[lld] r248106 - COFF: Fix race condition.

[Rui Ueyama via llvm-commits]

On Mon, Sep 21, 2015 at 3:35 PM, Sean Silva <chisophugis at gmail.com> wrote:

>
>
> On Mon, Sep 21, 2015 at 8:20 AM, Rui Ueyama <ruiu at google.com> wrote:
>
>> On Sun, Sep 20, 2015 at 6:42 PM, Rui Ueyama <ruiu at google.com> wrote:
>>
>>> We can, but I think we don't have to, because I didn't see any
>>> measurable difference between this code and the old racy one.
>>>
>> ... in terms of speed.
>>
>
> On x86 at a hardware level a similar amount of synchronization is required
> for both cases, so this isn't surprising (regardless of whether the access
> is atomic or not, the cache coherence mechanisms kick in). Simply replacing
> a non-atomic access with an atomic access is not enough. From inspection of
> the code, there seems to be quite a bit of work being done between
> increments of this value, so it's probably not going to matter, but if you
> someday change this to use fine-grained parallelism so that it scales
> linearly with # cores (it is highly parallelizable, so this should be
> possible), it will probably be necessary to factor this in.
>
> For reference, I have attached a simple test program that shows the
> relative impact of the different choices.
>
> Sean:~/tmp % clang++ -std=c++11 -DNUM_THREADS=12
> -DNUM_OPS_PER_THREAD=5000000 -O3 testsynchronization.cpp && ./a.out
>
> Per thread, non-atomic: 1.00x (baseline)
>
> Per-thread, atomic:     3.61x slower than baseline
>
> Shared, non-atomic:     29.47x slower than baseline
>
> Shared, atomic:         112.72x slower than baseline
>
> This is on a Xeon E5-2697 v2 (12 physical cores).
>
> As you can see, the effect of making the access non-atomic is very small
> compared to the effect of avoiding cache coherence overhead (3-4x vs. 30x).
>

Thank you for the interesting numbers. Shared atomic variable are indeed
much slower than non-shared, non-atomic variables when you have a lot of
concurrent writes. If I'm really trying to get until the last drop from
multi-cores, I'd do as you suggested so that threads share almost nothing.
I didn't do that simply because the cost of updating the atomic counter is
in practice negligible in this code because we are doing so many things
other than updating the atomic variable. I don't generally optimize unless
doing that makes measurable performance difference.


> CC'ing Dmitry who may have more insight to give.
>
> -- Sean Silva
>
>
>>
>>
>>
>>> 2015/09/20 18:32 "Sean Silva" <chisophugis at gmail.com>:
>>>
>>> It seems unfortunate for us to need any synchronization at all just to
>>>> generate a unique ID. Can we just have a bunch of counters, one per thread,
>>>> then initialize them to `ThreadIndex << 50` or something? (make sure they
>>>> are each on their own cache line)
>>>>
>>>> -- Sean Silva
>>>>
>>>> On Sat, Sep 19, 2015 at 6:44 PM, Rui Ueyama via llvm-commits <
>>>> llvm-commits at lists.llvm.org> wrote:
>>>>
>>>>> Author: ruiu
>>>>> Date: Sat Sep 19 20:44:44 2015
>>>>> New Revision: 248106
>>>>>
>>>>> URL: http://llvm.org/viewvc/llvm-project?rev=248106&view=rev
>>>>> Log:
>>>>> COFF: Fix race condition.
>>>>>
>>>>> NextID is updated inside parallel_for_each, so it needs mutual
>>>>> exclusion.
>>>>>
>>>>> Modified:
>>>>>     lld/trunk/COFF/ICF.cpp
>>>>>
>>>>> Modified: lld/trunk/COFF/ICF.cpp
>>>>> URL:
>>>>> http://llvm.org/viewvc/llvm-project/lld/trunk/COFF/ICF.cpp?rev=248106&r1=248105&r2=248106&view=diff
>>>>>
>>>>> ==============================================================================
>>>>> --- lld/trunk/COFF/ICF.cpp (original)
>>>>> +++ lld/trunk/COFF/ICF.cpp Sat Sep 19 20:44:44 2015
>>>>> @@ -73,7 +73,7 @@ private:
>>>>>    bool forEachGroup(std::vector<SectionChunk *> &Chunks, Comparator
>>>>> Eq);
>>>>>    bool partition(ChunkIterator Begin, ChunkIterator End, Comparator
>>>>> Eq);
>>>>>
>>>>> -  uint64_t NextID = 1;
>>>>> +  std::atomic<uint64_t> NextID = { 1 };
>>>>>  };
>>>>>
>>>>>  // Entry point to ICF.
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> llvm-commits mailing list
>>>>> llvm-commits at lists.llvm.org
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>>>>>
>>>>
>>>>
>>
>
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