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This is one of the reasons I have a strong aversion to "cloud" technologies like docker and kubernetes. You take networking, something with decades of developme

This is one of the reasons I have a strong aversion to "cloud" technologies like docker and kubernetes. You take networking, something with decades of development and hardware support, and you put it all in the CPU. To be clear, Linux has a very robust networking stack. But it will never come close to the natting and routing performance of an actual router. And so we develop things like DPDK to spend even more CPU just to keep things usable, but it still feels like a big step backwards. A typical k8s deployment runs in containers that are in VMs. So each packet you want to send to or from a container needs to touch a cpu and traverse a networking stack six times. That's dumb.

heinrichhartman 6y ago
Interesting. I always suspected something like this was going on. Virtual networking / "SDN" just sounded slow to me. Do you know a good source, to learn about the lower-level Linux/container/docker/k8s networking?
- zamadatix 6y ago
  I got the most value poking around with network namespaces manually via the "ip" command. There are also true VRFs in Linux these days which are much lighter but not nearly as common, most everything is a namespace with either bridging or NAT and some fancy configuration interface to drive that state.
- cheeseprocedure 6y ago
  packagecloud's in-depth writeups on the Linux networking stack are a great read: https://blog.packagecloud.io/eng/2016/06/22/monitoring-tuning-linux-networking-stack-receiving-data/ https://blog.packagecloud.io/eng/2016/06/22/monitoring-tunin... https://blog.packagecloud.io/eng/2017/02/06/monitoring-tuning-linux-networking-stack-sending-data/ https://blog.packagecloud.io/eng/2017/02/06/monitoring-tunin... The illustrated guide to receiving data is also solid: https://blog.packagecloud.io/eng/2016/10/11/monitoring-tuning-linux-networking-stack-receiving-data-illustrated/ https://blog.packagecloud.io/eng/2016/10/11/monitoring-tunin...
dan_quixote 6y ago
> That's dumb. Is it? It's certainly inefficient compared to dedicated hardware. But so is anything relying on a CPU - we could just use ASICs for everything. But then every logical change requires weeks/months/years of development and manufacturing. The goal of k8s, VMs, etc is flexibility. I can set up a 100-node k8s cluster with less-than-perfectly-efficient networking stack in mere minutes. Good luck matching that with dedicated hardware.
- supahfly_remix 6y ago
  For vCPUs on the same physical multicore CPU, packets are sent by shared memory constructs which is very fast. It's effectively sending a pointer to the memory. I have seen transfers like this up to 80 Gbps, much faster than the physical NIC.
  - dahfizz 6y ago
    The transitions from host -> VM is not slow, it's the fact that the packet now has to touch the VM's CPU and go through it's whole routing and natting tables again. And then again for the container. The transition from physical host memory -> vnic is analogous to just sending the packet over a wire. That's not the slow part of modern networks.
- dahfizz 6y ago
  > But so is anything relying on a CPU - we could just use ASICs for everything. But then every logical change requires weeks/months/years of development and manufacturing. Right, except the asics already exist and you're actively choosing the less efficient, more expensive option. > The goal of k8s, VMs, etc is flexibility. I don't think this is necessarily bad, as long as you understand the tradeoff. You're choosing a fundamentally slower architecture to make management easier. It's a choice of prioritizing the developer experience over the user experience.
  - oneplane 6y ago
    In this case it's not even user experience vs. developer experience as you can have both; it's just the cost for performance gets higher as the efficiency decreases. On the other hand: the cost for development goes down if you don't need to pay for extra steps taken by an extra person. If you take a 10-step process that is run by 5 people and reduce it to a 5-step process run by 3, you have 2 more people to do other stuff, or roles that you don't have to create/fill in the first place.
wmf 6y ago
This is partly true and partly not. All the physical routing and switching is still there and it's more efficient than ever. What we've done is added more functionality. Additional layers of abstraction like VMs or containers are generally going to have some performance cost. It's possible to optimize basic packet forwarding in a hypervisor/containervisor using leaner options like ipvlan or even SR-IOV. If you replace hardware firewalls with something like k8s network policy then you are technically replacing hardware with software but it isn't that slow if configured right (hello Cilium) and you're probably implementing a more sophisticated policy anyway.
onei 6y ago
How did you get to six? Is that just sending a single packet without dealing with responses? Never thought about this before so I'm curious.
- dahfizz 6y ago
  Source container -> VM -> host -> host -> VM -> destination container
  - deeblering4 6y ago
