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Comment by andrewstuart | Hacker News Reader

>>Unikernels can be difficult to debug.

This is an irrelevant objection that seems to get massive traction.

Just like some politicians have scathing criticisms of other politicians, which despite being not true, are easily and widely believed.

It's not even a relevant point. Nothing I ever did with Unikernels hit this as even an issue on the horizon. If you want to knock down the technology, this is the most effective way to do it because it very effectively frightens off people considering the technology.

The origin of the "Unikernels can be difficult to debug." is from someone who works at a company that bet everything on containers. Nuff said.

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Instead of straw-manning the GP, why not address the very specific concerns raised? If there's an error you're seeing in a cluster of, say, 30 machines that only seems to happen once every few days, how are we supposed to debug this?

Since the application and kernel are joined together so closely, there aren't any parallel processes that could be monitoring the behavior of the unikernel app. You can't log in on the affected machine and pull logs really, because the process may not be able to even write a log about what's failing when its in the process of failing.

If you have a situation where a single server is having performance issues in a unikernel approach, there is no UNIX-style environment to log in and poke the hardware configuration and health, whereas on a more normal stack you could log in and see that NetworkManager has degraded the connection down from 10Gbps to 100Mbps because it can't negotiate a higher speed successfully, anymore, and then you can configure your alerting system to watch that stat, remove any boxes in that state from the load balancer, and inform DevOps about the bad hardware.

All of that to then say that I really do like the idea of Unikernels, and I believe they have their place, but that place tends to be very well-defined and infrequently changing problem domains that need very high performance, such as in supercomputers.

It doesn't work as well elsewhere because the application, by being joined so closely to the kernel, is also responsible for these things operating systems have traditionally taken care of for us, and problems in the application code can have an even more catastrophic effect than normal. Most applications are neither well-defined enough, time-invariant enough, or latency-constrained enough for the unikernel tradeoff to make sense.

One of the things that struck me when I first joined Google was the extent to which logging, debugging, and tracing information was built into the application itself, as well as the extent to which this information was routinely useful in a way that basic UNIX performance tools are not.

They have, for example, a distributed tracing system [1] integrated into their RPC layer that lets you instantly view the complete set of RPCs kicked off by a request, the services touched, and statistics for latency either for a single request or across many requests. This means that if you're a developer trying to integrate a new service into the search stack, you can get an answer within a couple of minutes as to "Your service will be on the critical path, so you need to count microseconds in implementation" vs. "Your service is off the critical path, go wild" vs. "You have a budget of 25ms, as long as you come in under that you should be fine." It lets you tell whether your application requests were slow because you triggered an edge-case in your code vs. because that BigTable your dependency relies upon was undergoing a compaction. Combined with some other application-level logging, you could tell if your ranking algorithm was slow vs. it happened to share a machine with a process that hogged the machine resources.

It's probably one of the features I miss most now that I'm no longer at Google and doing my own startup, and I've tried to apply the same development philosophy to my own code. It's been invaluable, for instance, to tell whether I'm not getting results because I can't parse a page but forgot to write the error-handler to mark it as done & unparseable vs. because a complicated algorithm I use is accidentally exponential-time in some cases vs. because an HTTP connection is taking too long to complete vs. because CloudFlare blocks robots and the site really doesn't want to be crawled so I should give up on it vs. because an algorithm I wrote to avoid overloading hosts has a bug that makes it get stuck performing requests once every month. Some of these could perhaps be caught with lsof or top, but they'd give me nowhere near as precise info when trying to track down the problem.

All this is to say that perhaps the application (or a library used by it) really is the right place for those tools to go, because it knows far more about its operation and the likely questions you'll ask than a generic system-wide tool. This'd require a change in how we write applications and what we consider to be the responsibilities of an application developer, which probably explains why we don't have mainstream adoption yet. But in a world that's moving from multiple-apps-on-one-computer to multiple-computers-for-one-app, it makes sense, and so I wonder if the long-term trend will go in that direction.

