Enkh Tumenbayar,

v1.4, 9 july 2002

This HOWTO reviews main issues (and I hope solutions) related to SMP configuration under Linux.

1. Licensing

2. Introduction

3. Questions related to any architectures

4. x86 architecture specific questions

5. Sparc architecture specific questions

6. PowerPC architecture specific questions

7. Alpha architecture specific questions

8. Useful pointers

9. Glossary

10. What's new ?

11. List of contributors

1. Licensing

This document is made available under the terms of the GNU Free Documentation License. You should have received a copy with it. If not, it is available online at

2. Introduction

Linux works on SMP (Symmetric Multi-Processors) machines. SMP support was introduced with kernel version 2.0, and has improved steadily ever since.

HOWTO maintained by Enkh Tumenbayar ( The latest edition of this HOWTO can be found at

If you want to contribute to this HOWTO, I would prefer a diff against the SGML version. If you send me an email about this HOWTO, please include a tag like [Linux SMP HOWTO] in the Subject: field of your e-mail. It helps me to automatically sort mails (and you will have a faster reply ;)).

This HOWTO is an improvement of a first draft made by Chris Pirih and maintained by David Mentre.

All information contained in this HOWTO is provided "as is." All warranties, expressed, implied or statutory, concerning the accuracy of the information of the suitability for any particular use are hereby specifically disclaimed. While every effort has been taken to ensure the accuracy of the information contained in this HOWTO, the authors assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein.

3. Questions related to any architectures

3.1 Kernel Side

  1. Does Linux support multi-threading? If I start two or more processes, will they be distributed among the available CPUs?

    Yes. Processes and kernel-threads are distributed among processors. User-space threads are not.

  2. What kind of architectures are supported in SMP?

    From Alan Cox:

    SMP is supported in 2.0 on the hypersparc (SS20, etc.) systems and Intel 486, Pentium or higher machines which are Intel MP1.1/1.4 compliant. Richard Jelinek adds: right now, systems have been tested up to 4 CPUs and the MP standard (and so Linux) theoretically allows up to 16 CPUs.

    SMP support for UltraSparc, SparcServer, Alpha and PowerPC machines is in available in 2.2.x.

    From Ralf Bächle:

    MIPS, m68k and ARM does not support SMP; the latter two probly won't ever.

    That is, I'm going to hack on MIPS-SMP as soon as I get a SMP box ...

  3. Does SMP distribute the threads among the processors or is the library the one in charge of it?

    (Matti Aarnio) The way Linux implements threads is to treat them at scheduling the same way as any process - thread just happens to share several resources of the originating process; memory space, file descriptors. See clone(2) for part of explanation.

  4. How do I make a Linux SMP kernel?

    Most Linux distributions don't provide a ready-made SMP-aware kernel, which means that you'll have to make one yourself. If you haven't made your own kernel yet, this is a great reason to learn how. Explaining how to make a new kernel is beyond the scope of this document; refer to the Linux Kernel Howto for more information. (C. Polisher)

    Configure the kernel and answer Y to CONFIG_SMP.

    If you are using LILO, it is handy to have both SMP and non-SMP kernel images on hand. Edit /etc/lilo.conf to create an entry for another kernel image called "linux-smp" or something.

    The next time you compile the kernel, when running a SMP kernel, edit linux/Makefile and change "MAKE=make" to "MAKE=make -jN" (where N = number of CPU + 1, or if you have tons of memory/swap you can just use "-j" without a number). Feel free to experiment with this one.

    Of course you should time how long each build takes :-) Example:

    make config
    time -v sh -c 'make dep ; make clean install modules modules_install'

    If you are using some Compaq MP compliant machines you will need to set the operating system in the BIOS settings to "Unix

    In kernel series 2.0 up to but not including 2.1.132, uncomment the SMP=1 line in the main Makefile (/usr/src/linux/Makefile).

    In the 2.2 version, configure the kernel and answer "yes" to the question "Symmetric multi-processing support" (Michael Elizabeth Chastain).


    enable real time clock support by configuring the "RTC support" item (in "Character Devices" menu) (from Robert G. Brown). Note that inserting RTC support actually doesn't afaik prevent the known problem with SMP clock drift, but enabling this feature prevents lockup when the clock is read at boot time. A note from Richard Jelinek says also that activating the Enhanced RTC is necessary to get the second CPU working (identified) on some original Intel Mainboards.


    (x86 kernel) do NOT enable APM (advanced power management)! APM and SMP are not compatible, and your system will almost certainly (or at least probably ;)) crash while booting if APM is enabled (Jakob Oestergaard). Alan Cox confirms this : 2.1.x turns APM off for SMP boxes. Basically APM is undefined in the presence of SMP systems, and anything could occur.


