[LLVMdev] RFC - Improvements to PGO profile support

[Chandler Carruth]

On Tue, Mar 24, 2015 at 12:50 PM, Xinliang David Li <xinliangli at gmail.com>
wrote:

> On Tue, Mar 24, 2015 at 12:45 PM, Chandler Carruth <chandlerc at google.com>
> wrote:
>
>>
>> On Tue, Mar 24, 2015 at 11:46 AM, Xinliang David Li <xinliangli at gmail.com
>> > wrote:
>>
>>> On Tue, Mar 24, 2015 at 11:29 AM, Chandler Carruth <chandlerc at google.com
>>> > wrote:
>>>
>>>> Sorry I haven't responded earlier, but one point here still doesn't
>>>> make sense to me:
>>>>
>>>> On Tue, Mar 24, 2015 at 10:27 AM, Xinliang David Li <davidxl at google.com
>>>> > wrote:
>>>>
>>>>> Diego and I have discussed this according to the feedback received. We
>>>>> have revised plan for this (see Diego's last reply).  Here is a more
>>>>> detailed re-cap:
>>>>>
>>>>> 1) keep MD_prof definition as it is today; also keep using the
>>>>> frequency propagation as it is (assuming programs with irreducible
>>>>> loops are not common and not important. If it turns out to be
>>>>> otherwise, we will revisit this).
>>>>> 2) fix all problems that lead to wrong 'frequency/count' computed from
>>>>> the frequency propagation algorithm
>>>>>    2.1) relax 32bit limit
>>>>>
>>>>
>>>> I still don't understand why this is important or useful.... Maybe I'm
>>>> just missing something.
>>>>
>>>> Given the current meaning of MD_prof, it seems like the result of
>>>> limiting this to 32-bits is that the maximum relative ratio of
>>>> probabilities between two successors of a basic block with N successors is
>>>> (2 billion / N):1 -- what is the circumstance that makes this resolution
>>>> insufficient?
>>>>
>>>> It also doesn't seem *bad* per-se, I just don't see what it improves,
>>>> and it does cost memory...
>>>>
>>>
>>> right -- there is some ambiguity here -- it is needed if we were to
>>> change MD_prof's definition to represent branch count.  However, with the
>>> new plan, the removal of the limit only applies to the function entry count
>>> representation planned.
>>>
>>
>> Ah, ok, that makes more sense.
>>
>> I'm still curious, is the ratio of 2 billion : 1 insufficient between the
>> hottest basic block in the inner most loop and the entry block? My
>> intuition is that this ratio encapsulates all the information we could
>> meaningfully make decisions based upon, and I don't have any examples where
>> it falls over, but perhaps you have some examples?
>>
>
> The ratio is not the problem. The problem is that we can no longer
> effectively differentiate hot functions. 2 billion vs 4 billion will look
> the same with the small capping.
>

The current design for the entry frequency is that it should be interpreted
relative to the global hotness of the function. Today, we only have
attributes "hot" and "cold" which are terrible models of this, but if you
imagine having a function-level metadata signifying the detailed function
profile weight, then you could interpret the basic block frequencies
between two functions only after normalizing with these function-level
weights.

Does that make sense?

(Note, I'm not actually saying that I think this is necessarily the right
design, just that I believe it is the intent of the current design, and if
it is flawed I think that flaw hasn't yet been effectively shown.)
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