[lld] r248106 - COFF: Fix race condition.

[Sean Silva via llvm-commits]

On Mon, Sep 21, 2015 at 3:56 PM, Rui Ueyama <ruiu at google.com> wrote:

> I mean I would do that if there's a portable way in C++11 to get the
> current thread ID as a small integer.
>

Or an integer *at all*. Apparently it was decided to use an opaque class
for this with no way to access it as an integer/void*:
http://en.cppreference.com/w/cpp/thread/thread/id

-- Sean Silva


> If we have such function, we could write like this.
>
>   thread_local uint64_t NextID = 1;
>
>  ...
>
>   ID = (getThreadID() << 32) + NextID++;
>
> But I don't think such function exists.
>
> On Mon, Sep 21, 2015 at 3:49 PM, Rui Ueyama <ruiu at google.com> wrote:
>
>> 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
>>>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>>>>>>>
>>>>>>
>>>>>>
>>>>
>>>
>>
>
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