    (x86 kernel) enable "MTRR (Memory Type Range Register) support". Some BIOS are buggy as they do not activate cache memory for the second processor. The MTRR support contains code that solves such processor misconfiguration.

    You must rebuild all your kernel and kernel modules when changing to and from SMP mode. Remember to make modules and make modules_install (from Alan Cox).

    If you get module load errors, you probably did not rebuild and/or re-install your modules. Also with some 2.2.x kernels people have reported problems when changing the compile from SMP back to UP (uni-processor). To fix this, save your .config file, do make mrproper, restore your .config file, then remake your kernel (make dep, etc.) (Wade Hampton). Do not forget to run lilo after copying your new kernel.


    make config # or menuconfig or xconfig
    make dep
    make clean
    make bzImage # or whatever you want
    # copy the kernel image manually then RUN LILO 
    # or make lilo
    make modules
    make modules_install

  5. How do I make a Linux non-SMP kernel?

    In the 2.0 series, comment the SMP=1 line in the main Makefile (/usr/src/linux/Makefile).

    In the 2.2 series, configure the kernel and answer "no" to the question "Symmetric multi-processing support" (Michael Elizabeth Chastain).

    You must rebuild all your kernel and kernel modules when changing to and from SMP mode. Remember to make modules and make modules_install and remember to run lilo. See notes above about possible configuration problems.

  6. How can I tell if it worked?

     cat /proc/cpuinfo 

    Typical output (dual PentiumII):

    processor       : 0
    cpu             : 686
    model           : 3
    vendor_id       : GenuineIntel
    bogomips        : 267.06
    processor       : 1
    cpu             : 686
    model           : 3
    vendor_id       : GenuineIntel
    bogomips        : 267.06

  7. What is the status of converting the kernel toward finer grained locking and multithreading?

    Linux kernel version 2.2 has signal handling, interrupts and some I/O stuff fine grain locked. The rest is gradually migrating. All the scheduling is SMP safe.

    Kernel version 2.3 (next 2.4) has really fine grained locking. In the 2.3 kernels the usage of the big kernel lock has basically disappeared, all major Linux kernel subsystems are fully threaded: networking, VFS, VM, IO, block/page caches, scheduling, interrupts, signals, etc. (Ingo Molnar)

  8. What has changed between 2.2.x and 2.4.x kernels?

    (Mark Hahn) In many parts of the kernel, there's little relation between 2.2 and 2.4. One of the biggest changes is SMP - not just the evolutionary fine-graining of locks, but the radically revamped VM, memory management, interrupt handling that's basically unrelated to 2.2, fairly revolutionary net changes (thread and zero-copy), etc.

    In short, 2.2 doesn't use the hardware like 2.4 does.

  9. Does Linux SMP support processor affinity?

    Standard kernel

    No and Yes. There is no way to force a process onto specific CPU's but the linux scheduler has a processor bias for each process, which tends to keep processes tied to a specific CPU.


    Yes. Look at PSET - Processor Sets for the Linux kernel:

    The goal of this project is to make a source compatible and functionally equivalent version of pset (as defined by SGI - partially removed from their IRIX 6.4 kernel) for Linux. This enables users to determine which processor or set of processors a process may run on. Possible uses include forcing threads to separate processors, timings, security (a `root' only CPU?) and probably more.

    It is focused around the syscall sysmp(). This function takes a number of parameters that determine which function is requested. Functions include:

    • binding a process/thread to a specific CPU
    • restricting a CPU's ability to execute some processes
    • restricting a CPU from running at all
    • forcing a cpu to run _only_ one process (and its children)
    • getting information about a CPU's state
    • creating/destroying sets of processors, to which processes may be bound

  10. Where should one report SMP bugs to?

    Please report bugs to

  11. What about SMP performance?

    If you want to gauge the performance of your SMP system, you can run some tests made by Cameron MacKinnon and available at

    Also have a look at this article by Bryant, Hartner, Qi and Venkitachalam that compares 2.2 and 2.3/2.4 UP and SMP kernels : SMP Scalability Comparisons of Linux¨ Kernels 2.2.14 and 2.3.99 (Ray Bryant) (You'll find also a copy here)

3.2 User Side

  1. Do I really need SMP?

    If you have to ask, you probably don't. :) Generally, multi-processor systems can provide better performance than uni-processor systems, but to realize any gains you need to consider many other factors besides the number of CPU's. For instance, on a given system, if the processor is generally idle much of the time due to a slow disk drive, then this system is "input/output bound", and probably won't benefit from additional processing power. If, on the other hand, a system has many simultaneously executing processes, and CPU utilization is very high, then you are likely to realize increased system performance. SCSI disk drives can be very effective when used with multiple processors, due to the way they can process multiple commands without tying up the CPU. (C. Polisher)

  2. Do I get the same performance from 2-300 MHz processors as from one 600 MHz processor?

    This depends on the application, but most likely not. SMP adds some overhead that a faster uniprocessor box would not incur (Wade Hampton). :)

  3. How does one display mutiple cpu performance?

    Thanks to Samuel S. Chessman, here are some useful utilities:

    Character based:

    Basically, it's procps v1.12.2 (top, ps, et. al.) and some patches to support SMP.

