Re: [PATCH] f2fs: introduce nid cache

[Chao Yu]

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

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,
>>>
> 
> .
>