Subject: Re: [PATCH 12/17] Documentation: describe split commit-graphs

From: SZEDER Gábor <[email-masked]>
Date: Wed, 8 May 2019 19:20:29 +0200

On Wed, May 08, 2019 at 08:53:57AM -0700, Derrick Stolee via GitGitGadget wrote:
> From: Derrick Stolee <[email-masked]>
> 
> The design for the split commit-graphs uses file names to force
> a "stack" of commit-graph files. This allows incremental writes
> without updating the file format.
> 
> Signed-off-by: Derrick Stolee <[email-masked]>
> ---
>  Documentation/technical/commit-graph.txt | 142 +++++++++++++++++++++++
>  1 file changed, 142 insertions(+)
> 
> diff --git a/Documentation/technical/commit-graph.txt b/Documentation/technical/commit-graph.txt
> index fb53341d5e..ca1661d2d8 100644
> --- a/Documentation/technical/commit-graph.txt
> +++ b/Documentation/technical/commit-graph.txt
> @@ -127,6 +127,148 @@ Design Details
>    helpful for these clones, anyway. The commit-graph will not be read or
>    written when shallow commits are present.
>  
> +Split Commit Graphs
> +-------------------
> +
> +Typically, repos grow with near-constant velocity (commits per day). Over
> +time, the number of commits added by a fetch operation is much smaller than
> +the number of commits in the full history. The split commit-graph feature
> +allows for fast writes of new commit data without rewriting the entire
> +commit history -- at least, most of the time.
> +
> +## File Layout
> +
> +A split commit-graph uses multiple files, and we use a fixed naming
> +convention to organize these files. The base commit-graph file is the
> +same: `$OBJDIR/info/commit-graph`. The rest of the commit-graph files have
> +the format `$OBJDIR/info/commit-graphs/commit-graph-<N>` where N is a
> +positive integer. The integers must start at 1 and grow sequentially
> +to form a stack of files.
> +
> +Each `commit-graph-<N>` file has the same format as the `commit-graph`
> +file, including a lexicographic list of commit ids. The only difference
> +is that this list is considered to be concatenated to the list from
> +the lower commit-graphs. As an example, consider this diagram of three
> +files:
> +
> + +-----------------------+
> + |  commit-graph-2       |
> + +-----------------------+
> +	  |
> + +-----------------------+
> + |                       |
> + |  commit-graph-1       |
> + |                       |
> + +-----------------------+
> +	  |
> + +-----------------------+
> + |                       |
> + |                       |
> + |                       |
> + |  commit-graph         |
> + |                       |
> + |                       |
> + |                       |
> + +-----------------------+
> +
> +Let X0 be the number of commits in `commit-graph`, X1 be the number
> +of commits in commit-graph-1, and X2 be the number of commits in
> +commit-graph-2. If a commit appears in position i in `commit-graph-2`,
> +then we interpret this as being the commit in position (X0 + X1 + i),
> +and that will be used as its "graph position". The commits in
> +commit-graph-2 use these positions to refer to their parents, which
> +may be in commit-graph-1 or commit-graph. We can navigate to an
> +arbitrary commit in position j by checking its containment in the
> +intervals [0, X0), [X0, X0 + X1), [X0 + X1, X0 + X1 + X2).
> +
> +When Git reads from these files, it starts by acquiring a read handle
> +on the `commit-graph` file. On success, it continues acquiring read
> +handles on the `commit-graph-<N>` files in increasing order. This
> +order is important for how we replace the files.
> +
> +## Merging commit-graph files
> +
> +If we only added a `commit-graph-<N>` file on every write, we would
> +run into a linear search problem through many commit-graph files.
> +Instead, we use a merge strategy to decide when the stack should
> +collapse some number of levels.
> +
> +The diagram below shows such a collapse. As a set of new commits
> +are added, it is determined by the merge strategy that the files
> +should collapse to `commit-graph-1`. Thus, the new commits, the
> +commits in `commit-graph-2` and the commits in `commit-graph-1`
> +should be combined into a new `commit-graph-1` file.
> +
> +			    +---------------------+
> +			    |                     |
> +			    |    (new commits)    |
> +			    |                     |
> +			    +---------------------+
> +			    |                     |
> + +-----------------------+  +---------------------+
> + |  commit-graph-2       |->|                     |
> + +-----------------------+  +---------------------+
> +	  |                 |                     |
> + +-----------------------+  +---------------------+
> + |                       |  |                     |
> + |  commit-graph-1       |->|                     |
> + |                       |  |                     |
> + +-----------------------+  +---------------------+
> +	  |                   commit-graph-1.lock
> + +-----------------------+
> + |                       |
> + |                       |
> + |                       |
> + |  commit-graph         |
> + |                       |
> + |                       |
> + |                       |
> + +-----------------------+
> +
> +During this process, the commits to write are combined, sorted
> +and we write the contents to the `commit-graph-1.lock` file.
> +When the file is flushed and ready to swap to `commit-graph-1`,
> +we first unlink the files above our target file. This unlinking
> +is done from the top of the stack, the reverse direction that
> +another process would use to read the stack.
> +
> +During this time window, another process trying to read the
> +commit-graph stack could read `commit-graph-1` before the swap
> +but try to read `commit-graph-2` after it is unlinked. That
> +process would then believe that this stack is complete, but
> +will miss out on the performance benefits of the commits in
> +`commit-graph-2`. For this reason, the stack above the
> +`commit-graph` file should be small.

