Re: [PATCH v1] unpack-trees: avoid duplicate ODB lookups during checkout

["René Scharfe" <l.s.r@web.de>]

Am 06.04.2017 um 22:37 schrieb git@jeffhostetler.com:
> From: Jeff Hostetler <jeffhost@microsoft.com>
> 
> Teach traverse_trees_recursive() to not do redundant ODB
> lookups when both directories refer to the same OID.
> 
> In operations such as read-tree, checkout, and merge when
> the differences between the commits are relatively small,
> there will likely be many directories that have the same
> SHA-1.  In these cases we can avoid hitting the ODB multiple
> times for the same SHA-1.
> 
> TODO This change is a first attempt to test that by comparing
> TODO the hashes of name[i] and name[i-i] and simply copying
> TODO the tree-descriptor data.  I was thinking of the n=2
> TODO case here.  We may want to extend this to the n=3 case.
> 
> ================
> On the Windows repo (500K trees, 3.1M files, 450MB index),
> this reduced the overall time by 0.75 seconds when cycling
> between 2 commits with a single file difference.
> 
> (avg) before: 22.699
> (avg) after:  21.955
> ===============
> 
> ================
> Using the p0004-read-tree test (posted earlier this week)
> with 1M files on Linux:
> 
> before:
> $ ./p0004-read-tree.sh
> 0004.5: switch work1 work2 (1003037)       11.99(8.12+3.32)
> 0004.6: switch commit aliases (1003037)    11.95(8.20+3.14)
> 
> after:
> $ ./p0004-read-tree.sh
> 0004.5: switch work1 work2 (1003037)       11.17(7.84+2.76)
> 0004.6: switch commit aliases (1003037)    11.13(7.82+2.72)
> ================
> 
> Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
> ---
>   tree-walk.c    |  8 ++++++++
>   tree-walk.h    |  1 +
>   unpack-trees.c | 13 +++++++++----
>   3 files changed, 18 insertions(+), 4 deletions(-)
> 
> diff --git a/tree-walk.c b/tree-walk.c
> index ff77605..3b82f0e 100644
> --- a/tree-walk.c
> +++ b/tree-walk.c
> @@ -92,6 +92,14 @@ void *fill_tree_descriptor(struct tree_desc *desc, const unsigned char *sha1)
>   	return buf;
>   }
>   
> +void *copy_tree_descriptor(struct tree_desc *dest, const struct tree_desc *src)
> +{
> +	void *buf = xmalloc(src->size);
> +	memcpy(buf, src->buffer, src->size);
> +	init_tree_desc(dest, buf, src->size);
> +	return buf;
> +}
> +
>   static void entry_clear(struct name_entry *a)
>   {
>   	memset(a, 0, sizeof(*a));
> diff --git a/tree-walk.h b/tree-walk.h
> index 68bb78b..ca4032b 100644
> --- a/tree-walk.h
> +++ b/tree-walk.h
> @@ -43,6 +43,7 @@ int tree_entry(struct tree_desc *, struct name_entry *);
>   int tree_entry_gently(struct tree_desc *, struct name_entry *);
>   
>   void *fill_tree_descriptor(struct tree_desc *desc, const unsigned char *sha1);
> +void *copy_tree_descriptor(struct tree_desc *dest, const struct tree_desc *src);
>   
>   struct traverse_info;
>   typedef int (*traverse_callback_t)(int n, unsigned long mask, unsigned long dirmask, struct name_entry *entry, struct traverse_info *);
> diff --git a/unpack-trees.c b/unpack-trees.c
> index 3a8ee19..50aacad 100644
> --- a/unpack-trees.c
> +++ b/unpack-trees.c
> @@ -554,10 +554,15 @@ static int traverse_trees_recursive(int n, unsigned long dirmask,
>   	newinfo.df_conflicts |= df_conflicts;
>   
>   	for (i = 0; i < n; i++, dirmask >>= 1) {
> -		const unsigned char *sha1 = NULL;
> -		if (dirmask & 1)
> -			sha1 = names[i].oid->hash;
> -		buf[i] = fill_tree_descriptor(t+i, sha1);
> +		if (i > 0 && (dirmask & 1) && names[i].oid && names[i-1].oid &&

Can .oid even be NULL?  (I didn't check, but it's dereferenced in the 
sha1 assignment below, so I guess the answer is no and these two checks 
are not needed.)

> +			!hashcmp(names[i].oid->hash, names[i-1].oid->hash)) {

Calling oidcmp would be shorter.

> +			buf[i] = copy_tree_descriptor(&t[i], &t[i-1]);

buf keeps track of the allocations that need to be freed at the end of 
the function.  I assume these buffers are read-only.  Can you use an 
alias here instead of a duplicate by calling init_tree_desc with the 
predecessor's buffer and setting buf[i] to NULL?  Or even just copying 
t[i - 1] to t[i] with an assignment?  That would be shorter and probably 
also quicker.

> +		} else {
> +			const unsigned char *sha1 = NULL;
> +			if (dirmask & 1)
> +				sha1 = names[i].oid->hash;
> +			buf[i] = fill_tree_descriptor(t+i, sha1);
> +		}
>   	}
>   
>   	bottom = switch_cache_bottom(&newinfo);
>