Future proof benchmark for bg-tree.

The benchmark in the cover letter is in fact a pretty bad case for
original mode, with a lot of EXTENT_ITEMs bumping up extent tree height,
along with small node size to make searching extent tree super slow.
(Although that doesn't make any difference for bg-tree).


Now here is the best case scenario for original mode.
Just using plain fallocate to fill 12T, so every EXTENT_DATA will be at
its maximum size, causing minimal noise for block group items iteration.

For short conclusion:
Bg-tree still faster than best case original extent tree, by around 30%.
Bg-tree should be fast enough to mount the 12T filled fs less than *ONE*
*SECOND*.

And due to the fact that bg-tree doesn't care how crowded the extent
tree is, its block group iteration time is just O(N).
So for 12T filled fs, bg-tree should always mount around the same speed,
no matter how many extents are.

For full details, please check the sheet:

https://docs.google.com/spreadsheets/d/1YfZXiGoL9EoPcWGh0x1gIldS1ymsPgqZFQdngM-Iep0/edit?usp=sharing


Thanks,
Qu

On 2019/1/2 下午1:29, Qu Wenruo wrote:
> This patchset can be fetched from:
> https://github.com/adam900710/linux/tree/bg_tree
> Which is based on v4.20-rc1 tag.
> 
> This patchset will hugely reduce mount time of large fs by putting all
> block group items into its own tree.
> 
> The old behavior will try to read out all block group items at mount
> time, however due to the key of block group items are scattered across
> tons of extent items, we must call btrfs_search_slot() for each block
> group.
> 
> It works fine for small fs, but when number of block groups goes beyond
> 200, such tree search will become a random read, causing obvious slow
> down.
> 
> On the other hand, btrfs_read_chunk_tree() is still very fast, since we
> put CHUNK_ITEMS into their own tree and package them next to each other.
> 
> 
> Following this idea, we could do the same thing for block group items,
> so instead of triggering btrfs_search_slot() for each block group, we
> just call btrfs_next_item() and under most case we could finish in
> memory, and hugely speed up mount (see BENCHMARK below).
> 
> The only disadvantage is, this method introduce an incompatible feature,
> so existing fs can't use this feature directly.
> Either specify it at mkfs time, or use btrfs-progs offline convert tool
> (*).
> 
> *: Mkfs and convert tool are doing the same work, however I haven't
> decide if I should put this feature to btrfstune.
> 
> [[Benchmark]]
> Physical device:	HDD (7200RPM)
> Nodesize:		4K  (to bump up tree height)
> Used size:		250G
> Total size:		500G
> Extent data size:	1M
> 
> All file extents on disk is in 1M size, ensured by using fallocate.
> 
> Without patchset:
> Use ftrace function graph:
> 
>   3)               |  open_ctree [btrfs]() {
>   3)               |    btrfs_read_chunk_tree [btrfs]() {
>   3) * 69033.31 us |    }
>   3)               |    btrfs_verify_dev_extents [btrfs]() {
>   3) * 90376.15 us |    }
>   3)               |    btrfs_read_block_groups [btrfs]() {
>   2) $ 2733853 us  |    } /* btrfs_read_block_groups [btrfs] */
>   2) $ 3168384 us  |  } /* open_ctree [btrfs] */
> 
>  btrfs_read_block_groups() takes 87% of the total mount time,
> 
> With patchset, and use -O bg-tree mkfs option:
>   7)               |  open_ctree [btrfs]() {
>   7)               |    btrfs_read_chunk_tree [btrfs]() {
>   7) # 2448.562 us |    }
>   7)               |    btrfs_verify_dev_extents [btrfs]() {
>   7) * 19802.02 us |    }
>   7)               |    btrfs_read_block_groups [btrfs]() {
>   7) # 8610.397 us |    }
>   7) @ 113498.6 us |  }
> 
>   open_ctree() time is only 3% of original mount time.
>   And btrfs_read_block_groups() only takes 7.6% of total open_ctree()
>   execution time.
> 
> Changelog:
> RFC->v1:
>  - Fix memory leak for fs_info->bg_root at module unload time.
>  - Add sysfs features interface.
>  - Testing shows no regression, so no RFC tag now.
> 
> Qu Wenruo (2):
>   btrfs: Refactor btrfs_read_block_groups()
>   btrfs: Introduce new incompat feature, BG_TREE
> 
>  fs/btrfs/ctree.h                |   5 +-
>  fs/btrfs/disk-io.c              |  13 ++
>  fs/btrfs/extent-tree.c          | 300 ++++++++++++++++++++------------
>  fs/btrfs/sysfs.c                |   2 +
>  include/uapi/linux/btrfs.h      |   1 +
>  include/uapi/linux/btrfs_tree.h |   3 +
>  6 files changed, 208 insertions(+), 116 deletions(-)
>