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OpenCL

gautam_himanshu
Adept II

Re: HD 7970 LDS Bank Conflicts

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The LDS Bank conflicts are only related to within a workgroup. Any workgroup scheduled on a CU will have a part of LDS reserved for it. If the LDS requirements of workgroups is more, then less number of workgroups can be scheduled on a CU.

 

Section 4.11.2.3 In addition to registers, shared memory can also serve to limit the active

wavefronts/compute unit. Each compute unit has 32k of LDS, which is shared

among all active work-groups. LDS is allocated on a per-work-group granularity,

so it is possible (and useful) for multiple wavefronts to share the same local

memory allocation. However, large LDS allocations eventually limits the number

of workgroups that can be active

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LeeHowes
Staff
Staff

Re: HD 7970 LDS Bank Conflicts

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Bank conflicts are a half-wavefront issue problem, not a workgroup or even full wavefront problem. The way it works is that every cycle 16 lanes of requests are made by both one of SIMD 0 and 1 and one of or SIMD 2 and 3 in the CU. Those requests are serviced as 32-lanes per cycle by the LDS interface and hence conflicts (I think) can occur accross those 32 lanes and 32 banks.

Of course, bank conflicts that delay one wave can affect another wave on another SIMD unit in the CU because it creates a pipeline bubble.

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