关于970显存占用大于3.5GB之后速度骤减的官方解释

walla

除了2011dm在帖子http://bbs.3dmgame.com/thread-4608556-1-1.html提到过的，这里补充两点原文的说明，
由于９７０最后一组的Ｌ２缓存/ＲＯＰ被砍，而与最后一组对应的ＳＭＭｓ和ＤＲＡＭ却是完整的，因此这组接口（ｐｏｒｔ）可能会承受２倍请求（因为少了一组Ｌ２／ＲＯＰ处理，却有两个显存模块）。

平时没有满载还好，一旦第7端口（见下图）满载，第8端口产生新的请求，那其他6个端口处理时间就多了一倍，因为要等待第7端口处理2份任务，直接波及到剩下3.5GB数据的吞吐效率。所以ＮＶ想了个办法，在970里把之前3.5GB显存映射到一块儿，把最后的0.5GB独立开。一般使用只要不超过3.5GB，数据就都在之前这段显存里流通，避免把数据放到最后的0.5GB里。

而当显存占用超过3.5GB的时候，结果就像前文提到的第7端口的请求处理量比其他1-6个端口处理量多了一倍，于是整体处理速度骤降。单独访问最后的0.5GB显存的速度只有之前3.5GB速度的7分之一。因为它无法和之前3.5GB显存同步存取。



原文链接：http://www.pcper.com/reviews/Graphics-Cards/NVIDIA-Discloses-Full-Memory-Structure-and-Limitations-GTX-970

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But the GTX 970 and its altered design has to do things differently. If you walked across the memory interface in the exact same way, over the same 4GB capacity, the 7th crossbar port would tend to always get twice as many requests as the other port (because it has two memories attached).  In the short term that could be ok due to queuing in the memory path.  But in the long term if the 7th port is fully busy, and is getting twice as many requests as the other port, then the other six must be only half busy, to match with the 2:1 ratio.  So the overall bandwidth would be roughly half of peak. This would cause dramatic underutilization and would prevent optimal performance and efficiency for the GPU.

Let's be blunt here: access to the 0.5GB of memory, on its own and in a vacuum, would occur at 1/7th of the speed of the 3.5GB pool of memory.
To avert this, NVIDIA divided the memory into two pools, a 3.5GB pool which maps to seven of the DRAMs and a 0.5GB pool which maps to the eighth DRAM.  The larger, primary pool is given priority and is then accessed in the expected 1-2-3-4-5-6-7-1-2-3-4-5-6-7 pattern, with equal request rates on each crossbar port, so bandwidth is balanced and can be maximized. And since the vast majority of gaming situations occur well under the 3.5GB memory size this determination makes perfect sense. It is those instances where memory above 3.5GB needs to be accessed where things get more interesting.