Summary:
Add a config option to restrict the number of tree prefetches that can run in
parallel. Without this applications that recursively walk a checkout tree can
end up spawning a huge number of asynchronous prefetch operations. These
prefetch operations are quite expensive today, as we currently have to fetch
the full file contents in order to retrieve the file sizes. A recursive
directory walk can end up building up a huge backlog of in-progress tree
prefetches. This will slow down the directory walk, and it can take tens of
minutes to finish processing the prefetch backlog even after the directory
walk is aborted.
Reviewed By: chadaustin
Differential Revision: D19136685
fbshipit-source-id: cdc7a570d016fd7ca81a639cef83380b197acdfc