Re: [PATCH] lookup_object: remove hashtable_index() and optimize hash_obj()
Nicolas Pitre n...@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. -- To unsubscribe from this list: send the line unsubscribe git in the body of a message to majord...@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()
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;
--
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Re: [PATCH] lookup_object: remove hashtable_index() and optimize hash_obj()
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;
--
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[PATCH] lookup_object: remove hashtable_index() and optimize hash_obj()
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 n...@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;
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