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I'm not sure I follow the reasoning in the article. It says if ray tracing flops, AMD will take the lead, but then goes on to say that AMD believes ray tracing

by twtw 8y ago

I'm not sure I follow the reasoning in the article. It says if ray tracing flops, AMD will take the lead, but then goes on to say that AMD believes ray tracing will be important and is working on it.

devonkim 8y ago
It’s not too dissimilar from how AVX instructions are so poorly implemented on AMD CPUs - they may support it but it may not be core to their strategy (no pun intended).
- dragontamer 8y ago
  AVX instructions are actually done fine in my experience. 256-bit is emulated, but that's not really a big deal. Intel is faster for sure, but I never had issues running AVX code on AMD CPUs. The performance characteristics are different between Intel and AMD. In fact, AMD has some tricks up its sleeve. AMD Zen has two AES pipelines (while Intel Skylake only has one), so vectorized AES code is faster on AMD. There are some performance issues with vpgatherdd instructions, but Intel has those as uop emulated code too. So both Intel and AMD are equally to blame there. ---------- My "issues" with AMD CPUs are relatively tame. AMD's profiling tools are weaker than Intel's. (Ex: AMD has "Instruction Based Sampling" while Intel's "PEBS" (Precision Event Based Sampling) are a bit easier to use. True 256-bit execution would be nice, but its not a major hassle in most cases IMO: AMD's back-end is very thick, so you can still get a lot of ILP out of AVX2 instructions.
  - floatboth 8y ago
    > High Precision Event Timers Isn't HPET is just a basic system timer? (That is definitely present on Zen.) You might be thinking of something else? > 256-bit execution would be nice, but its not a major hassle in most cases Coming with Zen 2 anyway :)
    - dragontamer 8y ago
      > Isn't HPET is just a basic system timer? (That is definitely present on Zen.) You might be thinking of something else? You're right. I got the names confused. I've edited the post above to use the proper "PEBS" term. Intel's "Precise Event-Based Sampling" is what I was trying to talk about. Intel's PEBS can precisely tell you where a branch-mispredict happens. AMD's default event timers are inaccurate: your branch mispredictions will be all over "add" instructions, and other unrelated stuff. This is because a CPU is looking at roughly ~100 instruction windows (between the pipeline, decoder, and retirement... there's a lot of inaccuracy in determining "where did this branch misprediction happen??"). So when trying to track down a branch-misprediction on AMD systems, you have to switch to the harder to use "Instruction based Profiling" mode. Intel has a simpler PEBS switch which is easier to use IMO.
- CoolGuySteve 8y ago
  At some point I'm hoping to see AVX completely emulated in microcode and replaced with an embedded GPU core that can write to an L3 or L4 cache. This slow scaling of 128->256->512 bits in the instruction set is more or less a solved problem in the GPU space with shader compilers and AVX would mostly be redundant with GPUs if it weren't for the memory bandwidth constraint. ie: When it comes to vector processing, go big or go home. AVX/SSE are a compromise from back when CPU die space and bandwidth was more precious. Now that we have 8-32 cores on a die with a good bus between them, it seems like duplicating those AVX units 8-32 times is less optimal.
  - dragontamer 8y ago
    Please no. AVX's main advantage is that it is roughly 1-cycle away from your main registers, and 4-cycles away from L1 cache. Talking to and from L3 cache is on the order of 30 to 40 cycles, an order of magnitude slower. If your workload fits inside of 64kB, AVX is incredibly beneficial. If your workload fits within 8MB (L3 cache), you're starting to look at a point where maybe you should pipe that data to the GPU instead. A GPU call over PCIe is under 5 uS / 5000 nanoseconds, with a bandwidth of ~15GB/s. GPUs are certainly farther away than L3 cache, but if you're pushing L3 cache levels... you're getting close to the GPU anyway. --------- 128-bit SSE code is perfect for representing Complex Numbers (Two double-floats). 128-bit is also great for a x,y,z,w 32-bit vector. ------- GPUs are the "go big or go home" architecture. AVX's primary benefit is latency.
  - m0zg 8y ago
    What you think of as "vector" processing is currently being used by compilers to speed up things you didn't think were vectorizable. This is possible only because these instructions are pretty cheap latency-wise. By introducing huge latency, you'd be ruining performance of autovectorization, which accounts for a lot of the performance gains in the past decade.
    - deleted 8y ago
      [deleted]
    - CoolGuySteve 8y ago
ilaksh 8y ago
The author seems to think RTX is unimportant. He is wrong. Ray tracing or perhaps more likely path tracing plus the AI denoising is the next revolution in computer graphics and gaming. Now, up until this point, it has been totally impractical for games, but that isn't the case now. Let me try to explain why it's so important. Essentially this type of thing is the only good way to improve realism. And it allows rendering to move eventually from what is basically an aggregation of different types of lighting approximations towards a more unified global illumination system. It is going to require engine rewrites and a few more generations of hardware improvements to fully achieve that, but it will enable real-time rendering close to today's cinema quality and also streamline game development by removing the need for so much asset preparation related to lighting. Anyway I think that AMD absolutely needs to catch up in this area. Even though many people related to gaming or programming may have trouble recognizing the relevance because all of the lighting hacks are firmly baked into the culture and accepted. All those hacks will gradually become obsolete.
- bryanlarsen 8y ago
  Unless Moore's law stops, Ray tracing probably is the future. Regardless, Ray tracing in 2019 is pretty much useless. AMD needs to have Ray tracing ready for future chips, but in the mean time they can take advantage of Nvidia wasting a quarter of their die on a currently useless feature.
  - ilaksh 8y ago
    Ray tracing and path tracing are different. Path tracing with sophisticated denoising in 2019 is 100% useful today. It is just not quite practical yet to replace the engines in most games with it entirely. But it is absolutely useful. Please do some more research.

