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In January 2022, consumer-grade graphics cards support only four monitors each in the higher tiers. What is the technical limitation that dictates only four?

DisplayPort 1.4a MST can support 32.4 Gbit/s and in MST mode this will support up to four monitors 1080p@60 Hz.

The 11G-P4-6696-KR graphics card has three DisplayPort ports. What technical limitation prevents the vendor from allowing all three ports to support x4 1080p@60 Hz?

With 11 GiB of graphics memory I would expect that many frame buffers could be supported for simple things like web browsing and other office type applications.

1080p ARGB 8 bit depth (32 bits), color buffer only, is slightly under 8 MiB.

11 GiB / 8 MiB = 1408 frame buffers for basic desktop tasks. For the uninitiated, this is a gross simplification if the desktop composition environment which will have many rectangular surfaces that have textures applied to them, but this is a lightweight computation for this class of graphics device.

So with that in mind, the limiting factor must be something else in the graphics hardware, possibly the bandwidth of the ROPs. Another possible bandwidth limitation would be the encoder that has to transform the in-memory frame buffer representation into a format understood by the target monitor.

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    You wrote "64MiB" (mebibyte) but I think you are quoting mebibits. 1080p 32bpp is about 8.3MiB (1920 * 1080 * 32) (then divide by 8 to convert to bytes). "Mib" (lower case b for bit) does not seem to be in common use (?). I honestly don't know.
    – Yorik
    Commented Jan 4, 2022 at 19:06
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    No modern desktop environment just uses a framebuffer (or two, or three) per display unless all the compositing and rendering is being done in software (for example, you’re using the basic VESA driver in Windows, or doing OpenGL through Mesa on Linux). You need enough space in VRAM for each individual composited element, which is potentially dozens of things even on an ‘unused’ desktop. This is still probably not much of a limiting factor here, but it’s worth remembering.
    – Austin Hemmelgarn
    Commented Jan 5, 2022 at 12:13
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    I can get up to 10 monitors from 2 display ports and 2 hdmi, but I haven't tried to hookup more than 6. For development and word processing it does the job.
    – Chris Schaller
    Commented Jan 5, 2022 at 12:43
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    Physical space on the back of the GPU is a pretty solid reason. amazon.com/VisionTek-Radeon-7750-GDDR5-MiniDP/dp/B00C7EPSVS
    – MonkeyZeus
    Commented Jan 5, 2022 at 18:23
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    What if they supported five? Would you ask why not six? If six would you ask why not seven? Infinity clearly costs infinity and doesn't fit in the case. Zero is not useful, so people need somewhere between one and n monitor ports on a graphics card. Making n larger adds to the cost. People who need only one don't want to pay for five or ten. So manufacturers stop making their product more expensive once the feature set satisfies the majority of their target market.
    – J...
    Commented Jan 5, 2022 at 19:15

1 Answer 1

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Generally even a protocol such as DisplayPort requires some kind of transceiver electronics that are completely distinct from the other buses. The way a graphics card talks to a monitor over HDMI or DisplayPort is going to be significantly different from the way it talks to memory or the internal units. Either the voltage will be different, it will have different current usage, or will have error correction to handle going down a cable of questionable quality.

What you are probably seeing is some limitation in tagging hardware streams out to various display transmitters. There may be some complicated multiplexer/switching arrangement to merge MST streams together and the actual number of outputs from the main GPU to the multiplexer could only have 4 ports. There are apparently graphics cards that do support 6 streams so they likely have more ports going out from the GPU to the display transmitter electronics.

It is not a factor of memory bandwidth, nor likely anything to do with ROPs or anything internal to the GPU or memory bus, and it likely boils down to a marketting and technical choice to say that "most users probably only need X number of displays" along with "it's easy enough for the user to buy another card if they need more".

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    Unless there's a worldwide shortage of GPUs...
    – Sebastian Lenartowicz
    Commented Jan 5, 2022 at 17:37
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    @SebastianLenartowicz surely that could never happen with our robust supply chains and how reasonable people are at not wasting massive amounts of electicity and resources on fake internet points. It could never cause a shortage and massive increase in costs to normal users...
    – Mokubai ♦
    Commented Jan 5, 2022 at 17:45
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    I'd guess that something like 80-90% of people never use more than one monitor and less than 10% of those who want multiple monitors want more than 3. I'd also guess that most people who want +4 monitors would be like me and just get a single large 4k TV instead. It's just not practical for a business to cater to such small numbers of people wanting that many monitors, unless it's for bragging rights.
    – computercarguy
    Commented Jan 5, 2022 at 17:55
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    @Mokubai Not disagreeing with you in any way, just wondering what you mean by "fake internet points"? The first thing that came to mind was the incredible waste of electricity and resources caused by people mining Bitcoin, but I'm not sure if that's your intended meaning.
    – End Antisemitic Hate
    Commented Jan 5, 2022 at 23:52
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    @RockPaperLz-MaskitorCasket I was meaning it as a joke in general comparing bitcoin to point based scoring systems that websites use. They are all fake points used to track something, be it reputation, karma, or some supposed monetary value, but the "real world" value boils down to what people are willing to apply to it. Bitcoin is one of the most prominent offenders for resource waste and it amuses me to conflate points based on how efficiently you can waste electricity (or hard drives or other resources) with other systems used on the Internet.
    – Mokubai ♦
    Commented Jan 6, 2022 at 9:26

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