3 ms·

Just out of curiosity, what kind of speeds can be achieved by a pure software switch using say C or pure assembly?

p_l 6y ago
C probably won't give you much, difference. Assembly, carefully tweaked, might - the kind of stuff like Quake issuing FDIV every 16 pixels in software renderer, where the assembly intimately matches various aspects of microarchitecture.
- BubRoss 6y ago
  What are you basing this on exactly?
  - p_l 6y ago
    Despite years of propaganda, C is not well matched to CPUs currently in use (quite opposite in places), and the typical optimizations don't necessarily work when dealing with external I/O that you need to do in a switch. Essentially, even if you write in C, reaching higher speeds will involve using "assembly masquerading as C" rather than depending on compiler optimizations. Also, Snabb uses LuaJIT, which already generated quite tight code, so the performance gap that I suspect some imagine just isn't that wide.
    - fit2rule 6y ago
      >C is not well matched to CPUs + >depending on compiler optimizations + >Snabb uses LuaJIT .. quite tight code == You can write great C-based systems and avoid assembly if you a) know, always, what your compiler is doing and b) know, always, what your CPU is doing...
      - exikyut 6y ago
        I'd be interested to learn about significantly complex, nontrivial systems of say 100K to 1M LOC scale that require reasoning about every single instruction from the perspective of every other instruction, in order for the system to work.
        benou 6y ago
        You do not do that. Instead you optimize the short, critical part. Of course it does not apply to everything, you need a few hotspots, but it is quite common: audio/video codecs, scientific computation, games, crypto... And even networking.
        fit2rule 6y ago
        And with Lua (and C, and C++) its pretty easy to manage the complexity. Just put things where they belong.
      - p_l 6y ago
        My point was that the same optimizations that made C fast break down when you need to take into account often intricate dance between CPU caches, memory, I/O bus etc. - so that unless you go into cpu-model-specific assembly tweaks just using C might not bring you as much benefit. Is it possible to get better? yes. Would it count as "normal C"? I would say not really (if we say yes, then CL code on SBCL with huge custom VOPs counts)
    - BubRoss 6y ago
      I think the answer here is that LuaJIT is fast, and that well written native programs would still be faster, not that C "isn't well matched to CPUs". Modern optimizations are more about memory access patterns than anything else, with SIMD and concurrency beyond that. Focusing on assembly is really not the apex is used to be. For starters CPUs have multiple integer and floating point units, and they get scheduled in an out of order CPU. Out of order execution is as much about keeping the various units busy as it is about doing loads as soon as possible to avoid stalling. I think if you are going to claim that C or C derivatives aren't actually fast and the idea that they are is due to "propaganda" then you should back that up with something concrete, because it goes against a lot of established expertise.
wmf 6y ago
Around the same. VPP gets some benefit from prefetching which you might classify as inline asm.
benou 6y ago
We demonstrated 1 Tb/s back in 2017 on a high-end dual-socket Broadwell server, and the performance increased steadily since then (thanks to both HW and SW). It is hard to compare w/o knowing packet sizes (64-bytes? IMIX? 1500-bytes?), CPU type, #cores, workload (L2 patch? L2 switching with MAC-learning and 1M entries? IPv6 forwarding with 500K routes? etc.) This is why we have the FD.io CSIT project hosted by LinuxFoundation where we can compare lots of interesting, automated, reproducible benchmarking scenario in a controlled environment on various HW platforms: https://docs.fd.io/csit/master/report/ https://docs.fd.io/csit/master/report/

p_l 6y ago

C probably won't give you much, difference. Assembly, carefully tweaked, might - the kind of stuff like Quake issuing FDIV every 16 pixels in software renderer, where the assembly intimately matches various aspects of microarchitecture.

BubRoss 6y ago
What are you basing this on exactly?
- p_l 6y ago
  Despite years of propaganda, C is not well matched to CPUs currently in use (quite opposite in places), and the typical optimizations don't necessarily work when dealing with external I/O that you need to do in a switch. Essentially, even if you write in C, reaching higher speeds will involve using "assembly masquerading as C" rather than depending on compiler optimizations. Also, Snabb uses LuaJIT, which already generated quite tight code, so the performance gap that I suspect some imagine just isn't that wide.
  - fit2rule 6y ago
    >C is not well matched to CPUs + >depending on compiler optimizations + >Snabb uses LuaJIT .. quite tight code == You can write great C-based systems and avoid assembly if you a) know, always, what your compiler is doing and b) know, always, what your CPU is doing...
    - exikyut 6y ago
      I'd be interested to learn about significantly complex, nontrivial systems of say 100K to 1M LOC scale that require reasoning about every single instruction from the perspective of every other instruction, in order for the system to work.
      - benou 6y ago
        You do not do that. Instead you optimize the short, critical part. Of course it does not apply to everything, you need a few hotspots, but it is quite common: audio/video codecs, scientific computation, games, crypto... And even networking.
        fit2rule 6y ago
        And with Lua (and C, and C++) its pretty easy to manage the complexity. Just put things where they belong.
    - p_l 6y ago
      My point was that the same optimizations that made C fast break down when you need to take into account often intricate dance between CPU caches, memory, I/O bus etc. - so that unless you go into cpu-model-specific assembly tweaks just using C might not bring you as much benefit. Is it possible to get better? yes. Would it count as "normal C"? I would say not really (if we say yes, then CL code on SBCL with huge custom VOPs counts)
  - BubRoss 6y ago
    I think the answer here is that LuaJIT is fast, and that well written native programs would still be faster, not that C "isn't well matched to CPUs". Modern optimizations are more about memory access patterns than anything else, with SIMD and concurrency beyond that. Focusing on assembly is really not the apex is used to be. For starters CPUs have multiple integer and floating point units, and they get scheduled in an out of order CPU. Out of order execution is as much about keeping the various units busy as it is about doing loads as soon as possible to avoid stalling. I think if you are going to claim that C or C derivatives aren't actually fast and the idea that they are is due to "propaganda" then you should back that up with something concrete, because it goes against a lot of established expertise.

wmf 6y ago

Around the same. VPP gets some benefit from prefetching which you might classify as inline asm.

benou 6y ago

We demonstrated 1 Tb/s back in 2017 on a high-end dual-socket Broadwell server, and the performance increased steadily since then (thanks to both HW and SW). It is hard to compare w/o knowing packet sizes (64-bytes? IMIX? 1500-bytes?), CPU type, #cores, workload (L2 patch? L2 switching with MAC-learning and 1M entries? IPv6 forwarding with 500K routes? etc.) This is why we have the FD.io CSIT project hosted by LinuxFoundation where we can compare lots of interesting, automated, reproducible benchmarking scenario in a controlled environment on various HW platforms: https://docs.fd.io/csit/master/report/ https://docs.fd.io/csit/master/report/