks, > >>> > > > > . > >