by twtw 8y ago

I'm not sure I follow the reasoning in the article. It says if ray tracing flops, AMD will take the lead, but then goes on to say that AMD believes ray tracing will be important and is working on it.

devonkim 8y ago

It’s not too dissimilar from how AVX instructions are so poorly implemented on AMD CPUs - they may support it but it may not be core to their strategy (no pun intended).

dragontamer 8y ago
AVX instructions are actually done fine in my experience. 256-bit is emulated, but that's not really a big deal. Intel is faster for sure, but I never had issues running AVX code on AMD CPUs. The performance characteristics are different between Intel and AMD. In fact, AMD has some tricks up its sleeve. AMD Zen has two AES pipelines (while Intel Skylake only has one), so vectorized AES code is faster on AMD. There are some performance issues with vpgatherdd instructions, but Intel has those as uop emulated code too. So both Intel and AMD are equally to blame there. ---------- My "issues" with AMD CPUs are relatively tame. AMD's profiling tools are weaker than Intel's. (Ex: AMD has "Instruction Based Sampling" while Intel's "PEBS" (Precision Event Based Sampling) are a bit easier to use. True 256-bit execution would be nice, but its not a major hassle in most cases IMO: AMD's back-end is very thick, so you can still get a lot of ILP out of AVX2 instructions.
- floatboth 8y ago
  > High Precision Event Timers Isn't HPET is just a basic system timer? (That is definitely present on Zen.) You might be thinking of something else? > 256-bit execution would be nice, but its not a major hassle in most cases Coming with Zen 2 anyway :)
  - dragontamer 8y ago
    > Isn't HPET is just a basic system timer? (That is definitely present on Zen.) You might be thinking of something else? You're right. I got the names confused. I've edited the post above to use the proper "PEBS" term. Intel's "Precise Event-Based Sampling" is what I was trying to talk about. Intel's PEBS can precisely tell you where a branch-mispredict happens. AMD's default event timers are inaccurate: your branch mispredictions will be all over "add" instructions, and other unrelated stuff. This is because a CPU is looking at roughly ~100 instruction windows (between the pipeline, decoder, and retirement... there's a lot of inaccuracy in determining "where did this branch misprediction happen??"). So when trying to track down a branch-misprediction on AMD systems, you have to switch to the harder to use "Instruction based Profiling" mode. Intel has a simpler PEBS switch which is easier to use IMO.
CoolGuySteve 8y ago
At some point I'm hoping to see AVX completely emulated in microcode and replaced with an embedded GPU core that can write to an L3 or L4 cache. This slow scaling of 128->256->512 bits in the instruction set is more or less a solved problem in the GPU space with shader compilers and AVX would mostly be redundant with GPUs if it weren't for the memory bandwidth constraint. ie: When it comes to vector processing, go big or go home. AVX/SSE are a compromise from back when CPU die space and bandwidth was more precious. Now that we have 8-32 cores on a die with a good bus between them, it seems like duplicating those AVX units 8-32 times is less optimal.
- dragontamer 8y ago
  Please no. AVX's main advantage is that it is roughly 1-cycle away from your main registers, and 4-cycles away from L1 cache. Talking to and from L3 cache is on the order of 30 to 40 cycles, an order of magnitude slower. If your workload fits inside of 64kB, AVX is incredibly beneficial. If your workload fits within 8MB (L3 cache), you're starting to look at a point where maybe you should pipe that data to the GPU instead. A GPU call over PCIe is under 5 uS / 5000 nanoseconds, with a bandwidth of ~15GB/s. GPUs are certainly farther away than L3 cache, but if you're pushing L3 cache levels... you're getting close to the GPU anyway. --------- 128-bit SSE code is perfect for representing Complex Numbers (Two double-floats). 128-bit is also great for a x,y,z,w 32-bit vector. ------- GPUs are the "go big or go home" architecture. AVX's primary benefit is latency.
- m0zg 8y ago
  What you think of as "vector" processing is currently being used by compilers to speed up things you didn't think were vectorizable. This is possible only because these instructions are pretty cheap latency-wise. By introducing huge latency, you'd be ruining performance of autovectorization, which accounts for a lot of the performance gains in the past decade.
  - deleted 8y ago
    [deleted]
  - CoolGuySteve 8y ago

ilaksh 8y ago

The author seems to think RTX is unimportant. He is wrong. Ray tracing or perhaps more likely path tracing plus the AI denoising is the next revolution in computer graphics and gaming. Now, up until this point, it has been totally impractical for games, but that isn't the case now. Let me try to explain why it's so important. Essentially this type of thing is the only good way to improve realism. And it allows rendering to move eventually from what is basically an aggregation of different types of lighting approximations towards a more unified global illumination system. It is going to require engine rewrites and a few more generations of hardware improvements to fully achieve that, but it will enable real-time rendering close to today's cinema quality and also streamline game development by removing the need for so much asset preparation related to lighting. Anyway I think that AMD absolutely needs to catch up in this area. Even though many people related to gaming or programming may have trouble recognizing the relevance because all of the lighting hacks are firmly baked into the culture and accepted. All those hacks will gradually become obsolete.

bryanlarsen 8y ago
Unless Moore's law stops, Ray tracing probably is the future. Regardless, Ray tracing in 2019 is pretty much useless. AMD needs to have Ray tracing ready for future chips, but in the mean time they can take advantage of Nvidia wasting a quarter of their die on a currently useless feature.
- ilaksh 8y ago
  Ray tracing and path tracing are different. Path tracing with sophisticated denoising in 2019 is 100% useful today. It is just not quite practical yet to replace the engines in most games with it entirely. But it is absolutely useful. Please do some more research.