    For 2.2.x, Gregory R. Warnes as made a patch available at


    xosview-1.5.1 supports SMP. And kernels above 2.1.85 (included) the cpuX entry in /proc/stat file.

    The official homepage for xosview is:

    You'll find a version patched for 2.2.x kernels by Kumsup Lee :

    By the way, you can't monitor processor scheduling precisely with xosview, as xosview itself causes a scheduling perturbation. (H. Peter Anvin)

    And Rik van Riel tell us why:

    The answer is pretty simple. Basically there are 3 processes involved:
    1. the cpu hog (low scheduling priority because it eats CPU)
    2. xosview
    3. X

    The CPU hog is running on one CPU. Then xosview wakes up (on the other CPU) and starts sending commands to X, which wakes up as well.

    Since both X and xosview have a much higher priority than the CPU hog, xosview will run on one CPU and X on the other.

    Then xosview stops running and we have an idle CPU --> Linux moves the CPU hog over to the newly idle CPU (X is still running on the CPU our hog was running on just before).

  4. How can I enable more than 1 process for my kernel compile?


            # make [modules|zImage|bzImages] MAKE="make -jX"
            where X=max number of processes.
            WARNING: This won't work for "make dep".

    With a 2.2 like kernel, see also the file /usr/src/linux/Documentation/smp.txt for specific instruction.

    BTW, since running multiple compilers allows a machine with sufficient memory to use use the otherwise wasted CPU time during I/O caused delays, make MAKE="make -j 2" -j 2 actually helps even on uniprocessor boxes (from Ralf Bächle).

  5. Why is the time given by the time command inaccurate? (from Joel Marchand)

    In the 2.0 series, the result given by the time command is false. The sum user+system is right *but* the spreading between user and system time is false.

    More precisely: "The explanation is, that all time spent in processors other than the boot cpu is accounted as system time. If you time a program, add the user time and the system time, then you timing will be almost right, except for also including the system time that is correctly accounted for" (Jakob Østergaard).

    This bug is corrected in 2.2 kernels.

3.3 SMP Programming

Section by Jakob Østergaard.

This section is intended to outline what works, and what doesn't when it comes to programming multi-threaded software for SMP Linux.

Parallelization methods

  1. POSIX Threads
  2. PVM / MPI Message Passing Libraries
  3. fork() -- Multiple processes

Since both fork() and PVM/MPI processes usually do not share memory, but either communicate by means of IPC or a messaging API, they will not be described further in this section. They are not very specific to SMP, since they are used just as much - or more - on uniprocessor computers, and clusters thereof.

Only POSIX Threads provide us with multiple threads sharing ressources like - especially - memory. This is the thing that makes a SMP machine special, allowing many processors to share their memory. To use both (or more ;) processors of an SMP, use a kernel-thread library. A good library is the LinuxThreads, a pthread library made by Xavier Leroy which is now integrated with glibc2 (aka libc6). Newer Linux distributions include this library by default, hence you do not have to obtain a separate package to use kernel threads.

There are implementations of threads (and POSIX threads) that are application-level, and do not take advantage of the kernel-threading. These thread packages keep the threading in a single process, hence do not take advantage of SMP. However, they are good for many applications and tend to actually run faster than kernel-threads on single processor systems.

Multi-threading has never been really popular in the UN*X world though. For some reason, applications requiring multiple processes or threads, have mostly been written using fork(). Therefore, when using the thread approach, one runs into problems of incompatible (not thread-ready) libraries, compilers, and debuggers. GNU/Linux is no exception to this. Hopefully the next few sections will sched a little light over what is currently possible, and what is not.

The C Library

Older C libraries are not thread-safe. It is very important that you use GNU LibC (glibc), also known as libc6. Earlier versions are, of course possible to use, but it will cause you much more trouble than upgrading your system will, well probably :)

If you want to use GDB to debug your programs, see below.

Languages, Compilers and debuggers

There is a wealth of programming languages available for GNU/Linux, and many of them can be made to use threads one way or the other (some languages like Ada and Java even have threads as primitives in the language).

