5 ms·

My personal notes on this subject: * MIPS was perhaps the integer equivalent to FLOP, still used in modern microcontrollers because the 8051 at 12MHZ would onl

My personal notes on this subject: * MIPS was perhaps the integer equivalent to FLOP, still used in modern microcontrollers because the 8051 at 12MHZ would only execute 1MIPS (12 clocks per instruction). Modern 8051 chips obviously have sped up to 1 clock per instruction, but MIPS (and Dhrystone MIPS in particular) are still a common benchmark today. * FLOPs is very difficult to calculate in theory because modern CPUs have vector units, and multiple pipelines per core. You could have 3x AVX512 instructions in parallel on today's CPUs on a single core. * FLOPs we're traditionally a 64-bit operation for the supercomputer community. Today, most FLOPs are 32-bit for video games. Finally, the deep learning / neural net guys have popularized 16-bit flops, and even 8-bit iops. * 'The' flop is a misnomer because it's almost always the multiply-and-accumulate instruction: X = A + B * C. Which... Is two operations per instruction (per shader/SIMD lane). Eeehhh whatever. Who cares about these details? * As 'Dhrystone' is the benchmark for MIPS, the benchmark for 64-bit flops is Linpack.

segfaultbuserr 3y ago
> 'The' flop is a misnomer because it's almost always the multiply-and-accumulate instruction: X = A + B * C. Which... Is two operations per instruction (per shader/SIMD lane). Eeehhh whatever. Who cares about these details? If FMA is supported, it can either be counted as one or two operations, depending on the rule of the benchmark involved or the marketing of the processor. The marketing specification of a processor's theoretical peak performance sometimes counts a single-instruction FMA as two operations. On the other hand, for the purpose of code profiling, counting FMA as one operation is more realistic... As you said, who cares about these details?
- fluoridation 3y ago
  Given that the point of the FLOPS unit is to compare processors, it does make more sense to count complex instructions as more than a single floating-point operation. If one CPU could multiply a 4x4 matrix by a vector in a single instruction that can run a million times per second, and another CPU needed ~32 instructions and so can only multiply 500k matrices per second but retires 16 million instructions in that same second, it would be silly to compare instructions instead of multiplications and additions.
  - dragontamer 3y ago
    As a computer-engineer, the circuit design needed to make a fast multiplication operation (ie: Wallace Tree, and similar) are an order-of-magnitude larger than the circuit design needed for fast addition (ie: a Kogge-Stone Carry lookahead Adder). This idea that additions and multiplications can be combined like this as "equivalent operations" is kinda bullshit. But hey, if its "how its done" (and its done this way because multiply-then-add is how you do matrix-multiplications...) then so be it. Just remember that this is an arbitrary subdivision of a matrix multiplication operation, that may not have much relevance as a benchmark outside of matrix multiplications.
    - fluoridation 3y ago
      It was just an example, not necessarily a realistic one. The point is that we want to compare how quickly a processor will compute our problem, not how many instructions it's going to execute. If it was a car you want to compare things like its top speed and acceleration, not something inane like engine revolutions per kilometer. You measure and compare things that are relevant to the user, not implementation details.
      - dragontamer 3y ago
        Okay, I think I see what you're saying now. I misinterpreted your statements earlier.
      - magicalhippo 3y ago
        > not something inane like engine revolutions per kilometer Unrelated to the thread but I just read that Top Fuel dragster engines only do about 240 revolutions over the 1000 foot race[1]. Never thought of it like that before so blew my mind a little. [1]: https://en.wikipedia.org/wiki/Top_Fuel#Performance https://en.wikipedia.org/wiki/Top_Fuel#Performance
  - Findecanor 3y ago
    FLOPS is a measure of throughput. The time-difference between a `fmul` and a `fmadd` is often in latency. A modern processor is typically superscalar, with multiple individually pipelined floating point units that can each be issued one instruction per cycle: regardless of whether that is a `fmadd`, a `fadd or something else. If you'd issue a `fmadd` to a unit, then you are prevented from issuing a `fadd` to it in that same cycle, and vice versa. That's one op, not two. However, floating point units are sometimes heterogeneous. I think the question should be, rather: should you count the throughput of any ops, or of arbitrary ops. In other words, if a CPU runs at 1 MHz and has two units but where only one can run `fmul`, should you count the CPU as having one or two MFLOPS?
gumby 3y ago
> MIPS was perhaps the integer equivalent to FLOP, still used in modern microcontrollers because the 8051 at 12MHZ would only execute 1MIPS (12 clocks per instruction) Actually the origin of this term was VAX MIPS (VAX 780 specifically) because that was a ubiquitous, pretty fast for its time minicomputer. There were faster machines, and slower mainframes still being built, but that was what the late 70s were like. When the 8051 was released in 1980 it surely didn’t run at 12 MHz! Back then the Z80 sold because it could run 8080 code at a blistering 2 MHz. BTW the benchmark for FLOPS in those days was Whetstone, hence the otherwise weird name “Dhrystone”
- dragontamer 3y ago
  The 1981 manual for the 8051 contains numerous references to 12MHz. http://bitsavers.informatik.uni-stuttgart.de/components/intel/8051/MCS-51_Users_Manual_Jan81.pdf http://bitsavers.informatik.uni-stuttgart.de/components/inte... It was 12T clocked: even though the clock was 12MHz, it would only operate at 1MHz / 1MIPS, because it took 12-clock-ticks to even perform one addition. IIRC, there was a standard crystal (11.0592 MHz crystal?? I forget exactly) for the communications at the time. So going just above 11 MHz (or really, just above 11.0592 MHz) was needed for reliable serial comms.
  - gumby 3y ago
    Wow, right on page 1-2! I'm surprised -- I don't remember anything running that fast back then. Thanks. (Love those old Intel books too) Nevertheless, FWIW, MIPS started out as Vax MIPS, and at first people often used to write "VAX MIPS".
    - vardump 3y ago
      8051 might run at 12 MHz, but actual instructions run in multiples of 12 clock cycles. Realistic code on a 8051 runs only about 450k-800k instructions per second @12 MHz.