[1] https://static.googleusercontent.com/media/research.google.c...

I'm aware of those sorts of application level logs. I used that sort of thing when I worked at Uber, and I saw it built up over time as more classes of errors were encountered. :)

But there are a whole class of application failures that are handled out-of-band, which is what I was referring to: If you need to debug issues involving hardware then not having an interface to that hardware separate from your application can make it harder to inspect and determine what failure you're running into.

And again I point to application failures within the application frameworks themselves having issues (such as logs not being written out at all) that having an out-of-band monitoring process with a mostly-uncorrelated probability of failure seems to me to be very necessary for handling these less common but potentially more catastrophic issues.

I'll admit that this next statement is a bit of a Call to Authority fallacy, but you'll note that not even Uber or Google use unikernels in production, and I believe these two problem domains are why.

Perhaps, though it seems that if you want to debug hardware, the place to do that is within the hypervisor itself. And if you want out-of-band monitoring, the place to do that is from a supervisor process running on a different box, a la Erlang supervisor trees.

Very little of this infrastructure has been built yet, which is why we don't see this in production deployments. I'm talking decades out - I'll predict that the software industry will eventually tend towards this architecture because it's more efficient and better fits what we actually do with software now, but until it's a pressing pain point there's little urgency in getting there. Much like how all the folks who predicted in the 80s that CPUs would tend toward RISC were right, but it took 30 years and the ARM/mobile revolution to get there, and most desktops still have a hardware x86 emulation layer on top of RISC microcode.

I also agree that application -> cluster of machines is where we're going. I think it needs to be baked into an specialized programming language to really work well, though, and all of these things we rely on the OS for would have to be handled.

But unikernels running on top of a hypervisor seems like a terrible solution to the problem? You're trading away the convenience of the OS tooling that's built up over the past 40+ years to get closer to the metal and squeeze more performance out, then injecting virtualization in between and throwing that performance gain back away? Why not just make a regular application at that point and not throw away all of the runtime debugging support? A unikernel application running on top of a hypervisor basically turns the unikernel part into just an incredibly large and inefficient libc, in my mind.

> Why not just make a regular application at that point and not throw away all of the runtime debugging support?

A few reasons:

- There exist large public clouds willing to run any code you want under a hypervisor, but services that give you shell access to a user under a traditional kernel have mostly died out. This can partially be justified by security concerns: traditional syscall interfaces tend to be more complex and thus have more attack surface than the VM<->hypervisor interface. Hypervisors also tend to make it easier to divide system resources, e.g. by giving each VM a fixed RAM allocation.

- Some clouds, like EC2 with the Elastic Network Adapter, give virtual machines direct access to (custom) networking hardware, rather than making them trap to the hypervisor for every send and receive. This should mitigate much of the performance overhead of using a VM, at least as far as networking is concerned.

- Anyway, unikernels can put everything from filesystems and TCP to threading and even page table management "in-process"; this can reduce the number of syscalls that have to be performed and thus syscall overhead, even for operations that do ultimately delegate to "syscalls" in the hypervisor. In other words, they shouldn't be compared to just libc; they're also taking over many of the functions of a traditional kernel (just not all of them).

>but services that give you shell access to a user under a traditional kernel have mostly died out.

Shared hosting is alive and well, shell access is becoming more common to my knowledge.

>Hypervisors also tend to make it easier to divide system resources, e.g. by giving each VM a fixed RAM allocation.

CGroups. LXC and Docker are capable of RAM limits nowadays.

>traditional syscall interfaces tend to be more complex and thus have more attack surface than the VM<->hypervisor interface

A syscall interface is an assembly instruction and several registers that may point to some memory. The kernel is very thorough in checking the validity of such pointers (unless you use an ancient non-LTS kernel)

> give virtual machines direct access to (custom) networking hardware, rather than making them trap to the hypervisor for every send and receive.

Yes, VFIO and IOMMU have been around for a while. They do get pretty close to native performance (close enough for gaming atleast). It's not exactly new tech and full VMs with Linux images have been able to utilize full speed networking for a while now too.

