On Sat, Mar 14, 2020 at 10:56:06AM +0800, Qu Wenruo wrote: > > > 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? If you find some clear example where the items are scattered over the tree then yes. For the rest it would be good to put a comment that the readahead really helps. Thanks.