Consider the following sequence of events:

  1. There are three commit-graph files in the repository.

  2. A git process opens the base commit-graph and commit-graph-1 for
     reading.  It doesn't yet open commit-graph-2, because the (for
     arguments sake not very fair) scheduler takes the CPU away.

  3. Meanwhile, a 'git fetch', well, fetches from a remote, and
     upon noticing that it got a lot of commits it decides to collapse
     commit-graph-1 and -2 and the new commits, writing a brand new
     commit-graph-1.

  4. A second fetch fetches from a second remote, and writes
     commit-graph-2 (no collapsing this time).

  5. Now the crappy scheduler finally decides that it's time to wake
     up the waiting git process from step 2, which then finds the new
     commit-graph-2 file and opens it for reading.

  6. At this point this poor git process has file handles for:
  
     - the base commit-graph file, which is unchanged.

     - the old commit-graph-1 which has since been replaced, and does
       not yet contain info about the old commit-graph-2 or the
       commits received in the first fetch.

     - the new commit-graph-2, containing info only about commits
       received in the second fetch, and whose parents' graph
       positions point either to the base commitg-graph (good, since
       unchanged) or to the new commit-graph-1 (uh-oh).

What happens next?  If this process tries to access the parent of a
commit from commit-graph-2, and the metadata about this parent is in
the new commit-graph-1, then I expect all kinds of weird bugs.

But will a git process ever try to access a commit that didn't yet
existed in the repository when it started opening the commit-graph
files?

> +## Merge Strategy
> +
> +When writing a set of commits that do not exist in the
> +commit-graph stack of height N, we default to creating
> +a new file at level N + 1. We then decide to merge
> +with the Nth level if one of two conditions hold:
> +
> +  1. The expected file size for level N + 1 is at
> +     least half the file size for level N.
> +
> +  2. Level N + 1 contains more than MAX_SPLIT_COMMITS
> +     commits (64,0000 commits).
> +
> +This decision cascades down the levels: when we
> +merge a level we create a new set of commits that
> +then compares to the next level.
> +
> +The first condition bounds the number of levels
> +to be logarithmic in the total number of commits.
> +The second condition bounds the total number of
> +commits in a `commit-graph-N` file and not in
> +the `commit-graph` file, preventing significant
> +performance issues when the stack merges and another
> +process only partially reads the previous stack.
> +
> +The merge strategy values (2 for the size multiple,
> +64,000 for the maximum number of commits) could be
> +extracted into config settings for full flexibility.
> +
>  Related Links
>  -------------
>  [0] https://bugs.chromium.org/p/git/issues/detail?id=8
> -- 
> gitgitgadget
>