This section will, however, currently only describe C and C++. If you have experience in SMP Programming with other languages, please enlighten us.

GNU C and C++, as well as the EGCS C and C++ compilers work with the thread support from the standard C library (glibc). There are however a few issues:

  1. When compiling C or C++, use the -D_REENTRANT define in the compiler command line. This is necessary to make certain error-handling functions work like the errno variable.
  2. When using C++, If two threads throw exceptions concurrently, the program will segfault. The compiler does not generate thread-safe exception code. The workaround is to put a pthread_mutex_lock(&global_exception_lock) in the constructor(s) of every class you throw(), and to put the corresponding pthread_mutex_unlock(...) in the destructor. It's ugly, but it works. This solution was given by Markus Ferch.

The GNU Debugger GDB as of version 4.18, should handle threads correctly. Most Linux distribution offer a patched, thread-aware gdb.

It is not necessary to patch glibc in any way just to make it work with threads. If you do not need to debug the software (this could be true for all machines that are not development workstations), there is no need to patch glibc.

Note that core-dumps are of no use when using multiple threads. Somehow, the core dump is attached to one of the currently running threads, and not to the program as a whole. Therefore, whenever you are debugging anything, run it from the debugger.

Hint: If you have a thread running haywire, like eating 100% CPU time, and you cannot seem to figure out why, here is a nice way to find out what's going on: Run the program straight from the shell, no GDB. Make the thread go haywire. Use top to get the PID of the process. Run GDB like gdb program pid. This will make GDB attach itself to the process with the PID you specified, and stop the thead. Now you have a GDB session with the offending thread, and can use bt and the like to see what is happening.

Other libraries

ElectricFence: This library is not thread safe. It should be possible, however, to make it work in SMP environments by inserting mutex locks in the ElectricFence code.

Other points about SMP Programming

  1. Where can I found more information about parallel programming?

    Look at the Linux Parallel Processing HOWTO

    Lots of useful information can be found at Parallel Processing using Linux

    Look also at the Linux Threads FAQ

  2. Are there any threaded programs or libraries?

    Yes. For programs, you should look at: Multithreaded programs on linux (I love hyperlinks, did you know that ? ;))

    As far as library are concerned, there are:

    OpenGL Mesa library

    Thanks to David Buccarelli, Andreas Schiffler and Emil Briggs, it exists in a multithreaded version (right now [1998-05-11], there is a working version that provides speedups of 5-30% on some OpenGL benchmarks). The multithreaded stuff is now included in the regular Mesa distribution as an experimental option. For more information, look at the Mesa library


    Pentium Pro Optimized BLAS and FFTs for Intel Linux

    Multithreaded BLAS routines are not available right now, but a dual proc library is planned for 1998-05-27, see Blas News for details.

    The GIMP

    Emil Briggs, the same guy who is involved in multithreaded Mesa, is also working on multithreaded The GIMP plugins. Look at for more info.

3.4 MultiProcessor Specification Support (MPS)

(Randy Dunlap) Linux supports MPS (MP spec.) version 1.1 and 1.4.

Linux doesn't have full support for all of MPS version 1.4.

Experience has shown that Linux usually works best when the BIOS is configure for MP Spec. version 1.1 if that is an option in your system's BIOS. I don't see why the MP Spec. version should matter to Linux, but it would be an interesting exercise to find out the differences as presented by BIOS tables, to determine why Linux fails with MP Spec. version 1.4 in some cases, and to fix Linux so that this wouldn't matter.

This document summarizes the major changes in MP spec. version 1.4 and their support status in Linux.

Symmetric I/O Mode

The hardware must support a mode of operation in which the system can switch easily to Symmetric I/O mode from PIC or Virtual Wire mode. When the operating system is ready to swtich to MP operation, it writes a 01H to the IMCR register, if that register is implemented, and enables I/O APIC Redirection Table entries. The hardware must not require any other action on the part of software to make the transition to Symmetric I/O mode.

Linux recognizes and supports this MP configuration mode.

Floating Point Exception Interrupt

For PC/AT compatibility, the bootstrap processor must support DOS-compatible FPU execution and exception handling while running in either of the PC/AT-compatible modes. This means that floating point error signals from the BSP must be routed to the interrupt request 13 signal, IRQ13, when the system is in PIC or virtual wire mode. While floating point error signals from an application processor need not be routed to IRQ13, platform designers may choose to connect the two. For example, connecting the floating point error signal from application processors to IRQ13 can be useful in the case of a platform that supports dynamic choice of BSP during boot.