Also note that virtio adapters are close to baremetal even without passthrough of the adapter.

>Anyway, unikernels can put everything from filesystems and TCP to threading and even page table management "in-process"; this can reduce the number of syscalls that have to be performed and thus syscall overhead

For all the stuff you mentioned syscall overhead isn't the driving performance factor unless you're google or facebook scale. TCP, threading, filesystems do spend most of their time waiting for DMA or other interrupts.

Page tables is usually not even a syscall and rather a interrupt from the CPU, the performance difference should be negligible.

> In other words, they shouldn't be compared to just libc; they're also taking over many of the functions of a traditional kernel (just not all of them).

It should be compared to libc because in the ideal deployment scenario there should be no difference.

Coming from a different domain (game development) I suspect there will always be a place for both approaches. Games can be quite complex to debug and profile and the complexity increases for networked multiplayer games. Typically many different tools are used each of which has strengths and weaknesses.

It's pretty standard for non final builds of games to have a number of custom debugging and performance tools built in. These may display information in game or capture to a local log but commonly (especially for console games) they connect to an external tool running on another machine over a socket or custom debugging interface. This is both because it can be easier to build UIs on a PC and because, particularly for performance profiling, displaying the information locally can impact what you're trying to measure significantly.

Consoles are normally debugged using a remote debugger running on a PC but that's often useful even for debugging PC games. Often console devkits had special hardware to support this which wouldn't be present in retail hardware which made debugging issues that only showed up on retail hardware challenging.

While engines typically have some graphics debugging and profiling tools built in, it is also common to use external tools, often connecting from another machine. There are many of these: PIX, Visual Studio Graphics Debugger, NVIDIA Nsight, Intel Graphics Performance Analyzer, GPUView, Windows Performance Analyzer... They all have strengths and weaknesses so it's common to use more than one of them.

Prior generation consoles were often unikernel-like as all OS type functionality was statically linked into your executable and you'd boot right into your game. Current generation consoles have something more like a full OS but you still get much more clearly defined guaranteed minimum resources and deal with hardware at a lower level than on a PC.

Overall there's a wide range of both custom and generic but specialized tooling used for debugging and performance analysis and that's been a fairly stable reality for many years. I don't think this is a situation where one particular paradigm has to win out in the long run, more a case of using the right tool for the job.

What you say is not wrong but people have very good debuggin tools for kernels. You can surely make a small debuggin monitoring layer in the unikernel and instrumentation. If it is not already done, Im sure it will be soon.

People need to debug their application. If you need to convince people that this is a myth, you need to tell people how to debug and inspect their application and system.

If anything your own response here is like the typical politician- avoiding the issue, providing anecdotes that promotes your view, going on the offense and changing the subject.

>> People need to debug their application.

I'm not saying applications don't need to be debugged, that would be silly. What I am saying is that this "Unikernels are hard to debug" is a hand wavey way of knocking down the technology, and it has really worked - witness this subthread.

The unikernel is the final build, the package, the output of application development and debugging which happens upstream, before you put it into the package.

There's always ways you can expose the inner workings of a unikernel to gain insight into whatever might be going wrong for whatever reason - if you really need to do it at the stage that it is a running unikernel - but in most cases its just not necessary because application development and debugging happens, as I say, upstream of the final building of the thing into a unikernel.

and debugging which happens upstream

I believe the objection is clearly about debugging in production, not during development. This is absolutely essential if you want to be able to fix things in production that broke unexpectedly long after development/deployment.

Yeah, it's really strange that he's not addressing this, since the unikernel answer is relatively straight forward--you just compile in the debugging abilities (e.g., a protected endpoint that lets you inspect the application dynamically). That said, I don't know of any unikernels that support this.

This likely means that you work on systems whose needs are very different from the average needs.

