[PATCH] D11579: [InstrProfiling] Fix data race on profile counters by using AtomicRMW.

Thu Jul 30 22:15:56 PDT 2015

On Thu, Jul 30, 2015 at 8:10 PM, Jake VanAdrighem <jvanadrighem at gmail.com>
wrote:

>
>
> On Wed, Jul 29, 2015 at 7:27 PM, Sean Silva <chisophugis at gmail.com> wrote:
>
>>
>>
>> On Wed, Jul 29, 2015 at 6:55 PM, Bob Wilson <bob.wilson at apple.com> wrote:
>>
>>>
>>> On Jul 29, 2015, at 4:56 PM, Alexey Samsonov <vonosmas at gmail.com> wrote:
>>>
>>> Do relaxed atomics actually introduce that much of slowdown?
>>>
>>>
>>> I would definitely want to see some data showing that they do not slow
>>> things down before we decide to do this unconditionally. We’ve discussed
>>> this issue several times in the past. My recollection is those discussions
>>> ended with an acknowledgement there is a tradeoff between speed and
>>> accuracy and that we don’t all agree on where we want to be on that
>>> spectrum. Adding options to let people choose would be one solution. Good
>>> data, on a variety of platforms, showing that it doesn’t make much
>>> difference would be another way to resolve it.
>>>
>>
>> +1
>>
>> In my testing, the overhead of the existing instrumentation is about a 2x
>> slowdown, which is starting to get close to the range it would be very
>> difficult to play an instrumented game. I wouldn't want to make this much
>> slower. I'm glad to test this for you; I'll try to get around to this this
>> week.
>>
>>
> Sean and I tested Alexey's patch on one of our large titles and got
> somewhere in the area of 2.5 to 3x worse performance than without
> AtomicRMW. For the game we tested, it was basically unplayable.
>

We also noticed that the top 100 functions (out of 10's of thousands; i.e.
<1%) accounted for over 97% of the total counts (and just the top 10 cover
more than 50% IIRC). The few we manually looked into seemed like they would
be trivially inlined anyway and/or their high count otherwise doesn't seem
to really contribute much useful information to the optimizer since we
should already be "getting those right" (stuff like simple constructors,
getters, or vector operators).

Obviously this is a discussion for another thread, but there seems to be
enormous scope for reduction in our profiling overhead for those interested
in doing that; even just a simple file of 100 functions passed to the
compiler instructing those functions to not be instrumented would decrease
the profiling overhead by over 2 orders of magnitude for this title, based
on the above data.

-- Sean Silva

>
> Jake Van Adrighem
>
>
>> Also, in the past David Li suggested that his findings were that not
>> using atomic operations "only contribute
>> to very small count variations"
>> https://groups.google.com/d/msg/llvm-dev/ScLa2xIdo9s/Ow1FPDVVRIoJ
>> <https://urldefense.proofpoint.com/v2/url?u=https-3A__groups.google.com_d_msg_llvm-2Ddev_ScLa2xIdo9s_Ow1FPDVVRIoJ&d=AwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=mQ4LZ2PUj9hpadE3cDHZnIdEwhEBrbAstXeMaFoB9tg&m=SLAHuMS097rE0HJBsM_g5H8e-AltKY6DSKt4fmyXXdg&s=j6dXaeF32Dce88n0W1vgcjiII4AQnur8LuEPiKzgwCc&e=>
>> CC'ing David in case he has more input to the discussion.
>>
>> -- Sean Silva
>>
>>
>>
>>>
>>>
>>> You're intentionally introducing a data race, this doesn't look good to
>>> me at all. However, I'm not confident about
>>> what's allowed in LLVM IR - it's not C++ where any source-level data
>>> race is UB, but not an x86 assembly either.
>>>
>>> On Wed, Jul 29, 2015 at 4:43 PM, Justin Bogner <mail at justinbogner.com>
>>> wrote:
>>>
>>>> Alexey Samsonov <vonosmas at gmail.com> writes:
>>>> > samsonov created this revision.
>>>> > samsonov added reviewers: dnovillo, bogner.
>>>> > samsonov added a subscriber: llvm-commits.
>>>> >
>>>> > Since we introduced counters for functions in COMDAT sections (e.g.
>>>> > inline functions from STL headers), these headers can easily be
>>>> > incremented concurrently by multiple threads. Replace load-add-store
>>>> > with a single "atomicrmw add" with monotonic memory ordering.
>>>>
>>>> This significantly changes the performance characteristics of this code,
>>>> pessimizing single-threaded users and potentially making the
>>>> multithreaded performance issues even worse.
>>>>
>>>> It's fine to add an option to lower these to atomics, since this does
>>>> guarantee accuracy, but I think we need a switch to choose which kind of
>>>> lowering we're interested in in that case.
>>>>
>>>> > http://reviews.llvm.org/D11579
>>>> <https://urldefense.proofpoint.com/v2/url?u=http-3A__reviews.llvm.org_D11579&d=AwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=mQ4LZ2PUj9hpadE3cDHZnIdEwhEBrbAstXeMaFoB9tg&m=lDx1rX-3B32oZTA_vCe21kRN0Y14ujW2ePnnU3JiUX4&s=tiiUXsu0al8aXyrPKSc28tcPx4YG5wxgSglC63-ASTQ&e=>
>>>> >
>>>> > Files:
>>>> >   lib/Transforms/Instrumentation/InstrProfiling.cpp
>>>> >
>>>> > Index: lib/Transforms/Instrumentation/InstrProfiling.cpp
>>>> > ===================================================================
>>>> > --- lib/Transforms/Instrumentation/InstrProfiling.cpp
>>>> > +++ lib/Transforms/Instrumentation/InstrProfiling.cpp
>>>> > @@ -147,9 +147,9 @@
>>>> >    IRBuilder<> Builder(Inc->getParent(), *Inc);
>>>> >    uint64_t Index = Inc->getIndex()->getZExtValue();
>>>> >    Value *Addr = Builder.CreateConstInBoundsGEP2_64(Counters, 0,
>>>> Index);
>>>> > -  Value *Count = Builder.CreateLoad(Addr, "pgocount");
>>>> > -  Count = Builder.CreateAdd(Count, Builder.getInt64(1));
>>>> > -  Inc->replaceAllUsesWith(Builder.CreateStore(Count, Addr));
>>>> > +  Builder.CreateAtomicRMW(AtomicRMWInst::Add, Addr,
>>>> Builder.getInt64(1),
>>>> > +                          llvm::Monotonic);
>>>> > +  assert(Inc->use_empty() && "InstrProfIncrementInst has uses!");
>>>> >    Inc->eraseFromParent();
>>>> >  }
>>>>
>>>>
>>>
>>>
>>> --
>>> Alexey Samsonov
>>> vonosmas at gmail.com
>>> _______________________________________________
>>> llvm-commits mailing list
>>> llvm-commits at cs.uiuc.edu
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>>>
>>>
>>>
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>>>
>>
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