Re: [GSoC Patch 1/3] commit: introduce helpers for generation slab

From: Derrick Stolee <stolee@gmail.com>
To: Abhishek Kumar <abhishekkumar8222@gmail.com>, git@vger.kernel.org
Cc: jnareb@gmail.com
Subject: Re: [GSoC Patch 1/3] commit: introduce helpers for generation slab
Date: Thu, 4 Jun 2020 10:36:49 -0400	[thread overview]
Message-ID: <be28ab7b-0ae4-2cc5-7f2b-92075de3723a@gmail.com> (raw)
In-Reply-To: <20200604072759.19142-2-abhishekkumar8222@gmail.com>

On 6/4/2020 3:27 AM, Abhishek Kumar wrote:
> The struct member generation refers to "generation number" (or more
> broadly, a reachablity index value) used by commit-graph to reduce time
> taken to walk commits. However, generation is not useful in other
> contexts and bloats the struct.
> 
> Let's move it to a commit-slab and shrink the struct by four bytes.
> 
> Signed-off-by: Abhishek Kumar <abhishekkumar8222@gmail.com>
> ---
>  commit-graph.c | 27 +++++++++++++++++++++++++++
>  commit-graph.h |  5 +++++
>  commit.h       |  3 ---
>  3 files changed, 32 insertions(+), 3 deletions(-)
> 
> diff --git a/commit-graph.c b/commit-graph.c
> index e3420ddcbf..63f419048d 100644
> --- a/commit-graph.c
> +++ b/commit-graph.c
> @@ -87,6 +87,33 @@ static int commit_pos_cmp(const void *va, const void *vb)
>  	       commit_pos_at(&commit_pos, b);
>  }
>  
> +define_commit_slab(generation_slab, uint32_t);
> +static struct generation_slab generation_slab = COMMIT_SLAB_INIT(1, generation_slab);
> +
> +uint32_t generation(const struct commit *c)
> +{
> +	uint32_t *gen = generation_slab_peek(&generation_slab, c);
> +
> +	return gen ? *gen : GENERATION_NUMBER_INFINITY;
> +}

This is good: if we don't have the value, then use INFINITY.
In the header file, perhaps include a warning comment that a
caller _must_ first parse the commit or else we have no guarantee
that the generation slab is populated. This matches the current
expectations before accessing the generation member.

> +static void set_generation(const struct commit *c, const uint32_t generation)
> +{
> +	unsigned int i = generation_slab.slab_count;
> +	uint32_t *gen = generation_slab_at(&generation_slab, c);
> +
> +	/*
> +	 * commit-slab initializes with zero, overwrite this with
> +	 * GENERATION_NUMBER_INFINITY
> +	 */
> +	for (; i < generation_slab.slab_count; ++i) {
> +		memset(generation_slab.slab[i], GENERATION_NUMBER_INFINITY,
> +		       generation_slab.slab_size * sizeof(uint32_t));
> +	}

Here is an example where combining the graph_pos and generation
slabs into one would be helpful. The only reason the generation
would be INFINITY is if graph_pos is COMMIT_NOT_FROM_GRAPH. If
the two values are side-by-side, we could just check graph_pos
first and return INFINITY instead of paying this initialization
cost as the slab grows.

I would also like to avoid initializing the slab if there is
no commit-graph present. I wonder if we can populate the slab
while parsing the commit-graph and check here if the slab is
NULL before doing any other logic? (I'm not sure if this is
possible, but it would be nice.)

> diff --git a/commit-graph.h b/commit-graph.h
> index 4212766a4f..653bd041ad 100644
> --- a/commit-graph.h
> +++ b/commit-graph.h
> @@ -8,6 +8,10 @@
>  #include "object-store.h"
>  #include "oidset.h"
>  
> +#define GENERATION_NUMBER_INFINITY 0xFFFFFFFF
> +#define GENERATION_NUMBER_MAX 0x3FFFFFFF
> +#define GENERATION_NUMBER_ZERO 0
> +
>  #define GIT_TEST_COMMIT_GRAPH "GIT_TEST_COMMIT_GRAPH"
>  #define GIT_TEST_COMMIT_GRAPH_DIE_ON_LOAD "GIT_TEST_COMMIT_GRAPH_DIE_ON_LOAD"
>  #define GIT_TEST_COMMIT_GRAPH_CHANGED_PATHS "GIT_TEST_COMMIT_GRAPH_CHANGED_PATHS"
> @@ -137,4 +141,5 @@ void free_commit_graph(struct commit_graph *);
>   */
>  void disable_commit_graph(struct repository *r);
>  
> +uint32_t generation(const struct commit *c);
>  #endif
> diff --git a/commit.h b/commit.h
> index 1b2dea5d85..cc610400d5 100644
> --- a/commit.h
> +++ b/commit.h
> @@ -11,9 +11,6 @@
>  #include "commit-slab.h"
>  
>  #define COMMIT_NOT_FROM_GRAPH 0xFFFFFFFF
> -#define GENERATION_NUMBER_INFINITY 0xFFFFFFFF
> -#define GENERATION_NUMBER_MAX 0x3FFFFFFF
> -#define GENERATION_NUMBER_ZERO 0

I appreciate that you are able to relocate these constants to
a more appropriate location.

Thanks,
-Stolee