From: Derrick Stolee <stolee@gmail.com>
Date: Tue, 03 Apr 2018 12:15:09 GMT
Subject: Re: [PATCH 0/3] Lazy-load trees when reading commit-graph
Message-ID: <00d5cd22-3492-73cf-3493-1f6f1ec15922@gmail.com>
In-Reply-To: <20180403120057.173849-1-dstolee@microsoft.com>

On 4/3/2018 8:00 AM, Derrick Stolee wrote:
> There are several commit-graph walks that require loading many commits
> but never walk the trees reachable from those commits. However, the
> current logic in parse_commit() requires the root tree to be loaded.
> This only uses lookup_tree(), but when reading commits from the commit-
> graph file, the hashcpy() to load the root tree hash and the time spent
> checking the object cache take more time than parsing the rest of the
> commit.
>
> In this patch series, all direct references to accessing the 'tree'
> member of struct commit are replaced instead by one of the following
> methods:
>
> 	struct tree *get_commit_tree(struct commit *)
> 	struct object_id *get_commit_tree_oid(struct commit *)
>
> This replacement was assisted by a Coccinelle script, but the 'tree'
> member is overloaded in other types, so the script gave false-positives
> that were removed from the diff.
>
> After all access is restricted to use these methods, we can then
> change the postcondition of parse_commit_in_graph() to allow 'tree'
> to be NULL. If the tree is accessed later, we can load the tree's
> OID from the commit-graph in constant time and perform the lookup_tree().
>
> On the Linux repository, performance tests were run for the following
> command:
>
>      git log --graph --oneline -1000
>
> Before: 0.83s
> After:  0.65s
> Rel %: -21.6%
>
> Adding '-- kernel/' to the command requires loading the root tree
> for every commit that is walked. There was no measureable performance
> change as a result of this patch.
>
> This patch series depends on v7 of ds/commit-graph.
>
> Derrick Stolee (3):
>    commit: create get_commit_tree() method
>    treewide: use get_commit_tree() for tree access
>    commit-graph: lazy-load trees
>

This patch series is also available as a GitHub pull request [1]

[1] https://github.com/derrickstolee/git/pull/4