Agner`s CPU blog

The Ryzen has a micro-operation cache which can hold 2048 micro-operations or instructions. This is sufficient to hold the critical innermost loop in most programs. There has been discussions of whether the Ryzen would be able to run four instructions per clock cycle or six, because the documents published by AMD were unclear at this point. Well, my testing shows that it was not four, and not six, but five. As long as the code is running from the micro-operations cache, it can execute five instructions per clock, where Intel has only four. Code that doesn't fit into the micro-operations cache run from the traditional code cache at a maximum rate of four instructions per clock. However, the rate of fetching code from the code cache is not 32 bytes per clock, as some documents seem to indicate, but mostly around 16 bytes per clock. The maximum I have seen is 17.3 bytes per clock. This is a likely bottleneck since most instructions in vector code are more than four bytes long.

The combination of a compare instruction and a conditional jump can be fused together into a single micro-op. This makes it possible to execute a tiny loop with up to six instructions in one clock cycle per iteration. Except for tiny loops, the throughput for jumps is one jump per two clock cycles if the jump is taken, or two not-taken jumps per clock cycle.

256-bit vector instructions (AVX instructions) are split into two micro-ops handling 128 bits each. Such instructions take only one entry in the micro-operation cache. A few other instructions also generate two micro-ops. The maximum throughput of the micro-op queue after the decoders is six micro-ops per clock. The stream of micro-ops from this queue are distributed between ten pipelines: four pipes for integer operations on general purpose registers, four pipes for floating point and vector operations, and two for address calculation. This means that a throughput of six micro-ops per clock cycle can be obtained if there is a mixture of integer and vector instructions.

Let us compare the execution units of AMD's Ryzen with current Intel processors. AMD has four 128-bit units for floating point and vector operations. Two of these can do addition and two can do multiplication. Intel has two 256-bit units, both of which can do addition as well as multiplication. This means that floating point code with scalars or vectors of up to 128 bits will execute on the AMD processor at a maximum rate of four instructions per clock (two additions and two multiplications), while the Intel processor can do only two. With 256-bit vectors, AMD and Intel can both do two instructions per clock. Intel beats AMD on 256-bit fused multiply-and-add instructions, where AMD can do one while Intel can do two per clock. Intel is also better than AMD on 256-bit memory writes, where Intel has one 256-bit write port while the AMD processor has one 128-bit write port. We will soon see Intel processors with 512-bit vector support, while it might take a few more years before AMD supports 512-bit vectors. However, most of the software on the market lags several years behind the hardware. As long as the software uses only 128-bit vectors, we will see the performance of the Ryzen processor as quite competitive. The AMD can execute six micro-ops per clock while Intel can do only four. But there is a problem with doing so many operations per clock cycle. It is not possible to do two instructions simultaneously if the second instruction depends on the result of the first instruction, of course. The high throughput of the processor puts an increased burden on the programmer and the compiler to avoid long dependency chains. The maximum throughput can only be obtained if there are many independent instructions that can be executed simultaneously.

This is where simultaneous multithreading comes in. You can run two threads in the same CPU core (this is what Intel calls hyperthreading). Each thread will then get half of the resources. If the CPU core has a higher capacity than a single thread can utilize then it makes sense to run two threads in the same core. The gain in total performance that you get from running two threads per core is much higher in the Ryzen than in Intel processors because of the higher throughput of the AMD core (except for 256-bit vector code).

The Ryzen is saving power quite aggressively. Unused units are clock gated, and the clock frequency is varying quite dramatically with the workload and the temperature. In my tests, I often saw a clock frequency as low as 8% of the nominal frequency in cases where disk access was the limiting factor, while the clock frequency could be as high as 114% of the nominal frequency after a very long sequence of CPU-intensive code. Such a high frequency cannot be obtained if all eight cores are active because of the increase in temperature.

The varying clock frequency was a big problem for my performance tests because it was impossible to get precise and reproducible measurements of computation times. It helps to warm up the processor with a long sequence of dummy calculations, but the clock counts were still somewhat inaccurate. The Time Stamp Counter (TSC), which is used for measuring the execution time of small pieces of code, is counting at the nominal frequency. The Ryzen processor has another counter called Actual Performance Frequency Clock Counter (APERF) which is similar to the Core Clock Counter in Intel processors. Unfortunately, the APERF counter can only be read in kernel mode, unlike the TSC which is accessible to the test program running in user mode. I had to calculate the actual clock counts in the following way: The TSC and APERF counters are both read in a device driver immediately before and after a run of the test sequence. The ratio between the TSC count and the APERF count obtained in this way is then used as a correction factor which is applied to all TSC counts obtained during the running of the test sequence. This method is awkward, but the results appear to be quite precise, except in the cases where the frequency is varying considerably during the test sequence. My test program is available at www.agner.org/optimize/#testp

AMD has a different way of dealing with instruction set extensions than Intel. AMD keeps adding new instructions and remove them again if they fail to gain popularity, while Intel keeps supporting even the most obscure and useless undocumented instructions dating back to the first 8086. AMD introduced the FMA4 and XOP instruction set extensions with Bulldozer, and some not very useful extensions called TBM with Piledriver. Now they are dropping all these again. XOP and TBM are no longer supported in Ryzen. FMA4 is not officially supported on Ryzen, but I found that the FMA4 instructions actually work correctly on Ryzen, even though the CPUID instruction says that FMA4 is not supported.

Detailed results and list of instruction timings are in my manuals: www.agner.org/optimize/#manuals.