Subject: Re: [PATCH] refs.c: get_ref_cache: use a bucket hash

From: Thomas Gummerer <[email-masked]>
Date: Mon, 16 Mar 2015 18:19:10 +0100

Hi,

On 03/16, Andreas Krey wrote:
> get_ref_cache used a linear list, which obviously is O(n^2).
> Use a fixed bucket hash which just takes a factor of 100000
> (~ 317^2) out of the n^2 - which is enough.
>
> Signed-off-by: Andreas Krey <[email-masked]>
> ---
>
> This brings 'git clean -ndx' times down from 17 minutes
> to 11 seconds on one of our workspaces (which accumulated
> a lot of ignored directories). Actuallly using adaptive
> hashing or other structures seems overkill.
>
>  refs.c | 13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)
>
> diff --git a/refs.c b/refs.c
> index e23542b..8198d9e 100644
> --- a/refs.c
> +++ b/refs.c
> @@ -982,6 +982,8 @@ struct packed_ref_cache {
>  	struct stat_validity validity;
>  };
>
> +#define REF_CACHE_HASH 317
> +
>  /*
>   * Future: need to be in "struct repository"
>   * when doing a full libification.
> @@ -996,7 +998,7 @@ static struct ref_cache {
>  	 * is initialized correctly.
>  	 */
>  	char name[1];
> -} ref_cache, *submodule_ref_caches;
> +} ref_cache, *submodule_ref_caches[REF_CACHE_HASH];
>
>  /* Lock used for the main packed-refs file: */
>  static struct lock_file packlock;
> @@ -1065,18 +1067,19 @@ static struct ref_cache *create_ref_cache(const char *submodule)
>   */
>  static struct ref_cache *get_ref_cache(const char *submodule)
>  {
> -	struct ref_cache *refs;
> +	struct ref_cache *refs, **bucketp;
> +	bucketp = submodule_ref_caches + strhash(submodule) % REF_CACHE_HASH;
>

This breaks the test-suite for me, in the cases where submodule is
NULL.  How about something like this on top?

diff --git a/refs.c b/refs.c
index 8198d9e..311faf2 100644
--- a/refs.c
+++ b/refs.c
@@ -1068,7 +1068,9 @@ static struct ref_cache *create_ref_cache(const char *submodule)
 static struct ref_cache *get_ref_cache(const char *submodule)
 {
        struct ref_cache *refs, **bucketp;
-       bucketp = submodule_ref_caches + strhash(submodule) % REF_CACHE_HASH;
+       bucketp = submodule_ref_caches;
+       if (submodule)
+               bucketp += strhash(submodule) % REF_CACHE_HASH;

        if (!submodule || !*submodule)
                return &ref_cache;

>  	if (!submodule || !*submodule)
>  		return &ref_cache;
>
> -	for (refs = submodule_ref_caches; refs; refs = refs->next)
> +	for (refs = *bucketp; refs; refs = refs->next)
>  		if (!strcmp(submodule, refs->name))
>  			return refs;
>
>  	refs = create_ref_cache(submodule);
> -	refs->next = submodule_ref_caches;
> -	submodule_ref_caches = refs;
> +	refs->next = *bucketp;
> +	*bucketp = refs;
>  	return refs;
>  }
>
> --
> 2.3.2.223.g7a9409c
> --
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