Dual Screen / Dual GFX card issue

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I recently added an additional video card (Radeon HD 6450) to my system and instead of improving the response time its worse than before.

On the 1 GB card the video display is very laggy, on the lower spec card its fine (dragging windows from one to the other.)

Here’s my output is there anything that can be done to improve performance from the new card:

sudo lshw -C display
*-display               

       description: VGA compatible controller

       product: Caicos [Radeon HD 6450/7450/8450 / R5 230 OEM]
       vendor: Advanced Micro Devices, Inc. [AMD/ATI]
       physical id: 0
       bus info: pci@0000:01:00.0
       version: 00
       width: 64 bits
       clock: 33MHz
       capabilities: pm pciexpress msi vga_controller bus_master cap_list rom
       configuration: driver=radeon latency=0
       resources: irq:74 memory:c0000000-cfffffff memory:fea20000-fea3ffff 

ioport:e000(size=256) memory:fea00000-fea1ffff
  *-display

       description: VGA compatible controller
       product: RV620 LE [Radeon HD 3450]
       vendor: Advanced Micro Devices, Inc. [AMD/ATI]
       physical id: 0
       bus info: pci@0000:02:00.0
       version: 00
       width: 64 bits
       clock: 33MHz
       capabilities: pm pciexpress msi vga_controller bus_master cap_list rom
       configuration: driver=radeon latency=0
       resources: irq:76 memory:b0000000-bfffffff memory:fe920000-fe92ffff ioport:d000(size=256) memory:fe900000-fe91ffff

Answer

My best guess would be that this is caused by the discrepancy in VRAM.

The first GPU (Radeon HD 6450/7450/8450 / R5 230 OEM) has 1GB of VRAM, while the second GPU (Radeon HD 3450) has only 512MB.

When you put together multiple GPUs, the VRAM doesn’t add up, it is the same as the GPU with the lowest RAM. Essentially, you’re just boosting your processing power – VRAM bandwidth is the same as the VRAM on the lowest GPU.

In your case, since the second GPU has only 512MB (AFAIK), your 1st GPU is still running at its normal speed, but it has lost 768MB of memory bandwidth to work with and thus the second GPU is bottlenecking your system.

My best guess as to why the second (Radeon HD 3450) doesn’t lag is that it was made to run with 512MB of RAM, whereas the first GPU was made to run with 1GB of RAM.

Attribution
Source : Link , Question Author : user1049286 , Answer Author : RPiAwesomeness

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