Why less mobile memory (lpddr4) brandwidth than pc memory (ddr4) brandwidth?

Why less mobile memory (lpddr4) brandwidth than pc memory (ddr4) brandwidth?
Unity Evangelists say deffered rendering is not stable on mobile
because the memory brandwidth less than pc memory brandwidth.
but, brandwidth of lpddr4 and ddr4 is the same.
perhaps in mobile enviroment, the system memory and graphic memory are shared but pc is not.
Is My Guset Right? i'd like to see the official documentation or video explaining this.

 

The comparison isn't between LPDDR4 on mobile and DDR4 on PC, it's between LPDDR4 on mobile and GDDR5 (or even faster options like GDDR5X, GDDR6, and HBM2) on PC. On PC, dedicated GPUs have their own dedicated RAM that is much, much higher bandwidth than the system RAM.

For example on a mobile device using LPDD4 memory is going to be around 16 GB/s (@ 2133Mhz) to 24 GB/s (@ 3200Mhz). The newer LPDD4X you might see memory bandwidth in the neighborhood of 32 GB/s. A newer top end mobile device from the last month using LPDD5 might see closer to 50 GB/s. But the entry level GTX 1050 from 2015 has 112 GB/s of memory bandwidth. More recent RTX 3000 series GPUs start out 224 GB/s with the low end (though not yet released) RTX 3050, and quickly getting around the 450 GB/s range with the RTX 3060 Ti, to 1000 GB/s range at the top end RTX 3090 Ti. Basically an entire order of magnitude more bandwidth than mobile.

The other side of this is that 32 GB/s number for LPDD4X is what it can do when running at the peak possible rate. Mobile platforms can't do that continuously due to thermal and power limits, so you'll get that in short bursts and then the device will start lowering the clock rate to try to keep the temperature of the device down, so that number might quickly get cut in half or more. On desktop that's usually the rate it can sustain, often allowing for peaks that go even higher like on GPU memory which will overclock itself if there's thermal and power headroom. So your basic laptop using a single stick of DDR 2133Mhz RAM and an integrated GPU (which like mobile shares the system memory) might have nearly identical peak data rates as the older LPDD4, it'll still end up being faster as it'll be able to sustain that 16 GB/s forever.

It should also be noted that deferred renderers tend to run quite badly on integrated GPUs too.

 

Memory is pretty much always the bottleneck isn't it? It hasn't improved much up until very recently for GPUs, my main issue with higher resolutions.

 

i got it.
Thinking of lpddr vs ddr, I was stupid
The PC has vga memory specialized for graphics.But mobile only has system memory.
We should compare GDDR and LPDDR!
super thank you!