What’s the meaning of «-pae» at the end of kernel version?
Last time I installed Ubuntu there was a «-pae» at the end of the kernel version in the boot screen. Now that I’ve reinstalled Ubuntu from the same disk instead of «-pae» there is a «-generic» at the end of kernel version. What’s the meaning of those terms? Why do they different between two installations?
Note that PAE requires hardware support. If you install the pae kernel on hardware that doesn’t support it, your system may crash. Install the generic kernel in such situations.
3 Answers 3
PAE stands for P-hysical A-ddress E-xtension to access 4 GiB RAM on 32-bit systems. This is not needed for a 64-bit Ubuntu.
for 11.10 and earlier
For 32-bit Ubuntu a PAE kernel is automatically downloaded and installed on a system with more than 3 GB of RAM. Otherwise, and if no network connection is available the generic kernel is used.
for 12.04 LTS
Note that from Ubuntu and Kubuntu 12.04 LTS a PAE kernel only can be installed from the 32-bit installation CD. This may cause problems on old hardware when the CPU does not support PAE. Release Notes
In this rare cases we may have to install 32-bit Lubuntu or Xubuntu that still come with a non-PAE-kernel. We can install the Unity desktop later. Alternatively we may also install 32-bit 10.04 or 11.10 with the non-PAE kernel first. With an upgrade to 12.04 a non-PAE-kernel can also be upgraded to a non-PAE-kernel.
Ubuntu 12.04 LTS is the last release to support a non-PAE kernel.
Linux what is pae
The PAE option provides support for the physical address extensions capability of the Intel Pentium Pro and above CPUs, and allows for up to 64 gigabytes of memory to be used in systems capable of supporting it. With the PAE option, memory above 4 gigabytes is simply added to the general page pool. The system makes no distinction between memory above or below 4 gigabytes, and no specific facility is provided for a process or the kernel to access more memory than they would otherwise be able to access, through a sliding window or otherwise.
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Since KLD modules are not compiled with the same options headers that the kernel is compiled with, they must not be loaded into a kernel compiled with the PAE option.
Many devices or their device drivers are not capable of direct memory access to physical addresses above 4 gigabytes. In order to make use of direct memory access IO in a system with more than 4 gigabytes of memory when the PAE option is used, these drivers must use a facility for remapping or substituting physical memory which is not accessible to the device. One such facility is provided by the busdma interface. Device drivers which do not account for such devices will not work reliably in a system with more than 4 gigabytes of memory when the PAE option is used, and may cause data corruption. The PAE kernel configuration file includes the PAE option, and explicitly excludes all device drivers which are known to not work or have not been tested in a system with the PAE option and more than 4 gigabytes of memory.
Many parameters which determine how memory is used in the kernel are based on the amount of physical memory. The formulas used to determine the values of these parameters for specific memory configurations may not take into account the fact there may be more than 4 gigabytes of memory, and may not scale well to these memory configurations. In particular, it may be necessary to increase the amount of virtual address space available to the kernel, or to reduce the amount of a specific resource that is heavily used, in order to avoid running out of virtual address space. The KVA_PAGES option may be used to increase the kernel virtual address space, and the kern.maxvnodes sysctl(8) may be used to decrease the number of vnodes allowed, an example of a resource that the kernel is likely to overallocate in large memory configurations. For optimal performance and stability it may be necessary to consult the tuning(7) manual page, and make adjustments to the parameters documented there.
What is Kernel-PAE in Linux ?
The Physical Address Extension (PAE) is a feature implemented in modern x86 processors. PAE extends memory addressing capabilities, allowing more than 4 GB of random access memory (RAM) to be used.
NOTE: All the Linux machine with kernel version above 2.3.23 supports PAE kernel.
From Wiki
With PAE, IA-32 architecture is augmented with additional address lines used to select the additional memory, so physical address size increases from 32 bits to 36 bits. This increases the physical memory addressable by the system from 4 GB to 64 GB. The 32-bit size of the virtual address is not changed, so regular application software continues to use instructions with 32-bit addresses and (in a flat memory model) is limited to 4 gigabytes of virtual address space. Operating systems supporting this mode use page tables to map the regular 4 GB address space into the physical memory, which depending on the operating system may be as big as 64 GB. The mapping is typically applied separately for each process, so that the extra memory is useful even though no single regular application can access it all simultaneously.
