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Linux Kernel 2.4 Or 2.6 In Embedded System?
snikulin writes "My 6-year-old embedded software happily runs on kernel v2.4 on an XScale CPU. The software gets a bunch (tens of megabytes) of data from an FPGA over a PCI-X bus and pushes it out over GigE to data-processing equipment. The tool chain is based on the somewhat outdated gcc v2.95. Now, for certain technical reasons we want to jump from the ARM-based custom board to an Atom-based COM Express module. This implies that I'll need to re-create a Linux RAM disk from scratch along with the tool chain. The functionality of the software will be essentially the same. My question: is it worth it to jump to kernel 2.6, or better to stick with the old and proven 2.4? What will I gain and what will I lose if I stay at 2.4 (besides the modern gcc compiler and the other related dev tools)?"
if you're migrating, no doubt you're performing tests to ensure your product is still fit.
once you have your test plan ready, determining fitness against either kernel should be straight-forward.
2.4 is horrible to work with. It's missing so many features you expect from a POSIXy system that you constantly have to find work-arounds. Having a 2.4 kernel on the cluster during my PhD was enormous pain - I'd write code on FreeBSD, copy it to the cluster, and find half the features were missing. 2.6 is a lot better from a feature-standpoint, but is much heavier and isn't really suited to embedded systems anymore. If you're building the image yourself, why not go with FreeBSD or OpenBSD and get the best of both worlds - FreeBSD if you lean more towards features, OpenBSD if you want a smaller footprint?
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...then go with the newer kernel. 2.6 has _lots_ of improvements above 2.4. The security aspects may be of less interest in your application, but the performance probably won't be. I've always believed that it is better to regret having done something than to regret having not done it.
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I'd move on. Not for any particular feature, but to stay closer to the mainstream for the next years. The 2.4 kernel, not for any technical reason, becomes increasingly exotic as people move on to 2.6.
You'll have to maintain your existing 2.4 skills for another decade when all others have moved.
Markus
Old and proven on a different hardware. Chances are your new hardware will have some issues (if only caused by you misunderstanding something) and then it would help to have the latest kernel that more people are using.
Also, Atom is a newer processor, perhaps with PCI Express in the chipset - does 2.4 support that ?
I had the same question asked for an embedded project 3 years ago. And it was very clear cut then
2.6 you get (off the top of my head)
-Modern drivers (including USB/Network/etc)
-Various tick rates and tickless
-More support
-Several other improvements
So really don't bother w/ 2.4
how long until
http://www.linuxdevices.com/articles/AT7751365763.html
Without knowing your exact parameters though, it's hard to debate any specific advantages.
Well, I don't have that much experience with 2.4, and how much is 'backported' from 2.6, but IIRC you can use better IP filtering tools in 2.6. And are all drivers for various hardware written to work with 2.4 as well?
It doesn't sound like you use linux hardly for anything else than for using the drivers for the NIC, so if your system works now, then there's probably no explicit reason to change. What I would worry about though, are your future needs. Even if you don't need to upgrade now, it might just be the perfect time to do it.
c++;
2.95 is/was regarded as a "golden" version for its maturity and stability.
I'm not certain that newest 4.3x is that much better on small embedded system without SSE and FPU units to be worth a swap...
I suggest both a GCC 4 compiler (probably gcc-4.2 or 4.3) and a Linux 2.6 kernel (perhaps at least 2.6.25) with a fairly recent (ie 2.6 or 2.7) GNU libc Indeed, all this perhaps uses a bit more RAM, but you'll have more RAM than before, and it bring a lot of important functionalities & improvements (including bug fixes). If you need a specialized HTTP server, consider GNU libmicrohttpd Regard & Happy New Year 2009
Since first 2.6 release, most of the developer force is gone from 2.4. Although officially they "support" 2.4, expect the support to be practically nonexistent when you bump into problems. No one should have even considered using 2.4 for the last couple years now. It is simply too risky.
If you foresee needing to periodically update the firmware to along with a library or app, then I would say a definitive YES - use the 2.6 kernel (assuming your device is supported).
It might also be the case that the board you would like to use is not supported in the 2.4 kernel if it's new enough - kernel developers usually don't want to waste time backporting their code if they can avoid it.
Which introduces the most important issue - backporting is a PITA!! To make a long story short, if you need to track a library or app, such as an embedded JRE, or a hardware interface that requires a kernel module inserted, playing catchup and needing to backport at the same time is an awful game of one-step-forward two-steps-back. Avoid it at all costs. Backporting is not always guaranteed to work!
