Am 01.04.20 um 13:16 schrieb Jeff King:
> On Wed, Apr 01, 2020 at 06:35:23AM -0400, Jeff King wrote:
>
>>>> + /*
>>>> + * this cast works because we only look at the oid part of the entry,
>>>> + * and it comes first in the struct
>>>> + */
>>>> + khiter_t pos = kh_get_object_entry_set(&object_table,
>>>> + (struct object_entry *)oid);
>>>
>>> Dirty, but I can believe the comment.
>>
>> Our hashmap.c implementation gets around this by letting the equality
>> function take an extra parameter. It's annoying when you're writing
>> those functions, but it should allow this case without any casting (or
>> preemptively allocating a struct).
>
> And here's a patch trying that. Much to my surprise, it outperforms
> khash, which has generally been faster in previous tests.
>
> Here are the numbers I get:
>
> nr_objects master khash hashmap
> 2^20 0m4.317s 0m5.109s 0m3.890s
> 2^21 0m10.204s 0m9.702s 0m7.933s
> 2^22 0m27.159s 0m17.911s 0m16.751s
> 2^23 1m19.038s 0m35.080s 0m31.963s
> 2^24 4m18.766s 1m10.233s 1m6.793s
>
> And I didn't even have to pre-size the table. This really makes me
> wonder if there's some silly inefficiency in khash which we could be
> addressing. Or maybe open-addressing really does lose to chaining here,
> but I think we keep the load factor low enough that it should be a win.
Or we're just unlucky. I tried to find the difference between khash
with and without presizing using callgrind, but came up empty. It did
reveal that fast-import spends 70% of its cycles in a million memset()
calls issued (indirectly) by git_deflate_init() which in turn is called
by store_object() which is called from parse_and_store_blob(), though.
Why is the won second when handling 1M objects not showing in its
output? I suspect it's because it uses its custom allocator to gather
its data. So I ran the test with jemalloc2 preloaded:
nr_objects master khash khash+preload
2^20 0m5.812s 0m5.600s 0m5.604s
2^21 0m12.913s 0m10.884s 0m10.357s
2^22 0m31.257s 0m21.461s 0m21.031s
2^23 1m20.904s 0m40.181s 0m42.607s
2^24 3m59.201s 1m21.104s 1m23.814s
My measurements are noisy, but my point is simply that with a different
allocator you'd not even have seen any slowdown when switching to khash.
>
> ---
> diff --git a/fast-import.c b/fast-import.c
> index 202dda11a6..0ef6defc10 100644
> --- a/fast-import.c
> +++ b/fast-import.c
> @@ -39,12 +39,28 @@
>
> struct object_entry {
> struct pack_idx_entry idx;
> - struct object_entry *next;
> + struct hashmap_entry ent;
That uses 16 bytes more memory per entry on x64 than khash would.
That's 256MB for 2^24 objects -- not ideal, but bearable, I guess.
> uint32_t type : TYPE_BITS,
> pack_id : PACK_ID_BITS,
> depth : DEPTH_BITS;
> };
>
> +static int object_entry_hashcmp(const void *map_data,
> + const struct hashmap_entry *eptr,
> + const struct hashmap_entry *entry_or_key,
> + const void *keydata)
> +{
> + const struct object_id *oid = keydata;
> + const struct object_entry *e1, *e2;
> +
> + e1 = container_of(eptr, const struct object_entry, ent);
That's nicer that the pointer alchemy in the khash conversion for sure.
But why const? Can const change the layout of a structure? Scary.
René
