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All high performance code is IO bound. Even highly tuned c++ code spends most of it's time on the CPU waiting on cache misses. Its a pretty exceptional usecase

All high performance code is IO bound. Even highly tuned c++ code spends most of it's time on the CPU waiting on cache misses. Its a pretty exceptional usecase where your code is not IO bound

ssfrr 8y ago
Better cache access patterns are one of the reasons Julia's dot-broadcasting[1] is super cool. If you have big vectors `a` and `b`, the expression `sin.(a .+ exp.(b))` will do a single pass over your data, calling `sin(a[i]+exp(b[i]))` for each element, rather than creating big temporary arrays for the intermediate expressions and looping through multiple times. Then because putting all those dots can be unwieldy, there's the `@.` macro which puts a dot on all your function calls. [1]: https://julialang.org/blog/2017/01/moredots https://julialang.org/blog/2017/01/moredots
throwawaymath 8y ago
Wait what? I'm using "I/O bound" in the sense of CPU operations waiting on reads/writes to e.g. a disk. If you consider an operation waiting on the cache to be I/O bound, what do you consider to be CPU bound or cache bound? And what terminology would you use to refer to operations which are waiting on the disk as opposed to the cache? What about memory instead of the cache? I think it's useful to differentiate between an operation which is purely CPU bound (i.e. it's just constantly calculating without a reference) and an operation which is cache bound (faster than memory but still bound over the CPU). But calling operations I/O bound when they're sufficiently optimized that they live in the cache and don't even hit memory, let alone disk is an abuse of terminology. In the context of what I'm talking about, most HPC is absolutely not I/O bound unless it's using SATA/SAS drives instead of cache and memory. And circling back to my original point, most research labs which can afford it will sufficiently optimize their code and hardware so that they don't hit the disk unless they're working with terabytes of data. Python, C++ and R provide numerous packages between the three of them for numerical computing across each of these bottlenecks, so I don't think Julia can rely on differentiating itself by shining in an I/O bound setting (i.e., waiting on disk). And if it does, "hundreds of gigabytes" isn't really the data size in which people are (in my opinion) going to overcome the friction of a new language and ecosystem just to harvest those benefits.
- repsilat 8y ago
  It's not an especially weird use of terminology. "I/O bound" means "waiting on reads and/or writes." Cache or RAM or disk, it's all about communication throughput and latency, efficient access patterns etc. In a compute-bound workload the CPU spends the bulk of its time actually retiring instructions, not stalled waiting on data. Think about it from the perspective of what the FPU sees -- once it has done that FMA operation, does it have the data it needs to do the next one, or does it need to sit on its hands for a while? The cache hierarchy, cache-friendly data structures and algorithms -- they all aim to reduce time spent waiting on IO.
  - throwawaymath 8y ago
    I understand that, but I don't think it's useful to use use the term "I/O" in the theoretical sense of the concurrency problem. It's also not typical nomenclature - for example, see https://stackoverflow.com/questions/868568/what-do-the-terms-cpu-bound-and-i-o-bound-mean/868588 https://stackoverflow.com/questions/868568/what-do-the-terms.... We have terms like "CPU bound", "cache bound", and "memory bound", but we don't really have "disk bound" in common usage. This is because the common usage is "I/O bound". Theoretically speaking we can model any process as one which has to wait and one which doesn't have to wait. But in modern usage we have a variety of types and speeds for reads and writes. When I/O simply means reads and writes, you lose all the practical granularity you'd otherwise get by decomposing the reads/writes into different bottlenecks. It's philosophically elegant, but practically unhelpful for optimizing HPC and distributed systems when, as the responder said, it's rare to be CPU bound. I also think that the context of my original comment is pretty clearly using I/O in the modern sense of disk usage. Responding with a correction that everything is I/O bound is vacuous, not insightful.
    - ChrisRackauckas 8y ago
      >it's rare to be CPU bound. Not when solving (partial) differential equations, which is what I am using Julia for.
      - 8y ago
CyberDildonics 8y ago
I'm not sure how this makes sense. Tuning C++ for speed is mostly weeding out cache misses through memory access patterns. If memory is accesses linearly the prefetcher will get it ahead of time. If cache sizes are taken into account, you can not only cut down on memory latency, but memory bandwidth as well.

