**Summary**
After upgrading a host that is just a collection `Distributed` tables to 20.4.X and 20.5.2.7, we observe the new `MemoryTracking` setting increasing substantially during writes, leading to individual writes and reads yielding exceptions: ```Memory limit (total) exceeded: would use 28.28 GiB (attempt to allocate chunk of 4197840 bytes), maximum: 28.28```

System metrics show that no actual memory is being allocated to the same size.
<img width="993" alt="Screen Shot 2020-07-19 at 12 23 59 PM" src="https://user-images.githubusercontent.com/1189446/87883721-229e7280-c9be-11ea-851b-0e1511cc10bb.png">

Please let me know if there is any additional context, diagrams, or statements I should provide. I'm interested in knowing how to correlate the total `MemoryTracking` metric to anything else since system memory does not reflect the metric at all.

**Context**
This host has four different `Distributed` tables that are written to via the HTTP interface with `insert_distributed_sync=1` set.
The host itself has no local tables.

The hosts with the downstream local tables are all 20.5.2.7.

We have downgraded the `Distributed` host in question back to 20.3.9.70 in order to continue writing to the clusters.
return last_block_missing_values;
    }
private:
    const Block header;
    const RowInputFormatParams row_input_format_params;
    const FormatSettings format_settings;
    const InputProcessorCreator input_processor_creator;
    const size_t min_chunk_bytes;
    /*
     * This is declared as atomic to avoid UB, because parser threads access it
     * without synchronization.
     */
    std::atomic<bool> finished{false};
    BlockMissingValues last_block_missing_values;
    
    ReadBuffer & original_buffer;
    
    std::optional<size_t> next_block_in_current_unit;
    size_t segmentator_ticket_number{0};
    size_t reader_ticket_number{0};
    std::mutex mutex;
    std::condition_variable reader_condvar;
    std::condition_variable segmentator_condvar;
    
    ThreadPool pool;
    
    ThreadFromGlobalPool segmentator_thread;
    
    FormatFactory::FileSegmentationEngine file_segmentation_engine;
    enum ProcessingUnitStatus