curl-6 said:
Which is a pretty big boost because (A) 32MB is overkill for a 720p system, 360 got by with only 10MB, even after triple buffering to eliminate all screen tearing, there is still eDRAM to spare on Wii U, which brings us to (B) both the CPU and GPU have access to the eDRAM for operations that require fast memory access. |
Remember that 32MB is there also as the 1T memory in wii mode too and must electronically appear to be identical and that memory was more about low latency than memory bandwidth. The wii u has to read and write constantly to 2GB of memory with a bandwidth of 12.8gb/s. You can put some speed critical parts into the 32MB of edram memory especially the frame buffer but ultimately all the wii u can do is move data about in the 2GB of main memory at 12.8gb/s. 32MB is only 1/64th of actual memory. For comparison the main memory of 360 is 22.4gb/s and the bandwidth between edram and gpu is 32gb/s. PS3 is 22.4gb/s to graphics memory and 25.6 gb/s to main memory. So even the ps3 can do 4 operations approx to its main and graphic memory in the time the wii u can do one to its DDR memory. Shinshei said the 360 and wii u edram was comparable but the increase in size made it far more useful. To meet the wii's memory bandwidth requirements it needs to be about 4 gb/s approx while the wii u gpu is operating at 240mhz so probably about 10gb/s absolute minimum when operating at full speed. How much bandwith has it really got? No one seems to know so its an area of wild speculation but it has to appear identical to 1t ram when the gpu is slowed to 240mhz for wii mode. If Shinshei is saying its similar then perhaps 32gb/s but I've seen 40gb/s and 60gb/s also mentioned. This is probably linked to the debate about whether the wii u gpu is 176 gflops or 352 gflops. The wii u isn't performing anywhere near the claimed 352 gflops figure and not consuming the power either and would be horrifically bottlenecked by 12.8gb/s memory access. I'm thinking the eDRAM speed is actually at the higher end of speculation and the gpu is at 176 gflops this makes sense regarding the performance of the console.
The xbone needs about 125 gb/s memory bandwidth to match the 176gb/s of ps4 considering it has a 40% weaker gpu than ps4 and most of main memory is occupied with graphics data movement so its not bottlenecked. xbox one memory is 68 gb/s plus there is also the issue that many xbox one games suffer from more frame drops even when run at a lower resolution despite a slightly faster cpu setup than ps4. So clearly the 32MB of SRAM is not enough to fulfill the shortfall despite being far,far faster than eDRAM.
The 10MB edram on 360 was enough to give it a frame rate advantage over many ps3 games slightly as long the resolution was a fit for 10MB, the ps3 supports a much wider range of 1080p and 3D games that require larger frame buffers.
Also that 10MB edram for 1/2GB in 360 is a higher ratio than 32MB for 2GB admittedly the operating system may have less call on time critical memory access although I'm only guessing that.
Let's face it the consoles without small pockets of high speed memory but with reasonable bandwidth for main memory and/or graphics memory achieve a lot more. The PS4 clearly does, as does the ps3 despite having a much weaker gpu than 360 for properly optimised games as did the original xbox the generation before. A small amount of high bandwidth memory is really restricting for ambitious games and seems to have a common symptom, frame rate drops. Both xbox one and wii u suffer from it horribly. The 360 doesn't but then its main memory wasn't slow it was almost twice the bandwidth of wii u memory so for the 360 the eDRAM was a performance bonus that improved the console's games it wasn't used as a solution to using cheaper slower main memory.
Also the wii punched above its weight despite only having a 11gflops gpu (xenoblade) and that design has a dedicated 1meg texture cache, 2meg frame buffer, 24MB 1T-RAM and 64GB of DDR buffer memory that's 4 pools of memory in addition to all the other smaller caches in its design. Obviously mainly inherited from the gamecube it was based on but still huge bandwidth all things considered.
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