All high performance code is IO bound. Even highly tuned c++ code spends most of it's time on the CPU waiting on cache misses. Its a pretty exceptional usecase where your code is not IO bound

ssfrr 8y ago

Better cache access patterns are one of the reasons Julia's dot-broadcasting[1] is super cool. If you have big vectors `a` and `b`, the expression `sin.(a .+ exp.(b))` will do a single pass over your data, calling `sin(a[i]+exp(b[i]))` for each element, rather than creating big temporary arrays for the intermediate expressions and looping through multiple times. Then because putting all those dots can be unwieldy, there's the `@.` macro which puts a dot on all your function calls. [1]: https://julialang.org/blog/2017/01/moredots https://julialang.org/blog/2017/01/moredots

throwawaymath 8y ago

Wait what? I'm using "I/O bound" in the sense of CPU operations waiting on reads/writes to e.g. a disk. If you consider an operation waiting on the cache to be I/O bound, what do you consider to be CPU bound or cache bound? And what terminology would you use to refer to operations which are waiting on the disk as opposed to the cache? What about memory instead of the cache? I think it's useful to differentiate between an operation which is purely CPU bound (i.e. it's just constantly calculating without a reference) and an operation which is cache bound (faster than memory but still bound over the CPU). But calling operations I/O bound when they're sufficiently optimized that they live in the cache and don't even hit memory, let alone disk is an abuse of terminology. In the context of what I'm talking about, most HPC is absolutely not I/O bound unless it's using SATA/SAS drives instead of cache and memory. And circling back to my original point, most research labs which can afford it will sufficiently optimize their code and hardware so that they don't hit the disk unless they're working with terabytes of data. Python, C++ and R provide numerous packages between the three of them for numerical computing across each of these bottlenecks, so I don't think Julia can rely on differentiating itself by shining in an I/O bound setting (i.e., waiting on disk). And if it does, "hundreds of gigabytes" isn't really the data size in which people are (in my opinion) going to overcome the friction of a new language and ecosystem just to harvest those benefits.

repsilat 8y ago
It's not an especially weird use of terminology. "I/O bound" means "waiting on reads and/or writes." Cache or RAM or disk, it's all about communication throughput and latency, efficient access patterns etc. In a compute-bound workload the CPU spends the bulk of its time actually retiring instructions, not stalled waiting on data. Think about it from the perspective of what the FPU sees -- once it has done that FMA operation, does it have the data it needs to do the next one, or does it need to sit on its hands for a while? The cache hierarchy, cache-friendly data structures and algorithms -- they all aim to reduce time spent waiting on IO.
- throwawaymath 8y ago
  I understand that, but I don't think it's useful to use use the term "I/O" in the theoretical sense of the concurrency problem. It's also not typical nomenclature - for example, see https://stackoverflow.com/questions/868568/what-do-the-terms-cpu-bound-and-i-o-bound-mean/868588 https://stackoverflow.com/questions/868568/what-do-the-terms.... We have terms like "CPU bound", "cache bound", and "memory bound", but we don't really have "disk bound" in common usage. This is because the common usage is "I/O bound". Theoretically speaking we can model any process as one which has to wait and one which doesn't have to wait. But in modern usage we have a variety of types and speeds for reads and writes. When I/O simply means reads and writes, you lose all the practical granularity you'd otherwise get by decomposing the reads/writes into different bottlenecks. It's philosophically elegant, but practically unhelpful for optimizing HPC and distributed systems when, as the responder said, it's rare to be CPU bound. I also think that the context of my original comment is pretty clearly using I/O in the modern sense of disk usage. Responding with a correction that everything is I/O bound is vacuous, not insightful.
  - ChrisRackauckas 8y ago
    >it's rare to be CPU bound. Not when solving (partial) differential equations, which is what I am using Julia for.
    - 8y ago

CyberDildonics 8y ago

I'm not sure how this makes sense. Tuning C++ for speed is mostly weeding out cache misses through memory access patterns. If memory is accesses linearly the prefetcher will get it ahead of time. If cache sizes are taken into account, you can not only cut down on memory latency, but memory bandwidth as well.