In symmetric mode, a compliant system supports only on-chip floating point units, with error signaling via interrupt vector 16. Operating systems must use interrupt vector 16 to manage floating point exceptions when the system is in symmetric mode.

Linux does not use the floating point interrupt at all except in genuine i386 processor systems which are not SMP-capable. [In these systems, if they wire the FPU exception line in the PC/AT-compatible way, a run-time check for #MF exception availability is performed. If the #MF exception is available, then Linux handles this interrupt if it happens. (Maciej W. Rozycki)

Multiple I/O APIC Configurations

Multiple I/O APICs are supported in Linux.

MP Configuration Table

This table was made optional in MPS version 1.4. If the table isn't present, one of the default configurations should be used. An extended section was also added to it for new table entry types.

Linux supports the optional MP Configuration Table and uses a default configuration if the MP Config. Table is not present.

Linux tolerates extended section table entries by skipping over them if they are found. Data in the extended table entries is not used.

MP Configuration Table Header Fields

New or changed fields for MP Spec. version 1.4:

Extended MP Configuration Table Entries

Entry types for System Address Space Mapping, Bus Hierarchy Descriptor, and Compatibility Bus Address Space Modifier are defined.

Linux skips over (does not use) these extended MP Configuration table entries. Apparently this isn't critical to any shipping systems.

4. x86 architecture specific questions

4.1 Why it doesn't work on my machine?

  1. Can I use my Cyrix/AMD/non-Intel CPU in SMP?

    Yes. Current AMD Athlon MP processors support SMP with the AMD 760MP chipset. There are several boards available featuring this chipset, e.g. from Tyan, ASUS, etc. Athlon/SMP is supported by recent 2.4.x kernels and also by the latest 2.2.x kernels. (David Haring)

  2. Why doesn't my old Compaq work?

    Put it into MP1.1/1.4 compliant mode.

    check "Configure Hardware" -> "View / Edit details" -> "Advanced mode" (F7 I think) for a configuration option "APIC mode" and set this to "full Table mode". This is an official Compaq recommandation. (Daniel Roesen)

    (Adrian Portelli)To do this:

    1. Press F10 when the server boots to enter the System Configuration Utility
    2. Press Enter to dismiss the splash screen
    3. Immediately press CTRL+A
    4. A message will appear informing you that you are now in "Advanced Mode"
    5. Then select "Configure Hardware" -> "View / Edit details"
    6. You will then see the advanced settings (intermixed with the ordinary ones)
    7. Stroll down to "APIC Mode" and then select "Fully Mapped"
    8. Save changes and reboot

  3. I can't get my Compaq SystemPro work in SMP mode.

    (Maciej W. Rozycki) Chances are that your Compaq do not make use of 82489DX APICs as they were introduced quite late -- in late 1992 or early 1993. There used to be i486 machines that implemented the APIC architecture. 82489DX is the chip that was used for them and it contained a local APIC unit and an I/O APIC unit.

  4. Why doesnt my ALR work?

    From Robert Hyatt : ALR Revolution quad-6 seems quite safe, while some older revolution quad machines without P6 processors seem "iffy"...

  5. Why does SMP go so slowly? or Why does one CPU show a very low bogomips value while the first one is normal?

    From Alan Cox: If one of your CPU's is reporting a very low bogomips value the cache is not enabled on it. Your vendor probably provides a buggy BIOS. Get the patch to work around this or better yet send it back and buy a board from a competent supplier.

    A 2.0 kernel (> 2.0.36) contains the MTRR patch which should solve this problem (select option "Handle buggy SMP BIOSes with bad MTRR setup" in the "General setup" menu).

    I think buggy SMP BIOS handling is automatic in latest 2.2 kernels.

  6. I've heard IBM machines have problems

    Some IBM machines have the MP1.4 bios block in the EBDA, allowed but not supported below 2.2 kernels.

    There is an old 486SLC based IBM SMP box. Linux/SMP requires hardware FPU support.

  7. Is there any advantage of Intel MP 1.4 over 1.1 specification?

    Nope (according to Alan :) ), 1.4 is just a stricker specs of 1.1.

    Please see the Useful Pointers for comparison between MP 1.4 and 1.1.

  8. Why does the clock drift so rapidly when I run linux SMP?

    This is known problem with IRQ handling and long kernel locks in the 2.0 series kernels. Consider upgrading to a later 2.2 kernel.

    From Jakob Oestergaard: Or, consider running xntpd. That should keep your clock right on time. (I think that I've heard that enabling RTC in the kernel also fixes the clock drift. It works for me! but I'm not sure whether that's general or I'm just being lucky)

    There are some kernel fixes in the later 2.2.x series that may fix this.