The 2.4 kernel has a slightly faster boot time, while the 2.6 kernel has so many improvements that it's hard to shy away from. Do yourself a favour and go with a stable 2.6 kernel.
If you're doing embedded systems in mass market volume, it's a matter of hardware requirements and cost per unit. Then potentially staying with the 2.4 kernel may be a good choice. If what you're making is a small volume custom setup, I'd go with whatever is getting the most use and the most testing now, which is definately the 2.6 kernel.
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The largest benny for an embedded system with 2.6 is timing, really. The kernel is now, for the most part, 'almost' totally preemptable, bring sort real time to the kernel. Additionally, using RTAI Fusion, you can get hard real time. RTAI extensions have phased out support for the 2.4 kernel.
But one would have to ask, I suppose. If your just replacing a legacy component with the same thing, why change the code?
-- I'm the root of all that's evil, but you can call me cookie..
Lots of the improvements to 2.6 have probably been added to 2.4, but many come "native" to 2.6 so no outside patches are required. For example, kernel pre-emption, better scheduler, etc.. There are other intangibles too such as development time, testing, new toolchain, etc.., but you're already moving to a new processor and you'd have to do that anyway.
Sometime last year I was rebuilding some antique MIPS-based Linux from a 2.4 to a 2.6. Almost everything in the userspace was effortless (though much of it was based on Busybox); the main issue was related to some in-line assembler that took a while to figure out what it was doing. Once I did, I googled it and realized someone else had already solved a year or so ago.
So in short, no real benefit to sticking with 2.4 IMHO.
Others in this thread will adequately cover the feature differences between 2.4 and 2.6, though it sounds like 2.4 already covers your needs when it comes to functionality. This makes your question more of a management one than an engineering one.
With these types of decisions you need to look at what your constraints and requirements are, whether they be time, developer resources, product lifetime, estimated lifetime of leveraged technology (kernel 2.4 in this case), cash, etc. It sounds like you'll be doing the development yourself, but otherwise I can't tell what the rest of cycle looks like, so you need to clarify these things before making a decision.
Those are major considerations, but it gets more subtle when you consider things like how much time you'll save with future updates due to better development tools and support with a new kernel, etc., so you need to estimate also whether the time you spend up front will be saved down the line.
LS
There is a fine line between being a cultivated citizen and being someone else's crop. - A. J. Patrick Liszkie
So far all the positively moderated posts have advocated 2.6. Here's a slightly contrary view. You know 2.4. It seems to satisfy your needs. Why exactly are you considering change? Is there something 2.4 doesn't do that you want? I realise you might be asking in case there is some improvement that 2.6 may possibly provide that you've missed, but if the current setup does what you need then why would you even consider a change? My advice: stick with 2.4 unless 2.6 provides something additional that you definitely need.
There are some pretty compelling reasons to migrate, but looking at your specific application most of my favorite reasons don't apply. Since you're going to be changing your toolchain somewhat, the 2.6 migration isn't going to be that much more invasive. My reasons for wanting to change have mainly to do with filesystem improvements and USB improvements, which don't seem to have much traction for you. I'm assuming that you did your own hardware drivers for the PCI express data collection, so that shouldn't be a particularly big deal, except for having to redo for new hardware, which you have to do anyway.
So, like I said, I'd migrate but if you need to continue work with the old device for some reason I could see an argument for a continued freeze. The biggest downside to this is the larger jump you end up doing in another few years when you need to migrate for real functional reasons to some later kernel. It's always seemed easiest to me to embrace the opportunity to migrate if it makes reasonable sense.
GCC 3.4 is quite outdated. 2.95 is just plain old. Why not code in Fortran while you're at it?
My development group is also stuck with gcc 2.9x series because it's only compiler our toolchain maker (WindRiver) supports for VxWorks 5.X. I'm guessing he's in a similar situation. I can't complain though -- we've never had an issue with it.
This system is based on an Intel Atom CPU, so it does have SSE and and FPU.
For the main architectures (x86, x86-64, ARM, and maybe PowerPC), GCC has been getting significantly better with each release. For less well-supported architectures (like Hitachi's SH series, an uncommon embedded CPU, or really old architectures like 68k), there's usually an older version of GCC that works better than the current ones.
Given that this is a new(ish) CPU, newer versions of GCC are going to support it much better than older ones.
From a kernel point of view, it's likely to be easier to get everything running on an Atom board under Linux 2.6 than 2.4, simply because 2.6 has newer drivers.
However, the application itself may not compile correctly with GCC 4, or may compile but have incorrect behaviour. Changing the C compiler is one of those things that's probably not worth the effort unless you have to, or you have a decent test suite and a good idea of how all the code works.
