Re: [boost] [Performance of circular_buffer vs Queue][Multithreading may be broken]

-----Original Message----- From: Boost On Behalf Of Gavin Lambert via Boost Sent: 3 April 2020 00:16 To: boost@lists.boost.org Cc: Gavin Lambert Subject: Re: [boost] [Performance of circular_buffer vs Queue][Multithreading may be broken]

On 3/04/2020 08:41, Jan Hafer wrote:

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On 02.04.20 20:59, Mathias Gaunard wrote:

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So you're saying that circular_buffer is slower on a given thread when other threads are accessing their own circular_buffer in parallel? That sounds unlikely to be circular buffer's fault.

Yes and I dont know quite the reason for it. My Threads know their id to access a file-global data structure containing their queue/circular buffer. They start another after in a thread-safe way and exit on emptying the queue/circular buffer.

That sounds like you're allocating a single array of circular_buffers and then accessing them from different threads.

That's basically the worst possible thing to do; as Mathias was saying, that will end up sharing cache lines between different cores and your performance will tank.

At minimum, you should embed the circular_buffer into another struct that has sizeof() >= std::hardware_destructive_interference_size, and make an array of

I am following this discussion thread with no particular interest, but I can imagine that there are others who will find the discussion, theorizing and advice rather valuable. Can anyone add a link to it to the circular buffer documentation? Paul A. Bristow that.

But better still, embed the circular_buffer into your processing classes and

have arrays of them at all.

(If you're pre-C++17 and don't have std::hardware_destructive_interference_size, then using 64 works for most modern platforms.)

Ideally, the circular_buffer implementation itself should also separate all internal producer-thread members and consumer-thread members by std::hardware_destructive_interference_size and try very hard to not cross over. (Here the main thing that matters is write accesses.)

If you want to try using a more modern circular buffer that gets this correct, have a look at Boost.Lockfree's spsc_queue.

https://www.boost.org/doc/libs/1_72_0/doc/html/boost/lockfree/spsc_queue.ht ml

(While you're there, there's also an MPMC queue as well. Note that a lockfree queue will tend to be slower than std::queue in uncontended benchmarks, but

don't the

avoidance of locks can be superior in highly contended or other specialised scenarios.)

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