  9. Why are my CPU's numbered 0 and 2 instead of 0 and 1 (or some other odd numbering)?

    The CPU number is assigned by the MB manufacturer and doesn't mean anything. Ignore it.

  10. My quad-Xeon system hangs as soon as it has decompressed the kernel

    (Doug Ledford) Try recompiling LILO with LARGE_EBDA support and then making sure to always use make bzImage when compiling the kernel. That appears to have fixed the SMP boot hangs here on Intel multi-Xeon boards. However, please note that this also appears to break LILO in that the root= option no longer works, so make sure you rdev your kernel image at the same time you run lilo to make sure that the kernel loads the correct root filesystem at boot.

    (Robert M. Hyatt) With 3 cpus, do you have a terminator in the 4th slot?

  11. During boot machine hang signaling an "unexpected IO-APIC" warning

    Short Answer: Change your MP setting from 1.4 to 1.1 (BIOS option), and boot with "noapic" option at boot prompt.

    Long Answer: This message has nothing to do with your performance problems or why all interrupts go to one CPU. This message is for the ACPI(IO-APIC) maintainers to keep an eye on when there is new hardware. (Earle Nietzel)

    To summarize the article found in official kernel documentation:

    1. The "unexpected IO-APIC" is just an indicator that your motherboard is not on the whitelist.
    2. Cat your /proc/interrupts and if you see any line with IO-APIC then everything is fine because IO-APIC IRQ's are enabled.

  12. Do I need to do change MP from 1.4 to 1.1 and boot with (noapic) at the same time?

    It depends.

    I found that I do not need to turn off IO-APIC if I backed down from MP 1.4 and 1.1. Apparently some Xeon-based boards need to do both, but ASUS CUV4X boards do not. Turning off IO-APIC support needlessly imposes a probably small performance penalty on ASUS owners. (Vladimir G. Ivanovic)

    Some IBM Netfinity machines will have problems initializing the onboard SCSI controller if MPS 1.1 is selected. Each possible LUB of each possible device on each possible bus will be queried with a timeout. Booting takes a uselessly long time. (E. Robert Bogusta)

    There are reports that system with ASUS4X-DLS motherboard ran fine with IO-APIC enabled with MP 1.4.

    For CUV4X-D motherboard, disabling the IDE controllers you probably can boot with MP 1.4 and APIC enabled.

  13. Is there performance loss by running "noapic"?

    (David Mentre) It has minor impact, except if you have high interrupt load (i.e., nearly nobody).

  14. My motherboard is an ASUS-CUV4X-DLS with the VIA 694XDP chipset. If I boot with the noapic flag, the machine boots fine and /proc/cpuinfo show sboth processors. However, /proc interrupts does not show any sharing of the interrupts.

    Probably you need to upgrade your BIOS version to 1010.

  15. What are pros and cons of Xeons vs. Athlons?

    Xeon's chipset (440GX) and accompanying motherboard (supermicro S2DGE) I'd be using is probably (much?) more reliable and well-supported under Linux SMP than Athlons' (AMD 760/760MP) simply because they've been around longer and through many more iterations.

    Xeon's larger cache (1mb on the dual 400's I'm considering) might give performance enhancement (and given that I don't have only a single scientific code I'm planning to run on this, it's probably not helpful to test benchmark specifically for my code).

    Athlon's significiantly has faster clock rate (along with full-speed L2 cache in Thunderbirds, although at only 384kb) and much higher memory bandwidth with PC2100 DDR memory could help a lot.

    Cost is unclear until 760MP boards and PC2100 memory are released, but it will probably be  $950 to get two 1GHz 385km L2 Thunderbirds, dual motherboard and 512mb of ECC PC2100 vs  $750 to get two 400MHz 1mb L2 Xeons, dual motherboard and 512mb of ECC PC100. (Daniel Freedman)

  16. My system locks up during heavy NFS traffic

    Try the later 2.2.x kernels and the knfsd patches. This is currently under investigation. (Wade Hampton)

  17. My system locks up with no oops messages

    If you are using kernels 2.2.11 or 2.2.12, get the latest kernel. For example 2.2.13 has a number of SMP fixes. Several people have reported these kernels to be unstable for SMP. These same kernels may have NFS problems that can cause lockups. Also, use a serial console to capture your oops messages. (Wade Hampton)

    If the problem remains (and the other suggestions on this list didn't help either), then you could try the latest 2.3 kernels. They have more verbose (and more robust) SMP/APIC code, and automatic hard-lockup-prevention code which will produce meaningful oopses instead of a silent hang. (Ingo Molnar)

    (Osamu Aoki) You MUST also disable all BIOS related power save features. Example of good configuration (Dual Celeron 466 Abit BP6):

       ACPI:              Disabled
       POWER MANAGEMENT:  Disabled
       PM CONTROL by APM: No

    If power management features are activated, some random freeze can occur.

