What causes GPU global memory latency?












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I was asking myself what causes GPU global memory latency. I mean the global memory works e.g. on a frequency of 1700MHz (e.g.) and has a 352bit lane, this means it can move around 600GB/s. But then everyone states, this is the slowest memory on a GPU and you have around 200 - 800 cycles access time. If your core works with around 1000MHz this means you have around 2us per access, which is much slower than 600GB/s.



Where does this difference come from?










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  • Bandwidth is different from latency. Because GPUs prioritize bandwidth their memory controllers buffer requests and schedule them to reduce idle channel time. Multiple requests can be in-process concurrently; while one request is opening a row in a bank of one DRAM another can be transferring data and many others can be waiting. (Also 200 cycles at 1 GHz is only 200ns not 2µs.) The question deserve a real answer, but this is a quick hint.

    – Paul A. Clayton
    Jan 27 at 20:13
















0















I was asking myself what causes GPU global memory latency. I mean the global memory works e.g. on a frequency of 1700MHz (e.g.) and has a 352bit lane, this means it can move around 600GB/s. But then everyone states, this is the slowest memory on a GPU and you have around 200 - 800 cycles access time. If your core works with around 1000MHz this means you have around 2us per access, which is much slower than 600GB/s.



Where does this difference come from?










share|improve this question























  • Bandwidth is different from latency. Because GPUs prioritize bandwidth their memory controllers buffer requests and schedule them to reduce idle channel time. Multiple requests can be in-process concurrently; while one request is opening a row in a bank of one DRAM another can be transferring data and many others can be waiting. (Also 200 cycles at 1 GHz is only 200ns not 2µs.) The question deserve a real answer, but this is a quick hint.

    – Paul A. Clayton
    Jan 27 at 20:13














0












0








0








I was asking myself what causes GPU global memory latency. I mean the global memory works e.g. on a frequency of 1700MHz (e.g.) and has a 352bit lane, this means it can move around 600GB/s. But then everyone states, this is the slowest memory on a GPU and you have around 200 - 800 cycles access time. If your core works with around 1000MHz this means you have around 2us per access, which is much slower than 600GB/s.



Where does this difference come from?










share|improve this question














I was asking myself what causes GPU global memory latency. I mean the global memory works e.g. on a frequency of 1700MHz (e.g.) and has a 352bit lane, this means it can move around 600GB/s. But then everyone states, this is the slowest memory on a GPU and you have around 200 - 800 cycles access time. If your core works with around 1000MHz this means you have around 2us per access, which is much slower than 600GB/s.



Where does this difference come from?







memory gpu latency global






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share|improve this question











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asked Jan 25 at 6:02









Emerson FittipaldiEmerson Fittipaldi

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1













  • Bandwidth is different from latency. Because GPUs prioritize bandwidth their memory controllers buffer requests and schedule them to reduce idle channel time. Multiple requests can be in-process concurrently; while one request is opening a row in a bank of one DRAM another can be transferring data and many others can be waiting. (Also 200 cycles at 1 GHz is only 200ns not 2µs.) The question deserve a real answer, but this is a quick hint.

    – Paul A. Clayton
    Jan 27 at 20:13



















  • Bandwidth is different from latency. Because GPUs prioritize bandwidth their memory controllers buffer requests and schedule them to reduce idle channel time. Multiple requests can be in-process concurrently; while one request is opening a row in a bank of one DRAM another can be transferring data and many others can be waiting. (Also 200 cycles at 1 GHz is only 200ns not 2µs.) The question deserve a real answer, but this is a quick hint.

    – Paul A. Clayton
    Jan 27 at 20:13

















Bandwidth is different from latency. Because GPUs prioritize bandwidth their memory controllers buffer requests and schedule them to reduce idle channel time. Multiple requests can be in-process concurrently; while one request is opening a row in a bank of one DRAM another can be transferring data and many others can be waiting. (Also 200 cycles at 1 GHz is only 200ns not 2µs.) The question deserve a real answer, but this is a quick hint.

– Paul A. Clayton
Jan 27 at 20:13





Bandwidth is different from latency. Because GPUs prioritize bandwidth their memory controllers buffer requests and schedule them to reduce idle channel time. Multiple requests can be in-process concurrently; while one request is opening a row in a bank of one DRAM another can be transferring data and many others can be waiting. (Also 200 cycles at 1 GHz is only 200ns not 2µs.) The question deserve a real answer, but this is a quick hint.

– Paul A. Clayton
Jan 27 at 20:13










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