If doing an insert workload in WiredTiger with an append characteristic (and not bulk loading), the tree tends to grow deeper and deeper with WT_PM_SPLIT_MERGE pages. This happens because as we load items into the tree pages hit the memory_page_max limit and we force evict them. Each time a page is force evicted we create a new split merge parent page - that increases the depth of the tree.

We have code that manages the depth of split merge sub-trees by merging them together, but ideally the tree would not grow as deep.

One possible option would be to:

• Attempt to lock the parent page when doing a forced eviction
• If we don't get the lock, do what we do now
• If we do get the lock, then split the child page as before, but update the existing parent with the new child, rather than swapping in a new split-merge page.

I've attached a picture that attempts to explain more clearly.

NOTES:
This problem isn't isolated to append only workloads - though it's simpler to reason about the problem with an append workload. Any solution should be extendable to general insert workloads (e.g: hot spots in the middle of the tree).

To reproduce trees with deep split merge sub-trees you can use the following Python code snippet:

        self.session.create(self.uri, 'value_format=S,key_format=S')
        cursor = self.session.open_cursor(self.uri, None)
        for i in range(1, 10000000):
            cursor.set_key(key_populate(cursor, i))
            cursor.set_value(('%010d' % i) * 400)
            cursor.insert()
        cursor.close()