Re: [PATCH] lookup_object: remove hashtable_index() and optimize hash_obj()
On Wed, 11 Sep 2013, Jeff King wrote:
Show 15 quoted lines
> 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?
Show 29 quoted lines
> ---
> 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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> Nicolas Pitre <nico@fluxnic.net>