Of course, given that this is C, not C++, there's no reason you couldn't run Linux 2.6, compile everything except the app itself with GCC 4, and then compile the app with GCC 2.95.
If you want to minimize latency in your applications, chances are you'll like the new scheduler implemented in 2.6.23 and following. In general, 2.6 has better support for realtime (low-latency) applications: http://www.ibm.com/developerworks/linux/library/l-real-time-linux/index.html
I thought, aaaah, he's built a robust linux PC for his kid. But isn't insisting that his kiddy's first PC has a bang-up-to-date GCC compiler a little extreme?
Then I re-read it. Oops. Makes much more sense now. Not as cute, though.
Eric Baird
To go to 2.6 you have to rewrite ALL your custom drivers and the board configuration files. Yes is nicer in 2.6 - but it still has to be done. Just like we did in 2.2, 2.4 and probably again for >= 2.8.
It is a real pain that the kernel and apps change so rapidly. You regularly run into compatibility problems with:
I recommend:
The port from 2.4 to 2.6 can take 2 days to 2 weeks. If you can afford 2 weeks go for 2.6.
... and go with the 2.5 kernel.
And the Hebrew translation of the bible does it injustice.
Hi
We moved our project from 2.4 to 2.6 during development, because the maximum interrupt latency of 2.6 is so much better. We needed to handle UDP packets within 20 ms. max and occasionally on 2.4 we would have a 60 or more. Going to 2.6 solved our problems immediately, even with early versions.
See this Linux Journal article for more details on our project http://m.linuxjournal.com/article/7190
Bart van Deenen
Linux 2.4 might be "proven" on your old Xscale system, but I doubt anyone else has even _tried_ to use Linux 2.4 on something as new as Atom. Linux 2.4 will also lack support for any of the peripherals of your Atom com module.
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If you want people to stay with old code to run on their boxes, please leave GCC out of the mix.
GCC's had two major improvements over the versions:
1. Better language compliance. It matters *a*lot*. And frankly, you're not going to win an argument against that on a techie forum :-) This included a new hand-written C++ parser in g++ ~3.4 that closed out over 100 bugs at once. You don't ignore hand-written C++ parsers, they're complete bitches to do.
2. Better optimizer.
As for any library bloat, the basic C/C++ libraries are shared by nearly everyone on your system. You pay for them exactly once per running system (minus templates, but I don't know of anyone using template-heavy code on a normal Linux desktop installation).
If you were running Solaris, I'd just tell you to dtrace the system and find out what's making it slow. But, no, *sigh*,
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I have had issues with the VxWorks GCC 2.95.3 on several occasions when the compiler generated incorrect code resulting in crashes and lockups. The C code was correct, but the resulting MIPS assembly was incorrect. Each time, making slight changes to the C code would fix it, i.e. replace a for loop with a while loop. I say good riddance to VxWorks. The memory management in VxWorks 5.4 was atrocious and had to replace malloc with DLMalloc plus add a method of tracking memory usage on running systems (marking each memory block with the task and caller PC) so we could find and fix memory leaks on systems out in the field. The networking was also pretty bad in it.
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As someone facing a similar situation at his day job, 2.4 is painful in some regards. In my case, 2.6 allows me to do a non-standard initramfs (the stock is minimal and then you can load other initrd or initramfs images from the kernel options...) so that I can tapdance around the three differing hackish bootloaders they did in 2.4. This allows me to do major cleanups in what they did for doing NFS rootfs on the IXP2800 blades and on the X86 ones with minimal pain.
Most of the people commenting on 2.6 being too big are thinking of the whole size with everything loaded up. Minimal kernels with just your drivers loaded and only your drivers in the module build, you end up with only about 5-10% increase in footprint in memory and store space, with the ability to provide modern device support for things. In the case of what you mention, you're moving to an Atom based machine board. Given that you're moving to a modern board, the odds of things being "nicely" supported is lower with the 2.4 kernel.
Since you're manipulating large volumes of data over GigE, you're going to want to switch, probably even with the old ARM stuff if you can manage it. 2.6 provides much more responsive networking performance (so long as you do your network code right and don't dink with the scheduler (heh...let's just say I corrected a not so good idea there recently...)).
You may have to port a few custom drivers over to 2.6, but in the end, it'll work better since the driver architecture is better in 2.6.
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Well, it's not wise to change both the hardware and the software at the same time. You think it will reduce your time to market but it might increase it instead due to the numerous changes that will have to happen in your toolchain before getting anything barely working again.