    This is one of the benefits to running containers on bare metal.
  - sleepydog 6y ago
    It's not as bad as it sounds as a few of those transitions can be implemented via zero-copy APIs, so you're really just passing around pointers to shared memory.
    - dahfizz 6y ago
      The transitions happen efficiently, but the packet still has to spend cpu time traversing the network stack at all 6 hops. So the packet moves all the way through the routing / natting tables of the host, and then very quickly ends up in the VM. Then has to traverse the whole routing / natting tables of the VM.....
    - toast0 6y ago
      If there's NAT involved (as was posited in the start of the thread), then you can't just copy, you also need to modify.
  - Matthias247 6y ago
    Correct me if I'm wrong, but I don't think the container transition introduces any additional copies and hops. Containers are not VMs. They just use isolation that the kernel enforces. Whether you send a packet from inside or outside a container to the network should be exactly the same amount of work unless you opt-in to run some extra software-level NAT step.
nickpsecurity 6y ago
Given clouds are like mainframes reinvented, you might like reading on how the mainframes handled I/O: https://en.wikipedia.org/wiki/Channel_I/O https://en.wikipedia.org/wiki/Channel_I/O I always thought PC's could be setup to do something similar. The cores used for that could even be small and simple compared to main cores. Folks in early 2001 could be Bittorrenting their Linux distros or whatever they use it for with no user-visible lag. I'd like at least one each for user-input devices, graphics, storage, and networking. Other good reasons to isolate them a bit from each other. There's already ARM SoC's in embedded that have a good core for apps and weak one for I/O. Recently, the RISC-V chip with the minion cores. If embedded can do it, then there seems to be no technical limitation holding back desktops. Just marketing, backward compatibility, etc.
Aaronstotle 6y ago
I'm curious, what makes the natting/routing performance much better on a router? Is it that the router is an ASIC designed only to process packets vs a cpu that can execute any code?
- dsr_ 6y ago
  In particular, a modern high-end router has a CPU (which might be an x86 of some kind) which handles administrative and auxiliary tasks (NTP, ssh, DHCP reflection, whatever you want) while making one or more dedicated ASICs handle packets. When you manage the router, you're actually talking to the supervisory CPU.
- dahfizz 6y ago
  > Is it that the router is an ASIC designed only to process packets vs a cpu that can execute any code? Yes. Most routers + switches are "line rate" meaning the packets go through the switch at the speed of electricity, as if the switch wasn't there are all.
- rayiner 6y ago
  That's not really the whole explanation. There are a couple of things in play. First, while Linux can perform routing, it's mainly intended to be an end host. So for example software like VPP that is designed with the assumption that it can use polling and burn up entire CPU cores to optimize packet forwarding can perform 10x faster on the same hardware. Second, router ASICs can be designed to sped transistors on a predictable packet flow path, instead of intelligence like out-of-order execution to make general purpose code fast. For example, routers have big expensive content-addressable memories called TCAMs that are used to store things like routing tables and ACLs. The ASIC, moreover, can implement a highly tuned pipeline designed around the latencies in the underlying memories. E.g. you get a packet, grab the destination address, look up the next hop in the routing table, etc. Each step takes a predictable amount of time that you can account for and optimize. Third, parallelism is much cheaper in hardware than in general purpose CPUs. It takes a lot more transistors to be able to execute a second general-purpose instruction stream than to have a single-purpose circuit that does some work in parallel with something else.
Klinky 6y ago
Do most scenarios require that pedal-to-the-metal level of performance? A lot of applications are CPU-bound either within the application server or a dependent service(database). Your payloads/throughput may actually be rather low, and the inefficiency may be a tiny percentage of CPU usage compared to the application itself. If you need nonstop CDN levels of bandwidth, then you're probably going to go with a setup that has fewer layers, but CRUD applications don't exactly need terabits of bandwidth.
- dahfizz 6y ago
  As the article explains, it's not all about throughput or latency (though those do suffer), it's also about how much CPU time you spend because of the network architecture. If you're running a cpu bound app, all the more reason to avoid sending 6+ interupts per packet.
neurostimulant 6y ago
I think I read somewhere a while ago that kubernetes cluster will start experiencing big slowdown at around 500 pods due to overhead of its internal networking components. Not sure if it's still true now. I'm going to deploy a cluster that might contains more than 500 pods soon so I'm not really eager to find out the hard way.

