Re: [PATCH v2 4/4] unpack-trees: cheaper index update when walking by cache-tree

On Wed, Aug 8, 2018 at 8:46 PM Elijah Newren [off-list ref] wrote:

quoted

@@ -701,6 +702,24 @@ static int traverse_by_cache_tree(int pos, int nr_entries, int nr_names,
        if (!o->merge)
                BUG("We need cache-tree to do this optimization");

+       /*
+        * Try to keep add_index_entry() as fast as possible since
+        * we're going to do a lot of them.
+        *
+        * Skipping verify_path() should totally be safe because these
+        * paths are from the source index, which must have been
+        * verified.
+        *
+        * Skipping D/F and cache-tree validation checks is trickier
+        * because it assumes what n-merge code would do when all
+        * trees and the index are the same. We probably could just
+        * optimize those code instead (e.g. we don't invalidate that
+        * many cache-tree, but the searching for them is very
+        * expensive).
+        */
+       o->extra_add_index_flags = ADD_CACHE_SKIP_DFCHECK;
+       o->extra_add_index_flags |= ADD_CACHE_SKIP_VERIFY_PATH;
+

In sum of this whole patch, you notice that the Nway_merge functions
are still a bit of a bottleneck, but you know you have a special case
where you want them to put an entry in the index that matches what is
already there, so you try to set some extra flags to short-circuit
part of their logic and get to what you know is the correct result.

This seems a little scary to me.  I think it's probably safe as long
as o->fn is one of {oneway_merge, twoway_merge, threeway_merge,
bind_merge} (the cases you have in mind and which the current code
uses), but the caller isn't limited to those.  Right now in
diff-lib.c, there's a caller that has their own function, oneway_diff.
More could be added in the future.

If we're going to go this route, I think we should first check that
o->fn is one of those known safe functions.  And if we're going that
route, the comments I bring up on patch 2 about possibly avoiding
call_unpack_fn() altogether might even obviate this patch while
speeding things up more.

Yes I do need to check o->fn. I might have to think more about
avoiding call_unpack_fn(). Even if we avoid it though, we still go
through add_index_entry() and suffer the same checks every time unless
we do somethine like this (but then of course it's safer because
you're doing it in a specific x-way merge, not generic code like
this).

quoted

@@ -1561,6 +1581,13 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
                if (!ret) {
                        if (!o->result.cache_tree)
                                o->result.cache_tree = cache_tree();
+                       /*
+                        * TODO: Walk o.src_index->cache_tree, quickly check
+                        * if o->result.cache has the exact same content for
+                        * any valid cache-tree in o.src_index, then we can
+                        * just copy the cache-tree over instead of hashing a
+                        * new tree object.
+                        */

Interesting.  I really don't know how cache_tree works...but if we
avoided calling call_unpack_fn, and thus left the original index entry
in place instead of replacing it with an equal one, would that as a
side effect speed up the cache_tree_valid/cache_tree_update calls for
us?  Or is there still work here?

Naah. Notice that we don't care at all about the source's cache-tree
when we update o->result one (and we never ever do anything about
o->result's cache-tree during the merge). Whether you invalidate or
not, o->result's cache-tree is always empty and you still have to
recreate all cache-tree in o->result. You essentially play full cost
of "git write-tree" here if I'm not mistaken.
-- 
Duy