Applications absolutely need to be debugged in production, debugging does not only happen upstream before deployments. Unexpected things happen so you might need to list the current TCP connections, view the socket buffer sizes, produce a core dump, view metrics you didn't anticipate at development - even attach a debugger directly on a very bad day. (Please don't get hung up on these specific examples though - the point is more that the last decades have allowed us great system introspection into a production environment running on a traditional OS).

The perfect world where all that would be unnecessary for most people is not even close yet. People are largely not yet willing to give up on being able to ssh into their server and use the available tools to do troubleshooting , nor are they willing to change the development practice where that could largely be avoided.

Would love to see that guy parsing HTTP pages using some sort of manual software stack in python.

Exceptions? Could be raised at any level. Most of the time you can't predict at all which exceptions can happen.

Server-side errors? Could happen at any level. DNS switch/your provider forgot to restart DNS/Server out of RAM/...

Now try to do something along these lines at 1M requests/day. I mean, requests to several API providers who are probably not even providing good docs for their APIs.

There's no way you can have a scaled-up IT business without online debugging. Most of the time I spend on is debugging issues that I could absolutely not predict (or realistically, I could only predict them if my budget was a multiple of what it was) when I was writing the program.

Real-life problems are based on sets of contradicting requirements. And they are contradicting due to them being at the edge of our understanding; and that's why they are problems.

I'm a little confused. Are you arguing that unikernels are unfit for debugging or just that debugging is necessary in production? I agree with the latter but not the former.

I agree with this point, it seems most people don't get the fact that most of the debugging is happening in the development phase (before building an unikernel) and when you need to debug in production I think that's certainly not as difficult as it seems.

There's no issue with logging through network as opposed to logging to a file that you need to pull to see the contents (containers).

It's also common to have a debugging interface over network that you can use to inspect program state (eg. Clojure nREPL, node.js inspector protocol, and this can be built into any environment), I'm pretty sure even Rust can have networked REPL that can satisfy most debugging needs.

It's also common to have a debugging interface over network that you can use to inspect program state

Except in unikernels that have proudly eliminated the network stack.

What?!?

Unikernels have network stack, just not BSD sockets.

Writing to the network can be certainly be done in whatever debugging framwework, production variables they end up providing.

How do you imagine unikernels run in server applications without a network stack?

Easy, using remote debugging, just like we already can do with embedded hardware, JEE and ASP.NET servers.

It is just a matter of tool maturity.

> It is just a matter of tool maturity.

Err.. today's remote debugging is strongly based on the premises of the issues being isolated to some degree: broken app will not hit the network stack over which you run you remote session, misbehaving network stack doesn't impact serial communication, so using gdbserver is still possible (but already significantly harder), etc.

Will it be the same with unikernels?

I don't see why not, it is a matter of what the runtime allows and debugging tools.

When I talk to JEE servers via JMX I don't care if they run on as OS process, on a container, hypervisor or even a bare-metal JVM.

Likewise I get to enjoy cluster monitoring tools like Visual VM, Java Mission Control, New Relic APM, DataDog and many others, all blissful unaware of how the JEE server are actually deployed.

So I don't see why unikernels can't provide debugging access points for such kind of tooling.

> When I talk to JEE servers via JMX I don't care if they run on as OS process, on a container, hypervisor or even a bare-metal JVM.

But I think the whole point on the thread was that, making all this possible on unikernels, would require putting back a big part of the code that went away when making the unikernel lean and mean (and interesting because of that).

That is what people that favor an UNIX userland advocate.

An optional debug layer for JVM, Erlang, CLR running bare-metal (just as possible language examples) is tailored made for such runtimes and much thinner than all the services a general purpose OS needs to provide.

Honest question: Do you still have the ability to ls, find,grep, netstat, lsof stuff? I don't know if unikernels can be difficult to debug but if you told me I didn't have those I would be nervous at a glance at least. Maybe we've been mislead.

Most unikernels are a kernel specifically packaged for a certain application, so the answer would be no.

No, but they again you don't really need them.

Remote debugging and cluster monitoring tools like JEE servers provide would already be quite good.

Debugging UNIX way is not the only path.

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