heinrichhartman 6y ago

Interesting. I always suspected something like this was going on. Virtual networking / "SDN" just sounded slow to me. Do you know a good source, to learn about the lower-level Linux/container/docker/k8s networking?

zamadatix 6y ago
I got the most value poking around with network namespaces manually via the "ip" command. There are also true VRFs in Linux these days which are much lighter but not nearly as common, most everything is a namespace with either bridging or NAT and some fancy configuration interface to drive that state.
cheeseprocedure 6y ago
packagecloud's in-depth writeups on the Linux networking stack are a great read: https://blog.packagecloud.io/eng/2016/06/22/monitoring-tuning-linux-networking-stack-receiving-data/ https://blog.packagecloud.io/eng/2016/06/22/monitoring-tunin... https://blog.packagecloud.io/eng/2017/02/06/monitoring-tuning-linux-networking-stack-sending-data/ https://blog.packagecloud.io/eng/2017/02/06/monitoring-tunin... The illustrated guide to receiving data is also solid: https://blog.packagecloud.io/eng/2016/10/11/monitoring-tuning-linux-networking-stack-receiving-data-illustrated/ https://blog.packagecloud.io/eng/2016/10/11/monitoring-tunin...

dan_quixote 6y ago

> That's dumb. Is it? It's certainly inefficient compared to dedicated hardware. But so is anything relying on a CPU - we could just use ASICs for everything. But then every logical change requires weeks/months/years of development and manufacturing. The goal of k8s, VMs, etc is flexibility. I can set up a 100-node k8s cluster with less-than-perfectly-efficient networking stack in mere minutes. Good luck matching that with dedicated hardware.

supahfly_remix 6y ago
For vCPUs on the same physical multicore CPU, packets are sent by shared memory constructs which is very fast. It's effectively sending a pointer to the memory. I have seen transfers like this up to 80 Gbps, much faster than the physical NIC.
- dahfizz 6y ago
  The transitions from host -> VM is not slow, it's the fact that the packet now has to touch the VM's CPU and go through it's whole routing and natting tables again. And then again for the container. The transition from physical host memory -> vnic is analogous to just sending the packet over a wire. That's not the slow part of modern networks.
dahfizz 6y ago
> But so is anything relying on a CPU - we could just use ASICs for everything. But then every logical change requires weeks/months/years of development and manufacturing. Right, except the asics already exist and you're actively choosing the less efficient, more expensive option. > The goal of k8s, VMs, etc is flexibility. I don't think this is necessarily bad, as long as you understand the tradeoff. You're choosing a fundamentally slower architecture to make management easier. It's a choice of prioritizing the developer experience over the user experience.
- oneplane 6y ago
  In this case it's not even user experience vs. developer experience as you can have both; it's just the cost for performance gets higher as the efficiency decreases. On the other hand: the cost for development goes down if you don't need to pay for extra steps taken by an extra person. If you take a 10-step process that is run by 5 people and reduce it to a 5-step process run by 3, you have 2 more people to do other stuff, or roles that you don't have to create/fill in the first place.

wmf 6y ago

This is partly true and partly not. All the physical routing and switching is still there and it's more efficient than ever. What we've done is added more functionality. Additional layers of abstraction like VMs or containers are generally going to have some performance cost. It's possible to optimize basic packet forwarding in a hypervisor/containervisor using leaner options like ipvlan or even SR-IOV. If you replace hardware firewalls with something like k8s network policy then you are technically replacing hardware with software but it isn't that slow if configured right (hello Cilium) and you're probably implementing a more sophisticated policy anyway.

onei 6y ago

How did you get to six? Is that just sending a single packet without dealing with responses? Never thought about this before so I'm curious.

