On 3/20/2018 6:25 PM, Jonathan Tan wrote:
On Tue, 20 Mar 2018 16:03:25 -0400
Derrick Stolee <dstolee@xxxxxxxxxxxxx> wrote:
This patch updates the abbreviation code to use bsearch_hash() as defined
in [1]. It gets a nice speedup since the old implementation did not use
the fanout table at all.
You can refer to the patch as:
b4e00f7306a1 ("packfile: refactor hash search with fanout table",
2018-02-15)
Also, might be worth noting that this patch builds on
jt/binsearch-with-fanout.
Thanks. That commit is in master and v2.17.0-rc0, so hopefully it isn't
difficult to apply the patch.
One caveat about the patch: there is a place where I cast a sha1 hash
into a struct object_id pointer. This is because the abbreviation code
still uses 'const unsigned char *' instead of structs. I wanted to avoid
a hashcpy() in these calls, but perhaps that is not too heavy a cost.
I recall a discussion that there were alignment issues with doing this,
but I might have be remembering wrongly - in my limited knowledge of C
alignment, both "unsigned char *" and "struct object_id *" have the same
constraints, but I'm not sure.
Adding Brian M. Carlson in the CC line for advice on how to do this
translation between old sha1's and new object_ids. If it isn't safe,
then we could do a hashcpy() until the translation makes it unnecessary.
I should have compared the two methods before sending the patch, but
running the "git log --oneline --parents" test with a hashcpy() versus a
cast has no measurable difference in performance for Linux. Probably
best to do the safest thing here if there is no cost to perf.
+ const unsigned char *index_fanout = p->index_data;
[snip]
+ return bsearch_hash(oid->hash, (const uint32_t*)index_fanout,
+ index_lookup, index_lookup_width, result);
This cast to "const uint32_t *" is safe, because p->index_data points to
a mmap-ed region (which has very good alignment, as far as I know). I
wonder if we should document alignment guarantees on p->index_data, and
if yes, what guarantees to declare.
The existing application in find_pack_entry_one() [1] does similar
pack-index arithmetic before calling. A similar amount of arithmetic and
alignment concerns appear in the commit-graph patch. Where is the right
place to declare these alignment requirements?
In v2, I'll have find_pack_entry_one() call bsearch_pack() so this logic
is not duplicated.
Thanks,
-Stolee
[1]
https://github.com/git/git/blob/c6284da4ff4afbde8211efe5d03f3604b1c6b9d6/packfile.c#L1721-L1749
find_pack_entry_one() in packfile.c