  18. Debugging lockups

    (item by Wade Hampton)

    A good means of debugging lockups is to get the ikd patch from Andrea Arcangeli:

    There are several of debug options, but do NOT use the soft lockup option! For newer SMP boxes, turn kernel debugging then turn on the NMI oopser. To verify that the NMI oopser is working, after booting the new kernel, /cat /proc/interrupts and verify that you are getting NMIs. When the box locks up, you should get an OOPS.

    You may also try the %eip option. This allows the kernel to print on the console the %eip address every time a kernel function is called. When the box locks up, write down the first column ordered by the second column then lookup the addresses in the file. This works only in console mode.

    Also note that the use of a serial console can greatly facilitate debugging kernel lockups, not just SMP kernel lockups!

  19. "APIC error interrupt on CPU#n, should never happen" messages in logs

    A message like:

    APIC error interrupt on CPU#0, should never happen.
    ... APIC ESR0: 00000002
    ... APIC ESR1: 00000000

    indicates a 'receive checksum error'. This cannot be caused by Linux as the APIC message checksumming part is completely in hardware. It might be marginal hardware. As long as you dont see any instability, they are not a problem - APIC messages are retried until delivered. (Ingo Molnar)

4.2 Possible causes of crash

In this section you'll find some possible reasons for a crash of an SMP machine (credits are due to Jakob Østergaard for this part). As far as I (David) know, theses problems are Intel specific.

4.3 Motherboard specific information

Please note: Some more specific information can be found with the list of Motherboards rumored to run Linux SMP

Motherboards with known problems

4.4 Low cost SMP Linux box (dual Celeron box)

(Stéphane Écolivet)

The lowest cost SMP Linux boxes with nowadays buyable processors are dual Celeron systems. Such a system is not officially possible according to Intel. Better think about the second generation of Celeron, those with 128 Kb L2 cache.

Is it possible to run a dual Intel Celeron box ?

Official answer from Intel: no, Celeron cannot work in SMP mode.

Practical answer: it is possible, but requires hardware alteration for Slot 1 processors. Alteration is described by Tomohiro Kawada on his Dual Celeron System page. Of course, this kind of modification removes warranties... Some versions of Celeron processor are also available in Socket 370 format. In that case, alteration may just be done on the Socket 370 to Slot 1 adapter or may even be sold pre-wired for SMP use. (Andy Poling, Hans - Erik Skyttberg, James Beard)

There is also a motherboard (ABIT BP6) allowing two Celerons in Socket 370 format to be inserted (Martijn Kruithof, Ryan McCue). ABIT Computer BP6 verified tested and native to linux with dual ppga socket 370 (Andre Hedrick).

How does Linux behave on a dual Celeron system ?

Fine, thank you.

Celeron processors are known to be easily overclockable. And dualCeleron system ?

It may work. However, overclocking this kind of system is not as easy as overclocking a mono-processor one. It is definitly not a good idea for a production system. For personal use, dual Celeron 300A systems running rock-solid at 450 MHz have been reported. (numerous people)

And making a quad Celeron system ?

It is impossible. Celeron processors have nearly the same features as basic Pentium II chips. If you want more than 2 processors in your system, you'll have to look at Pentium Pro, Pentium II Xeon or Pentium III (?) boxes.

What about mixing Celeron and Pentium II processor ?

A system using a "re-enable" Celeron processor and a Pentium II processor with the same steppings may theorically work.

Alexandre Charbey as made such a system:

5. Sparc architecture specific questions

5.1 Which Sparc machines are supported ?

Quoting the UltraLinux web page (only SMP systems):

UltraLinux has ran on a 14 CPUs machine (see the dmesg output) and on a Starfire E10000 with 24 CPUs (see the dmesg output).

The SparcStation 10 and SparcStations 20 are SMP capable machine and according to the FAQABOSS the following combinations are known to work:

And, as stated earlier, CPU modules in SparcStations 10 and can run a different clock speeds, the following ones _SHOULD_ work:

How does it performs? Well, it is fast, really fast. Some of the java Demos can run faster on a dual HyperSparc 125Mhz 128MB ( ywing ) than on a dual celeron BP6 433@433Mhz 192MB ( calimero ). The same applies for the Gimp. When it comes to compiling calimero runs faster than ywing. Both computers running 2.2.16 kernel and calimero's hard disk subsystem is full SCSI.

