[PATCH] lookup_object: remove hashtable_index() and optimize hash_obj()

[PATCH] lookup_object: remove hashtable_index() and optimize hash_obj()

[Nicolas Pitre]

hashtable_index() appears to be a close duplicate of hash_obj().
Keep only the later and make it usable for all cases.

Also remove the modulus as this is an expansive operation.
The size argument is always a power of 2 anyway, so a simple
mask operation provides the same result.

On a 'git rev-list --all --objects' run this decreased the time spent
in lookup_object from 27.5% to 24.1%.

Signed-off-by: Nicolas Pitre <nico@fluxnic.net>
---

I discovered this patch in my git work tree dating from 2 years ago.

diff --git a/object.c b/object.c
index d8a4b1f..e2dae22 100644
--- a/object.c
+++ b/object.c
@@ -43,16 +43,16 @@ int type_from_string(const char *str)
 	die("invalid object type \"%s\"", str);
 }
 
-static unsigned int hash_obj(struct object *obj, unsigned int n)
+static unsigned int hash_obj(const unsigned char *sha1, unsigned int n)
 {
 	unsigned int hash;
-	memcpy(&hash, obj->sha1, sizeof(unsigned int));
-	return hash % n;
+	memcpy(&hash, sha1, sizeof(unsigned int));
+	return hash & (n - 1);
 }
 
 static void insert_obj_hash(struct object *obj, struct object **hash, unsigned int size)
 {
-	unsigned int j = hash_obj(obj, size);
+	unsigned int j = hash_obj(obj->sha1, size);
 
 	while (hash[j]) {
 		j++;
@@ -62,13 +62,6 @@ static void insert_obj_hash(struct object *obj, struct object **hash, unsigned i
 	hash[j] = obj;
 }
 
-static unsigned int hashtable_index(const unsigned char *sha1)
-{
-	unsigned int i;
-	memcpy(&i, sha1, sizeof(unsigned int));
-	return i % obj_hash_size;
-}
-
 struct object *lookup_object(const unsigned char *sha1)
 {
 	unsigned int i, first;
@@ -77,7 +70,7 @@ struct object *lookup_object(const unsigned char *sha1)
 	if (!obj_hash)
 		return NULL;
 
-	first = i = hashtable_index(sha1);
+	first = i = hash_obj(sha1, obj_hash_size);
 	while ((obj = obj_hash[i]) != NULL) {
 		if (!hashcmp(sha1, obj->sha1))
 			break;

Re: [PATCH] lookup_object: remove hashtable_index() and optimize hash_obj()

[Jeff King]

On Tue, Sep 10, 2013 at 06:17:12PM -0400, Nicolas Pitre wrote:

> hashtable_index() appears to be a close duplicate of hash_obj().
> Keep only the later and make it usable for all cases.

Thanks. This duplication has often bugged me when looking at that
hash table, but I just never actually wrote the patch.

> Also remove the modulus as this is an expansive operation.
> The size argument is always a power of 2 anyway, so a simple
> mask operation provides the same result.
> 
> On a 'git rev-list --all --objects' run this decreased the time spent
> in lookup_object from 27.5% to 24.1%.

Nice. This is a tiny bit subtle, though, as the power-of-2 growth
happens elsewhere, and we may want to tweak it later (the decorate.c
hash, for example, grows by 3/2).

Maybe it's worth squashing in one or both of the comments below as a
warning to anybody who tries to tweak it.

---
diff --git a/object.c b/object.c
index e2dae22..5f792cb 100644
--- a/object.c
+++ b/object.c
@@ -47,6 +47,7 @@ static unsigned int hash_obj(const unsigned char *sha1, unsigned int n)
 {
 	unsigned int hash;
 	memcpy(&hash, sha1, sizeof(unsigned int));
+	/* Assumes power-of-2 hash sizes in grow_object_hash */
 	return hash & (n - 1);
 }
 
@@ -94,6 +95,10 @@ static void grow_object_hash(void)
 static void grow_object_hash(void)
 {
 	int i;
+	/*
+	 * Note that this size must always be power-of-2 to match hash_obj
+	 * above.
+	 */
 	int new_hash_size = obj_hash_size < 32 ? 32 : 2 * obj_hash_size;
 	struct object **new_hash;
 

Re: [PATCH] lookup_object: remove hashtable_index() and optimize hash_obj()

[Nicolas Pitre]

On Wed, 11 Sep 2013, Jeff King wrote:

> On Tue, Sep 10, 2013 at 06:17:12PM -0400, Nicolas Pitre wrote:
> 
> > Also remove the modulus as this is an expansive operation.
> > The size argument is always a power of 2 anyway, so a simple
> > mask operation provides the same result.
> > 
> > On a 'git rev-list --all --objects' run this decreased the time spent
> > in lookup_object from 27.5% to 24.1%.
> 
> Nice. This is a tiny bit subtle, though, as the power-of-2 growth
> happens elsewhere, and we may want to tweak it later (the decorate.c
> hash, for example, grows by 3/2).
> 
> Maybe it's worth squashing in one or both of the comments below as a
> warning to anybody who tries to tweak it.

Agreed.

@Junio: are you willing to squash those in, or do you prefer a resent?

> ---
> diff --git a/object.c b/object.c
> index e2dae22..5f792cb 100644
> --- a/object.c
> +++ b/object.c
> @@ -47,6 +47,7 @@ static unsigned int hash_obj(const unsigned char *sha1, unsigned int n)
>  {
>  	unsigned int hash;
>  	memcpy(&hash, sha1, sizeof(unsigned int));
> +	/* Assumes power-of-2 hash sizes in grow_object_hash */
>  	return hash & (n - 1);
>  }
>  
> @@ -94,6 +95,10 @@ static void grow_object_hash(void)
>  static void grow_object_hash(void)
>  {
>  	int i;
> +	/*
> +	 * Note that this size must always be power-of-2 to match hash_obj
> +	 * above.
> +	 */
>  	int new_hash_size = obj_hash_size < 32 ? 32 : 2 * obj_hash_size;
>  	struct object **new_hash;
>  
> --
> To unsubscribe from this list: send the line "unsubscribe git" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> 

Re: [PATCH] lookup_object: remove hashtable_index() and optimize hash_obj()

[Junio C Hamano]

Nicolas Pitre <nico@fluxnic.net> writes:

>> Maybe it's worth squashing in one or both of the comments below as a
>> warning to anybody who tries to tweak it.
>
> Agreed.
>
> @Junio: are you willing to squash those in, or do you prefer a resent?

I think I've queued it ready to be squashed.  No need for resend.

Thanks.