Loop Around File Mapping Kills Performance

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I have a circular buffer which is backed with file mapped memory (the buffer is in the size range of 8GB-512GB).

I am writing to this memory is a sequential manner from the beginning to the end at which point it loops around back to the beginning.

It works fine until it reaches the end where it needs to perform two file mappings and loop around the memory, at which point IO performance is totally trashed for some reason I haven't been able to figure out. It's as if it wants to flush the entire file and not just the regions I am mapping.

    mapping(file_mapping& file, mode_t mode, std::size_t file_size, std::size_t offset, std::size_t size)
        : offset_(offset)
        , mode_(mode)
    {           
        const auto aligned_size         = page_ceil(size + page_size());
        const auto aligned_file_size    = page_floor(file_size);
        const auto aligned_file_offset  = page_floor(offset % aligned_file_size);
        const auto region1_size         = std::min(aligned_size, aligned_file_size - aligned_file_offset);
        const auto region2_size         = aligned_size - region1_size;

        for (auto n = 0; ; ++n)
        {   
            try
            {
                if (region2_size)
                {
                    const auto region1_address  = mapped_region(file, read_only, 0, (region1_size + region2_size) * 2).get_address();   
                    const auto region2_address  = reinterpret_cast<char*>(region1_address) + region1_size;
                    region1_ = mapped_region(file, mode, aligned_file_offset, region1_size, region1_address);
                    region2_ = mapped_region(file, mode, 0,                   region2_size, region2_address);
                }
                else
                {
                    region1_ = mapped_region(file, mode, aligned_file_offset, region1_size);
                    region2_ = mapped_region();
                }

                break;
            }
            catch(interprocess_exception&)
            {
                if (n >= 16)
                    throw;

                std::this_thread::sleep_for(std::chrono::milliseconds(4));
            }
        }
    }

Any ideas?

asked 34 secs ago

ronag

13.2k

Loop Around File Mapping Kills Performance