[Mesa-dev] [PATCH 1/6] glsl: Prehash in refcount hash table to reduce hashing

[Thomas Helland]

2017-05-22 21:19 GMT+02:00 Ian Romanick <idr at freedesktop.org>:
> On 05/22/2017 11:55 AM, Thomas Helland wrote:
>> _mesa_hash_table_search is one of our hottest function according to perf.
>> Callgrind shows the refcounting as one of the major users of the
>> searching functions. We can reduce the pain by prehashing, so that we
>> avoid hashing two times when inserting in the table.
>>
>> On a short shader-db run (with validation disabled) it makes
>> 1.2 million of 4.5 million calls to _hash_table_insert come from
>> the pre-hashed path.
>
> Did this have any measurable impact on the run time?
>

Not as I can tell. I believe it is mostly just accumulating a slight
gain from each patch. However, coming to think of it, the big one
here might actually be string_to_uint_map, as that one has to hash
strings instead of just pointers. I guess the pointer hashing is now
so fast that it doesn't really matter.

However, this is all a bit hard to measure. I would probably have
to instrument this, and run it on my desktop, maybe with
GALLIUM_NOOP set, in order to get conclusive results. My poor
old ivy-bridge ultrabook has seen better days and is giving me
quite hard-to-conlude-on results. What I can tell for sure though,
is that the number of executed instructions drops by about 0.1%
for this patch only.

I can do some tests on my desktop tomorrow, if you'd like that.
Then I would probably be able to get more conclusive results.

>> ---
>>  src/compiler/glsl/ir_variable_refcount.cpp | 7 +++++--
>>  1 file changed, 5 insertions(+), 2 deletions(-)
>>
>> diff --git a/src/compiler/glsl/ir_variable_refcount.cpp b/src/compiler/glsl/ir_variable_refcount.cpp
>> index 8306be10b9..bbb4c0ddd7 100644
>> --- a/src/compiler/glsl/ir_variable_refcount.cpp
>> +++ b/src/compiler/glsl/ir_variable_refcount.cpp
>> @@ -79,13 +79,16 @@ ir_variable_refcount_visitor::get_variable_entry(ir_variable *var)
>>  {
>>     assert(var);
>>
>> -   struct hash_entry *e = _mesa_hash_table_search(this->ht, var);
>> +   uint32_t hash = this->ht->key_hash_function(var);
>> +
>> +   struct hash_entry *e =
>> +      _mesa_hash_table_search_pre_hashed(this->ht, hash, var);
>>     if (e)
>>        return (ir_variable_refcount_entry *)e->data;
>>
>>     ir_variable_refcount_entry *entry = new ir_variable_refcount_entry(var);
>>     assert(entry->referenced_count == 0);
>> -   _mesa_hash_table_insert(this->ht, var, entry);
>> +   _mesa_hash_table_insert_pre_hashed(this->ht, hash, var, entry);
>>
>>     return entry;
>>  }
>>
>