dahfizz 6y ago
Source container -> VM -> host -> host -> VM -> destination container
- deeblering4 6y ago
  This is one of the benefits to running containers on bare metal.
- sleepydog 6y ago
  It's not as bad as it sounds as a few of those transitions can be implemented via zero-copy APIs, so you're really just passing around pointers to shared memory.
  - dahfizz 6y ago
    The transitions happen efficiently, but the packet still has to spend cpu time traversing the network stack at all 6 hops. So the packet moves all the way through the routing / natting tables of the host, and then very quickly ends up in the VM. Then has to traverse the whole routing / natting tables of the VM.....
  - toast0 6y ago
    If there's NAT involved (as was posited in the start of the thread), then you can't just copy, you also need to modify.
- Matthias247 6y ago
  Correct me if I'm wrong, but I don't think the container transition introduces any additional copies and hops. Containers are not VMs. They just use isolation that the kernel enforces. Whether you send a packet from inside or outside a container to the network should be exactly the same amount of work unless you opt-in to run some extra software-level NAT step.

nickpsecurity 6y ago

Given clouds are like mainframes reinvented, you might like reading on how the mainframes handled I/O: https://en.wikipedia.org/wiki/Channel_I/O https://en.wikipedia.org/wiki/Channel_I/O I always thought PC's could be setup to do something similar. The cores used for that could even be small and simple compared to main cores. Folks in early 2001 could be Bittorrenting their Linux distros or whatever they use it for with no user-visible lag. I'd like at least one each for user-input devices, graphics, storage, and networking. Other good reasons to isolate them a bit from each other. There's already ARM SoC's in embedded that have a good core for apps and weak one for I/O. Recently, the RISC-V chip with the minion cores. If embedded can do it, then there seems to be no technical limitation holding back desktops. Just marketing, backward compatibility, etc.

Aaronstotle 6y ago

I'm curious, what makes the natting/routing performance much better on a router? Is it that the router is an ASIC designed only to process packets vs a cpu that can execute any code?

dsr_ 6y ago
In particular, a modern high-end router has a CPU (which might be an x86 of some kind) which handles administrative and auxiliary tasks (NTP, ssh, DHCP reflection, whatever you want) while making one or more dedicated ASICs handle packets. When you manage the router, you're actually talking to the supervisory CPU.
dahfizz 6y ago
> Is it that the router is an ASIC designed only to process packets vs a cpu that can execute any code? Yes. Most routers + switches are "line rate" meaning the packets go through the switch at the speed of electricity, as if the switch wasn't there are all.
rayiner 6y ago
That's not really the whole explanation. There are a couple of things in play. First, while Linux can perform routing, it's mainly intended to be an end host. So for example software like VPP that is designed with the assumption that it can use polling and burn up entire CPU cores to optimize packet forwarding can perform 10x faster on the same hardware. Second, router ASICs can be designed to sped transistors on a predictable packet flow path, instead of intelligence like out-of-order execution to make general purpose code fast. For example, routers have big expensive content-addressable memories called TCAMs that are used to store things like routing tables and ACLs. The ASIC, moreover, can implement a highly tuned pipeline designed around the latencies in the underlying memories. E.g. you get a packet, grab the destination address, look up the next hop in the routing table, etc. Each step takes a predictable amount of time that you can account for and optimize. Third, parallelism is much cheaper in hardware than in general purpose CPUs. It takes a lot more transistors to be able to execute a second general-purpose instruction stream than to have a single-purpose circuit that does some work in parallel with something else.

Klinky 6y ago

Do most scenarios require that pedal-to-the-metal level of performance? A lot of applications are CPU-bound either within the application server or a dependent service(database). Your payloads/throughput may actually be rather low, and the inefficiency may be a tiny percentage of CPU usage compared to the application itself. If you need nonstop CDN levels of bandwidth, then you're probably going to go with a setup that has fewer layers, but CRUD applications don't exactly need terabits of bandwidth.

dahfizz 6y ago
As the article explains, it's not all about throughput or latency (though those do suffer), it's also about how much CPU time you spend because of the network architecture. If you're running a cpu bound app, all the more reason to avoid sending 6+ interupts per packet.

neurostimulant 6y ago

I think I read somewhere a while ago that kubernetes cluster will start experiencing big slowdown at around 500 pods due to overhead of its internal networking components. Not sure if it's still true now. I'm going to deploy a cluster that might contains more than 500 pods soon so I'm not really eager to find out the hard way.