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Granularity of parallel tasks

#1
04-09-2024, 05:57 AM
When you split tasks for parallel runs you notice the chunk sizes decide everything I recall seeing that in my own tests. Fine grained work breaks things into tiny bits that fly across cores fast but they pile up overhead quick. You end up with lots of chatter between parts and that slows the whole show. Coarse options lump bigger pieces together so communication drops yet you risk idle cores if loads skew. I tried both on my setups and coarse won for bigger jobs while fine suited quick bursts only. But balance comes hard when you mix them without care.
Or perhaps the overhead eats gains if tasks get too small you watch context switches multiply like crazy then. Processors stall waiting on tiny handoffs and that drains speed fast. You gain from better load spread with fine grains yet the cost in setup time adds up bad. Coarse grains cut those costs but they leave some cores twiddling when data uneven hits. I saw this drag in my old code runs where I adjusted sizes mid way and watched times shift dramatic. Maybe start with medium grains then tweak based on your hardware quirks to hit sweet spots easier.
Also communication patterns twist everything when granularity shifts you feel the network load spike or drop sudden. Fine tasks flood links with messages that clog pipes and force waits everywhere. Coarse ones keep traffic light but they demand more upfront planning to avoid bottlenecks in shared memory spots. I played with cache effects too and found fine grains thrash lines often while coarse preserves locality better overall. Your choice shapes how algorithms scale on real chips with limited buses and that matters for big data flows. Then you test and see idle times drop or rise depending on the grain you pick each round.
Now load balancing turns tricky with varying grains you adjust dynamically or suffer uneven runs that waste cycles. Fine grains allow easy steals from busy spots yet they multiply the steal attempts themselves. Coarse grains lock you into rigid chunks that resist quick fixes when one lags behind. I messed with schedulers in my projects and learned to monitor queue depths close to catch imbalances early. Perhaps combine approaches where initial coarse splits feed into finer ones later for hybrid wins that smooth things out. But hardware limits like interconnect speeds cap what you achieve no matter the tweaks you apply.
You explore these in architecture classes and realize granularity ties direct to speedup formulas that factor comm costs heavy. Fine options boost potential parallelism but only if overhead stays below thresholds you calculate roughly. Coarse keeps things simple for beginners yet caps the max threads you can throw at problems effectively. I compared benchmarks from papers and noticed real apps favor coarse for I O heavy work while compute kernels lean fine when memory access patterns allow. Then your own experiments reveal quirks like thread creation times that textbooks skip over often. Or test on clusters and watch how grain size interacts with latency in ways that surprise at first glance.
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ProfRon
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Granularity of parallel tasks

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