[LLVMdev] multithreaded performance disaster with -fprofile-instr-generate (contention on profile counters)
Bob Wilson
bob.wilson at apple.com
Fri Apr 25 09:44:38 PDT 2014
On Apr 24, 2014, at 1:33 AM, Dmitry Vyukov <dvyukov at google.com> wrote:
>>
>> I can see that the behavior of our current instrumentation is going to be a
>> problem for the kinds of applications that you’re looking at. If you can
>> find a way to get the overhead down without losing accuracy
>
> What are your requirements for accuracy?
> Current implementation does not provide 100% accuracy, so it's
> something less than 100%. What is it?
> What use cases for numeric counters (as opposed to bool flag) do we
> need to support? Is it only feedback-driven optimizations?
That’s a fair point. The current implementation is potentially inaccurate because the counter increments are not thread-safe. In a low-contention situation, that won’t matter much, but the counts could become quite inaccurate if there are multiple threads running the same code at the same time.
I don’t have a specific goal in mind right now for accuracy. We plan to use this instrumentation for both PGO and code coverage. Some coverage users only care about a boolean check but others want to see the actual execution counts. If we display the count values and they are significantly different from the real execution counts, that will lead to much confusion.
>
>
>> and without
>> hurting the performance for applications without significant contention,
>
> What is the acceptable threshold? 0.01% would be fine, right? What is
> the maximum value that you are ready to agree with?
Like I said, I don’t have a specific value in mind. My sense is that most of the applications we care about are quite different from the massively parallel code that Google cares about. We may have many threads but they’re all doing different things and contention is much less likely to be a problem.
I really think we need to see specifics before we can decide anything here.
>
>
>> then we can just adopt that. If you can’t do it without tradeoffs, then we
>> should have a separate option to let those who care switch between different
>> kinds of instrumentation.
>>
>> Using the PC to map to the source code is simply not going to work with
>> -fprofile-instr-generate. The mapping from counter values to the
>> user-visible execution counts is complex and relies on detailed knowledge of
>> the clang ASTs.
>>
>>
>>
>> % clang++ -O2 -lpthread coverage_mt_vec.cc && time ./a.out
>> TIME: real: 0.219; user: 0.430; system: 0.000
>> % clang++ -O2 -lpthread -fprofile-instr-generate coverage_mt_vec.cc && time
>> ./a.out
>> TIME: real: 3.743; user: 7.280; system: 0.000
>>
>> % cat coverage_mt_vec.cc
>> #include <pthread.h>
>> #include <vector>
>>
>> std::vector<int> v(1000);
>>
>> __attribute__((noinline)) void foo() { v[0] = 42; }
>>
>> void *Thread1(void *) {
>> for (int i = 0; i < 100000000; i++)
>> foo();
>> return 0;
>> }
>>
>> __attribute__((noinline)) void bar() { v[999] = 66; }
>>
>> void *Thread2(void *) {
>> for (int i = 0; i < 100000000; i++)
>> bar();
>> return 0;
>> }
>>
>> int main() {
>> static const int kNumThreads = 16;
>> pthread_t t[kNumThreads];
>> pthread_create(&t[0], 0, Thread1, 0);
>> pthread_create(&t[1], 0, Thread2, 0);
>> pthread_join(t[0], 0);
>> pthread_join(t[1], 0);
>> return 0;
>> }
>>
>>
>>
>>
>> On Fri, Apr 18, 2014 at 11:45 PM, Xinliang David Li <xinliangli at gmail.com>
>> wrote:
>>>
>>>
>>>
>>>
>>> On Fri, Apr 18, 2014 at 12:13 AM, Dmitry Vyukov <dvyukov at google.com>
>>> wrote:
>>>>
>>>> Hi,
>>>>
>>>> This is long thread, so I will combine several comments into single
>>>> email.
>>>>
>>>>
>>>>>> - 8-bit per-thread counters, dumping into central counters on
>>>>>> overflow.
>>>>> The overflow will happen very quickly with 8bit counter.
>>>>
>>>> Yes, but it reduces contention by 256x (a thread must execute at least
>>>> 256 loop iterations between increments). In practice, if you reduce
>>>> contention below some threshold, it does not represent a problem anymore.
>>>>
>>>>
>>>>
>>>>>> - per-thread counters. Solves the problem at huge cost in RAM
>>>>>> per-thread
>>>>> It is not practical. Especially for TLS counters -- it creates huge
>>>>> pressure on stack memory.
>>>>
>>>> Do we have any numbers about number of counters? If there are 100K 1-byte
>>>> counters, I would consider it as practical.
>>>>
>>>
>>> A medium sized app I looked at has about 10M counters (arcs only). It is
>>> also not uncommon to see such apps running with hundreds of threads.
>>>
>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>> In Google GCC, we implemented another technique which proves to be very
>>>>> effective -- it is called FDO sampling.
>>>>> Basically counters will be updated every N samples.
>>>>
>>>> How does it work?
>>>
>>>
>>> Similar to how occurrences based PMU sampling work. Setting sampling
>>> period to 100 can reduce the instrumentation overhead by close to 100x
>>> without introducing much precision loss.
>>>
>>>>
>>>>
>>>>
>>>>
>>>>>> It seems to me like we’re going to have a hard time getting good
>>>>>> multithreaded performance without significant impact on the single-threaded
>>>>>> behavior.
>>>>> I don't really agree.
>>>>
>>>>
>>>>> We are talking about developers here. Nobody would know the exact thread
>>>>> counts, but developers know the ballpark number
>>>>
>>>> I strongly believe that we must relief developers from this choice during
>>>> build time, and do our best to auto-tune (if the final scheme requires
>>>> tuning).
>>>
>>>
>>>
>>> That really depends. If the tuning space is small, it won't be a problem
>>> for the developer/builder.
>>>
>>>
>>>>
>>>> First, such questions puts unnecessary burden on developers. We don't ask
>>>> what register allocation algorithm to use for each function, right?
>>>
>>>
>>> Crazy developers can actually do that via internal options, but this is
>>> totally different case. People just needs one flag to turn on/off sharding.
>>> When sharding is on, compiler can pick the best 'N' according to some
>>> heuristics at compile time.
>>>
>>>
>>>>
>>>> Second, there are significant chances we will get a wrong answer, because
>>>> e.g. developer's view of how threaded is the app can differ from reality or
>>>> from our classification.
>>>> Third, the app can be build by a build engineer; or the procedure can be
>>>> applied to a base with 10'000 apps; or threaded-ness can change; or the app
>>>> can work in different modes; or the app can have different phases.
>>>>
>>>
>>> We have forgotten to mention the benefit of implementation simplicity. If
>>> the static/simple solution solves the problem for most of the use cases,
>>> designing fancy dynamic solution sounds like over-engineering to me. It
>>> (overhead reduction technique) may also get in the way of easier functional
>>> enhancement in the future.
>>>
>>> David
>>>
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20140425/b8dbdefc/attachment.html>
More information about the llvm-dev
mailing list