From what I understand, you have a big experience in 2.4 and Xscale. 2.4 Also works on x86, so you'll not have to re-learn everything from scratch by just changing the architecture. All your toolchains, boot scripts, packaging scripts, etc... will still work as they did before. Then, only once you get familiar with your new architecture and the minor changes you might observe in the boot sequence, build process etc... it will be the right time to evaluate a migration to 2.6. Once you put your finger there, you'll quickly discover that you need to upgrade your gcc, glibc, replace modutils with module-init-tools, experiment with hotplug and sysfs, maybe switch to udev, etc... Step by step you'll notice a big number of changes, but you will be able to proceed one at a time, which is not possible if you change the soft at the same time as the hardware.
Also there are other aspects to consider. 2.4 has been maintained for a very long time, and you're probably used to backport some mainline fixes to your own kernel from time to time. 2.6 is not maintained that long (avg 6 months), and changes so fast that you will not be able to backport fixes for many years. I'd strongly recommend to start with 2.6.27, because Adrian Bunk will maintain it for a long time, as he did with 2.6.16. Once 2.6.27 is not maintained anymore (in about 2 years) you'll have to decide whether you stick to 2.6.27 and try to backport fixes yourself or switch to 2.6.36 (just a guess).
Also, 2.4 accepts almost no new hardware nowadays. If your new platform works well, that's fine, but how can you be sure that next year your GigE NIC will not change to something not supported anymore ?
I would say that the only case where 2.4 would make sense for a long term starting from now is if you don't have the time to revalidate 2.6 or to wait for 2.6.27 to stabilize, and need to quickly release something which will sit at your customer's in a place where it cannot be upgraded. Something like "install and forget". But I don't feel like it's what you're looking for.
So, to summarize :
1) switch your architecture
2) switch your kernel
Whether an official release of your product exists between 1 and 2 is just a matter of your time constraints and customer demand.
Last, to show you you're not alone, I'm too considering switching our products to 2.6, but next release will still be 2.4. Too many changes for a short-term release, and 2.6.27 not ready yet to reach years of uptime (but it's getting better though). 2.6.25 was particularly good but not maintained anymore.
Hoping this helps,
Willy
I do embedded Linux and have done so on several generations of hardware and kernel.
There are a lot of slightly incorrect comments here about the size of the 2.6 kernel vs. 2.4. Once you know what your target hardware platform will is, and which subsystems you need, you can tailor a kernel to be usably small by configuring out the parts that you don't need. Don't need USB, or video4linux? Leave it out. Don't use modules, unless you need to control the load order, set parameters, or want to be able to swap hardware platforms, since they take up space in two places (ram disk and kernel memory).
One of the best reasons that I have for the 2.6 family is the new version of ram disk. You can almost trivially generate a run-time ram disk in the new cpio format, with a tweaked init that doesn't need to go to a disk for root, although some kind of non-volatile storage will certainly be needed.
It will be harder and more expensive to find really small RAM DIMMs for the Atom. Get a size that will be manufactured for a while. Same thing with your non-volatile storage. On a current project using a commercial board (including a GUI, commercial multi-language font set, media player, and a kernel with modules so we can move to other hardware, as needed), we've got a 512MByte DIMM for the Intel chip set with room to spare.
Another great reason for moving on is development. Have you tried to run a 2.4-based distribution on computers that you can buy today? It's doable, but painful. If you want to give your developers a (for example) Ubuntu 8.10 distro, which can be easily guested to freeze your build environment, they can run on the same kernel and libraries you do in the target for initial build and test.
Your question feels a bit of strange question to ask as surely anyone who has looked would notice a huge difference between the latest 2.6 (2.6.28) and the latest 2.4 (2.4.37).
Preemptible kernel (so lower latencies are possible)
Far more devices supported (both in terms of architectures and additional add on devices e.g. SATA support)
Better scheduler (initially made O(1) scales better under load and then fairer with CFS)
Task Control Groups
Better support for threads (schedules them in a more intelligent fashion)
Strict overcommit
Massive VM changes
Tickless/dynticks support
Asynchronous I/O support
Introduction of different I/O schedulers (deadline, cfq
Network stack improvements (faster, better under load e.g. NAPI support)
epoll support
Improved ACPI support
Network filesystem improvements
Initramfs support
There is a huge list of Linux kernel changes that happened between 2.4 and 2.5. There is also a good Linux kernel 2.5 changes page on IBM's developerworks. Kernelnewbies has an excellent summary of changes for each of the 2.6 kernels and a 2.5 changes page. LWN is also excellent for kernel news.
I hate it when people don't bother to state exactly the points they object to. What other changes (not listed above) do you think the question poster wouldn't benefit from? Follow the links to the full lists (don't just use the ones off the top of your head)...