Messages in this thread | | | Date | Wed, 8 Feb 2017 17:28:40 -0800 | From | Jaegeuk Kim <> | Subject | Re: [PATCH] f2fs: introduce nid cache |
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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? :(
> 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.
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, > >
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