You can verify whether your processor supports PAE using the below command
$ cat /proc/cpuinfo | grep flags | grep pae
flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss syscall nx rdtscp lm constant_tsc ida nonstop_tsc pni ssse3 cx16 sse4_1 sse4_2 popcnt lahf_lm
If you don’t see an output it means PAE kernel is not supported by your processor. As you need to look for » pae «under flags.
PAE, Physical Address Extension
If you encounter an error related to PAE while installing Lubuntu 18.04 LTS, the solution is as follows:
Boot the computer with a 32-bit Lubuntu DVD in the DVD drive. When the image of a keyboard and a little man in a circle appears, hit the tab key.
With the cursor on the top choice press F6 (If you use the minimal ISO then press Tab in stead of F6).
A menu with a number of options appear. The option ‘forcepae’ is not there, so press Escape to close the list.
Now a string of options is visible, often with ‘quiet’ or ‘quiet splash —‘ at the end. Add ‘forcepae’ to the string before and after the two or three dashes («forcepae — forcepae»).
Press return, and after a moment the installation begins. A possible warning about forcepae being experimental does not matter for Pentium M or Celeron M processors.
If everything appears to work you don’t have to read further.
The «forcepae» option must be entered twice, before and after the delimiter «— «, so that it is applied to both the kernel on the ISO and the kernel on the system after installation. This is a change from older installs where only one parameter was sufficient. This link describes use of the «—» in boot options.
Technical note: After booting, «dmesg | grep -i pae» will now show «PAE forced!». You can use this as a check.
If you are going to upgrade (as opposed to install) there is advice here.
Details
Physical Address Extension (PAE) is a feature found on almost all 32 bit processors produced after Pentium Pro, ie. younger than around 1995. Because PAE is close to being a standard it is now a requirement for Ubuntu: During installation the processor is prompted for the PAE flag, and only if present the process will carry on.
Lubuntu and Xubuntu offered a PAE and a non-PAE release up to and including 12.04, but from 12.10 only the PAE releases are maintained.
A number of older Pentium M processors produced around 2003-4 (the Banias family) do not display the PAE flag, and hence a normal installation fails. However, these processors are in fact able to run the latest (and PAE-demanding) kernels if only the installation process is modified a little. The problem is not missing PAE, it’s about the processor not displaying its full capabilities.
Pentium M’s of the Dothan family display the PAE flag correctly and support the latest Buntus without modifications. The same distinction (Banias versus Dothan) goes for the lower performing Celeron M processors.
In spite of their age many of the affected computers (IBM Thinkpads and Dell Latitudes, for example) are suitable for today’s use if given a light distro like Xubuntu or Lubuntu; among other advantages they have a low power consumption. This guide describes a workaround for installing the latest Lubuntu and bringing them back to life — actually the guide can be modified to work for any member of the Ubuntu family, but because we are dealing with old hardware we focus on Lubuntu.
For 14.04 LTS a boot option has been added which makes the install straightforward, as can be seen at the top of the page. This method works also for newer version (16.04 LTS, 18.04 LTS, . ).
General note: If the installation is failing, are you sure the problem is related to PAE? A lot of misunderstandings flourish, and lack of PAE support is too often blamed when an installation on old hardware fails. Only if you get the error kernel requires features not present on the CPU: PAE or similar this guide comes into play.
Now we know that the problem is in fact related to the PAE flag. This gives several options:
A. Install 18.04 LTS as described at the top of the page. For most users this is the easiest and best solution.
B. If for some reason the computer does not run 18.04 LTS a number of distros are available. for example Bodhi Linux but check that you select a version with a reasonable time left until end of life.
In general, long term releases of standard Ubuntu are supported for 5 years, while the corresponding releases of the Ubuntu family flavours are supported for 3 years. See this link, www.ubuntu.com/info/release-end-of-life.
C. Move the hard disk to a computer on which the processor has PAE capability and PAE flag (that is, almost everything else than a Banias). Install the system as usual but don’t add restricted drivers. After the install move the disk back.
D. If you are handy with hardware and you can get your hands on a Dothan-class Intel Pentium M CPU with a 400MHz front-side bus, replace the non-PAE Banias CPU with a Dothan. Dothans with 400MHz FSB are marked Pentium 7×5, i.e. 715, 725, 735, 745, 755 and 765.
PAE (последним исправлял пользователь 98-132-17-89 2018-08-17 19:37:52)
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