Re: [PATCH 3/6] index-pack: remove redundant child field

[Derrick Stolee <stolee@gmail.com>]

On 10/9/2019 7:44 PM, Jonathan Tan wrote:
> Instead, recompute ancestry if we ever need to reclaim memory.

I find this message lacking in important details:

1. Where do we recompute ancestry?
2. What are the performance implications of this change?
3. Why is it important that you construct a stack of deltas in prune_base_data()?

> Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
> ---
>  builtin/index-pack.c | 41 ++++++++++++++++++++++-------------------
>  1 file changed, 22 insertions(+), 19 deletions(-)
> 
> diff --git a/builtin/index-pack.c b/builtin/index-pack.c
> index 99f6e2957f..35f7f9e52b 100644
> --- a/builtin/index-pack.c
> +++ b/builtin/index-pack.c
> @@ -34,7 +34,6 @@ struct object_stat {
>  
>  struct base_data {
>  	struct base_data *base;
> -	struct base_data *child;
>  	struct object_entry *obj;
>  	void *data;
>  	unsigned long size;
> @@ -44,7 +43,6 @@ struct base_data {
>  
>  struct thread_local {
>  	pthread_t thread;
> -	struct base_data *base_cache;
>  	size_t base_cache_used;
>  	int pack_fd;
>  };
> @@ -380,27 +378,37 @@ static void free_base_data(struct base_data *c)
>  	}
>  }
>  
> -static void prune_base_data(struct base_data *retain)
> +static void prune_base_data(struct base_data *youngest_child)
>  {
>  	struct base_data *b;
>  	struct thread_local *data = get_thread_data();
> -	for (b = data->base_cache;
> -	     data->base_cache_used > delta_base_cache_limit && b;
> -	     b = b->child) {
> -		if (b->data && b != retain)
> -			free_base_data(b);
> +	struct base_data **ancestry = NULL;
> +	int nr = 0, alloc = 0;
> +	int i;
> +
> +	if (data->base_cache_used <= delta_base_cache_limit)
> +		return;
> +
> +	/*
> +	 * Free all ancestors of youngest_child until we have enough space,
> +	 * starting with the oldest. (We cannot free youngest_child itself.)
> +	 */
> +	for (b = youngest_child->base; b != NULL; b = b->base) {
> +		ALLOC_GROW(ancestry, nr + 1, alloc);
> +		ancestry[nr++] = b;
> +	}
> +	for (i = nr - 1;
> +	     i >= 0 && data->base_cache_used > delta_base_cache_limit;
> +	     i--) {
> +		if (ancestry[i]->data)
> +			free_base_data(ancestry[i]);
>  	}
> +	free(ancestry);
>  }
>  
>  static void link_base_data(struct base_data *base, struct base_data *c)
>  {
> -	if (base)
> -		base->child = c;
> -	else
> -		get_thread_data()->base_cache = c;
> -
>  	c->base = base;
> -	c->child = NULL;
>  	if (c->data)
>  		get_thread_data()->base_cache_used += c->size;
>  	prune_base_data(c);
> @@ -408,11 +416,6 @@ static void link_base_data(struct base_data *base, struct base_data *c)
>  
>  static void unlink_base_data(struct base_data *c)
>  {
> -	struct base_data *base = c->base;
> -	if (base)
> -		base->child = NULL;
> -	else
> -		get_thread_data()->base_cache = NULL;
>  	free_base_data(c);
>  }

Seems like this method should be removed and all callers should
call free_base_data() instead.

-Stolee