[llvm-dev] [RFC] Removing Waymarking from Use.

[Johannes Doerfert via llvm-dev]

On 4/3/20 11:56 AM, Ehud Katz via llvm-dev wrote:
> I'm in favor for this change.
>
> I've also done some testing myself and noticed only about 1-2% increase of
> memory, in exchange for about 1-3% increase of speed.
> I can't say that a speedup of 3% (at most, and usually 1%), worth working
> for, this is a simple change, that give a lot more than this; especially
> simpler code path (also easier debugging).
>
> As part of my measurements, while compiling "deal.II", the maximum number
> of simultaneous `Use` instances was 2,728,963 (and in most cases, about
> half of it).
> In a 64-bit process (Clang for that matter), we will have roughly 21 MB of
> memory spent on the extra pointer (of 8 bytes size).
> On a 16-core machine, for which, in many cases, a build will trigger that
> many simultaneous Clang processes, we will reach, in the worst case, 336 MB
> more memory spent on compilation.
>
> Is it worth it? I think it is. But I am not sure I see the whole picture -
> are there low-memory systems that need to run LLVM on?
>
> I am not sure what needs to be done to approve such a fundamental change;
> especially when we can't prove the Waymarking was needed at all.

I guess if no-one brings forth arguments (= results) for keeping it and

people continue to support replacing it, we will replace it. There should

be a grace period in which people have the chance to do their benchmarking

(basically what is happening), but I don't recall a problem being 
reported yet.


Cheers,

   Johannes


P.S. Thanks for sharing your findings!


> On Thu, Apr 2, 2020 at 8:10 PM Chris Lattner via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> On Apr 1, 2020, at 5:51 AM, Tyker1 at outlook.com via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>
>> Hi llvm-dev,
>>
>> I have a patch open for review that removes waymarking
>> https://reviews.llvm.org/D77144.
>>
>> This patch removes waymarking and replaces it with storing a pointer to
>> the User in the Use.
>>   when compiling the CTMark tests of the test suite, this give an average
>> of +1.8% max memory use and -1.1% compile time.
>>
>> Removing Waymarking also simplifies the code of Use and User.
>>
>>
>> Waymarking is very complicated, and I’d be generally +1 on removing it if
>> it is not paying for itself.  It is also nice to see compiler speedups!
>>
>> I’m curious about kimwitu++ though - when waymarking initially landed, it
>> was a significant memory size win for that.  Do you have any idea why
>> you’re not seeing the same difference?  Is it possible that different
>> things were being measured back then?
>>
>> -Chris
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