On 2020/3/14 上午5:09, Josef Bacik wrote:
> In debugging Zygo's huge commit delays I noticed we were burning a bunch of time
> doing READA in cases where we don't need to.  The way READA works in btrfs is
> we'll load up adjacent nodes and leaves as we walk down.  This is useful for
> operations where we're going to be reading sequentially across the tree.
> 
> But for delayed refs we're looking up one bytenr, and then another one which
> could be elsewhere in the tree.  With large enough extent trees this results in
> a lot of unneeded latency.
> 
> The same applies to build_backref_tree, but that's even worse because we're
> looking up backrefs, which are essentially randomly spread out across the extent
> root.  Thanks,

There are quite some other locations abusing READA.

E.g. btrfs_read_block_groups(), where we're just searching for block
group items. There is no guarantee that next block group item is in next
a few leaves.

I guess it's a good time to review all READA abuse. Or would you mind me
to do that?

Thanks,
Qu
> 
> Josef
>