On Wed, Jun 7, 2017 at 1:58 AM, Timofey Titovets <nefelim4ag@xxxxxxxxx> wrote:
> In worst case code do 4 comparison,
> just add some new checks to switch check branch faster
> now in worst case code do 3 comparison
Some comments below.
>
> Signed-off-by: Timofey Titovets <nefelim4ag@xxxxxxxxx>
> ---
> fs/btrfs/ctree.c | 20 +++++++++++---------
> 1 file changed, 11 insertions(+), 9 deletions(-)
>
> diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
> index a3a75f1de..318379531 100644
> --- a/fs/btrfs/ctree.c
> +++ b/fs/btrfs/ctree.c
> @@ -460,15 +460,17 @@ __tree_mod_log_insert(struct btrfs_fs_info *fs_info, struct tree_mod_elem *tm)
> while (*new) {
> cur = rb_entry(*new, struct tree_mod_elem, node);
> parent = *new;
> - if (cur->logical < tm->logical)
> - new = &((*new)->rb_left);
> - else if (cur->logical > tm->logical)
> - new = &((*new)->rb_right);
> - else if (cur->seq < tm->seq)
> - new = &((*new)->rb_left);
> - else if (cur->seq > tm->seq)
> - new = &((*new)->rb_right);
> - else
> + if (cur->logical != tm->logical) {
> + if (cur->logical < tm->logical)
> + new = &((*new)->rb_left);
So now when cur->logical is less then tm->logical, we do 2 comparisons
instead of 1.
Is this case much less common always? If it were it could be a good change.
What guarantees that this change will give better performance for
common workloads? What guarantees you the example I just gave isn't
typical for most/many common workloads?
And most importantly, did you actually do some benchmarks that justify
this change? Is this a hot code path that would benefit from such
changes?
(Since it's the tree mod log, seldom used, I really doubt it).
Same comments apply to all the other similar patches you have sent.
thanks
> + else
> + new = &((*new)->rb_right);
> + } else if (cur->seq != tm->seq) {
> + if (cur->seq < tm->seq)
> + new = &((*new)->rb_left);
> + else
> + new = &((*new)->rb_right);
> + } else
> return -EEXIST;
> }
>
> --
> 2.13.0
>
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--
Filipe David Manana,
“Whether you think you can, or you think you can't — you're right.”
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