What are the main challenges of parallel computing and how do you overcome them?

Parallel computing involves dividing a problem into smaller subproblems and executing them simultaneously across multiple processors or cores. This reduces overall computation time and enhances performance, and is applied in domains like scientific computing, machine learning, and cryptography.

Parallel computing is often useful when you have a large solution space with simple multi-step filters. The ability to put multiple cores to work at once can result in up to a 1/((1-p)+(p/s)) speed increase (Ahmdal's law). However if the problem allows for it, the use of quantum computing can result in further increased speeds. Superposition allows for a qbit to inhabit and process multiple current states at a time (similar to processes or threads without the overhead). This isn't to say quantum computing would be better in all applications as it is difficult to implement and can't solve many classical parallel problems, but it is an interesting option to consider.

Parallel computing offers increased speed, scalability, and efficiency. By leveraging concurrent hardware, it reduces execution time, handles complex problems that are infeasible on a single processor, and optimizes resource usage in distributed systems. It allows tasks like large dataset analysis or simulations to be done faster and more cost-effectively.

As parallel systems scale, particularly in high-performance computing environments, energy consumption and the associated heat dissipation become significant concerns. Designing energy-efficient parallel algorithms and implementing advanced cooling solutions are necessary to mitigate these issues. In one particularly demanding project involving real-time data processing, optimising algorithmic efficiency and employing liquid cooling techniques were instrumental in maintaining system performance while managing energy and thermal constraints. Efficiently managing the memory hierarchy in parallel computing systems, from registers to various levels of cache and main memory, is crucial. The challenge lies in optimising data locality.

Challenges include communication overhead, complexity, and hardware/software compatibility. Communication between processors can introduce latency and affect performance. The complexity of parallel computing makes development, testing, and debugging more difficult. Compatibility issues arise when specific hardware or software requirements for parallelism are unavailable or costly.

- In Multi-step workflows aggregating results between steps can be demanding in terms of time and resources, sometimes causing the main process to run out of resources. - The above is more problematic when having to maintain the order of the original data when sub processes finish at different times, and wait for one another in order for put the sorted data back in order. - Sub-processes may may encounter processing errors, potentially causing issues for the entire main process as node recovery may never be successful if the same conditions recur. This may result in the loss of resources or data. - Sub-processes that fail may generate error logs and dump files that remain on remote nodes potentially containing sensitive information.

In a project involving parallelizing image processing algorithms for real-time video analysis, we faced challenges related to resource management and algorithm design. To address resource management issues, we developed custom memory management strategies to optimize memory allocation and minimize data movement across processing units. Additionally, we restructured the image processing algorithms to exploit parallelism at the pixel level, enabling efficient parallel execution on multicore processors and GPUs. These efforts resulted in significant performance improvements and enhanced real-time processing capabilities for our video analysis system.

Careful design is essential for successful parallel computing. Choose the right model of parallelism (instruction-level, data-level, task-level) and use appropriate tools like MPI, OpenMP, CUDA, or Hadoop to facilitate development. Testing and optimization, using performance profilers or load-balancing strategies, are key to improving efficiency and ensuring correctness.

- Check for resource availability on the main as well as distributed processing nodes before starting processing. - Avoid creating log entires that contain sensitive information and remove "orphan" files left over after failed processing. - Monitor memory consumption and asses the adequacy of execution environment parameters to avoid memory overflows. - Limit the number of failed node retries. Alert stakeholders. - Check for data completeness and accuracy after processing. - Use resource managers (e.g., Yarn, etc.) and make sure to configure priorities, resource pools and all the tools they provide. - Avoid using skewed data at source, or strategize partitioning to avoid starving processing cores while overloading others.