segfaultbuserr 3y ago

> 'The' flop is a misnomer because it's almost always the multiply-and-accumulate instruction: X = A + B * C. Which... Is two operations per instruction (per shader/SIMD lane). Eeehhh whatever. Who cares about these details? If FMA is supported, it can either be counted as one or two operations, depending on the rule of the benchmark involved or the marketing of the processor. The marketing specification of a processor's theoretical peak performance sometimes counts a single-instruction FMA as two operations. On the other hand, for the purpose of code profiling, counting FMA as one operation is more realistic... As you said, who cares about these details?

fluoridation 3y ago
Given that the point of the FLOPS unit is to compare processors, it does make more sense to count complex instructions as more than a single floating-point operation. If one CPU could multiply a 4x4 matrix by a vector in a single instruction that can run a million times per second, and another CPU needed ~32 instructions and so can only multiply 500k matrices per second but retires 16 million instructions in that same second, it would be silly to compare instructions instead of multiplications and additions.
- dragontamer 3y ago
  As a computer-engineer, the circuit design needed to make a fast multiplication operation (ie: Wallace Tree, and similar) are an order-of-magnitude larger than the circuit design needed for fast addition (ie: a Kogge-Stone Carry lookahead Adder). This idea that additions and multiplications can be combined like this as "equivalent operations" is kinda bullshit. But hey, if its "how its done" (and its done this way because multiply-then-add is how you do matrix-multiplications...) then so be it. Just remember that this is an arbitrary subdivision of a matrix multiplication operation, that may not have much relevance as a benchmark outside of matrix multiplications.
  - fluoridation 3y ago
    It was just an example, not necessarily a realistic one. The point is that we want to compare how quickly a processor will compute our problem, not how many instructions it's going to execute. If it was a car you want to compare things like its top speed and acceleration, not something inane like engine revolutions per kilometer. You measure and compare things that are relevant to the user, not implementation details.
    - dragontamer 3y ago
      Okay, I think I see what you're saying now. I misinterpreted your statements earlier.
    - magicalhippo 3y ago
      > not something inane like engine revolutions per kilometer Unrelated to the thread but I just read that Top Fuel dragster engines only do about 240 revolutions over the 1000 foot race[1]. Never thought of it like that before so blew my mind a little. [1]: https://en.wikipedia.org/wiki/Top_Fuel#Performance https://en.wikipedia.org/wiki/Top_Fuel#Performance
- Findecanor 3y ago
  FLOPS is a measure of throughput. The time-difference between a `fmul` and a `fmadd` is often in latency. A modern processor is typically superscalar, with multiple individually pipelined floating point units that can each be issued one instruction per cycle: regardless of whether that is a `fmadd`, a `fadd or something else. If you'd issue a `fmadd` to a unit, then you are prevented from issuing a `fadd` to it in that same cycle, and vice versa. That's one op, not two. However, floating point units are sometimes heterogeneous. I think the question should be, rather: should you count the throughput of any ops, or of arbitrary ops. In other words, if a CPU runs at 1 MHz and has two units but where only one can run `fmul`, should you count the CPU as having one or two MFLOPS?

gumby 3y ago

> MIPS was perhaps the integer equivalent to FLOP, still used in modern microcontrollers because the 8051 at 12MHZ would only execute 1MIPS (12 clocks per instruction) Actually the origin of this term was VAX MIPS (VAX 780 specifically) because that was a ubiquitous, pretty fast for its time minicomputer. There were faster machines, and slower mainframes still being built, but that was what the late 70s were like. When the 8051 was released in 1980 it surely didn’t run at 12 MHz! Back then the Z80 sold because it could run 8080 code at a blistering 2 MHz. BTW the benchmark for FLOPS in those days was Whetstone, hence the otherwise weird name “Dhrystone”

dragontamer 3y ago
The 1981 manual for the 8051 contains numerous references to 12MHz. http://bitsavers.informatik.uni-stuttgart.de/components/intel/8051/MCS-51_Users_Manual_Jan81.pdf http://bitsavers.informatik.uni-stuttgart.de/components/inte... It was 12T clocked: even though the clock was 12MHz, it would only operate at 1MHz / 1MIPS, because it took 12-clock-ticks to even perform one addition. IIRC, there was a standard crystal (11.0592 MHz crystal?? I forget exactly) for the communications at the time. So going just above 11 MHz (or really, just above 11.0592 MHz) was needed for reliable serial comms.
- gumby 3y ago
  Wow, right on page 1-2! I'm surprised -- I don't remember anything running that fast back then. Thanks. (Love those old Intel books too) Nevertheless, FWIW, MIPS started out as Vax MIPS, and at first people often used to write "VAX MIPS".
  - vardump 3y ago
    8051 might run at 12 MHz, but actual instructions run in multiples of 12 clock cycles. Realistic code on a 8051 runs only about 450k-800k instructions per second @12 MHz.