One important detail when you plan to have different CPU modules in your computer is to have the same kind of modules, you cannot mix SuperSparc and HyperSparc for example, but you can have an odd number of CPUs, for example 3. They are said to be able to run modules at different clock speed as written in this article form AcesHardware , but I have not witnessed it. (Lionel, trollhunter Bouchpan-Lerus-Juery)

5.2 Specific problem related to Sparc SMP support

(David Miller) There should not be any worries.

The only known problem, and one we don't intend to fix, is that if you build an SMP kernel for 32-bit (ie. non-ultrasparc) systems, this kernel will not work on sun4c systems.

6. PowerPC architecture specific questions

6.1 Which PPC machines are supported ?

(Cort Dougan) Not supported: PPC RS/6000 systems

6.2 Specific problem related to PPC SMP support

Nothing. Usual SMP compiling (see above). As usual, be aware, modules are specific either for UP or SMP. Recompile them. (Paul Mackerras)

7. Alpha architecture specific questions

7.1 Which Alpha machines are supported ?

(Geerten Kuiper) SMP works for most, if not all, AXP servers.

(Jay A Estabrook) SMP does seem to work on most of our [Compaq] boxes with 2 or more CPUs. That includes :

It does not include :

(Alpha Processor Inc) SMP support has been qualified for all API SMP systems starting from later 2.2-series kernels (approximately kernel 2.2.7). At the time of writing, that is :

See API's support website for more info.

7.2 Specific problem related to Alpha SMP support

None (really ? :-)

8. Useful pointers

8.1 Various

8.2 Multithreaded programs and library

8.3 SMP specific patches

8.4 Parallelizing/Optimizing Compilers for 586/686 machines (Sumit Roy)

9. Glossary

9.1 Definitions

9.2 Concepts

10. What's new ?

v1.14, 9 july 2002

v1.12.1, 25 october 2000

v1.12, 22 october 2000

v1.11, 8 october 2000

v1.10, 5 october 2000

v1.9.1, 28 september 2000

v1.9, 13 january 2000

v1.8, 8 november 1999

v1.7, 6 november 1999

v1.6, 21 october 1999

v1.5, 4 october 1999

v1.4, 30 september 1999

v1.3, 29 september 1999

v1.2, 27 september 1999

v1.1, 26 september 1999

v1.00, 25 september 1999

v0.54, 13 march 1999

v0.53, 08 march 1999

v0.52, 07 march 1999

v0.51, 06 march 1999

v0.50, 03 february 1999

v0.49, 13 january 1999

v0.48, 10 december 1998

v0.47, 20 november 1998

v0.46, 10 november 1998

v0.45, 25 october 1998

v0.44, 14 october 1998

v0.43, 9 september 1998

v0.42, 2 september 1998

v0.41, 1 september 1998

v0.40, 27 august 1998

v0.39, 27 august 1998

v0.38, 8 august 1998

v0.37, 30 July 1998

v0.36, 26 July 1998

v0.35, 14 July 1998

v0.34, 10 june 1998

v0.33, 3 june 1998

v0.32, 27 may 1998

v0.31, 18 may 1998

v0.30, 12 may 1998

v0.29, 11 may 1998

v0.28, 09 may 1998

v0.27, 05 may 1998

11. List of contributors

Many thanks to those who help me to maintain this HOWTO:

  1. Tigran A. Aivazian
  2. John Aldrich
  3. Niels Ammerlaan
  4. H. Peter Anvin
  5. Osamu Aoki
  6. Guylhem Aznar
  7. Ralf Bächle
  8. James Beard
  9. Troy Benjegerdes
  10. Anton Blanchard
  11. Emil Briggs
  12. Robert G. Brown
  13. Ray Bryant
  14. Alexandre Charbey
  15. Michael Elizabeth Chastain
  16. Samuel S. Chessman
  17. Alan Cox
  18. Andrew Crane
  19. Cort Dougan
  20. Patrick Doyle
  21. Mark Duguid
  22. Stéphane Écolivet
  23. Johan Ekenberg
  24. Jocelyne Erhel
  25. Jay A Estabrook
  26. Byron Faber
  27. Mark Garlanger
  28. hASCII
  29. Wade Hampton
  30. Andre Hedrick
  31. Claus-Justus Heine
  32. Benedikt Heinen
  33. Florian Hinzmann
  34. Moni Hollmann
  35. Robert M. Hyatt
  36. Jeffrey H. Ingber
  37. Richard Jelinek
  38. Tony Kocurko
  39. Geerten Kuiper
  40. Martijn Kruithof
  41. Doug Ledford
  42. Kumsup Lee
  43. Hank Leininger
  44. Ryan McCue
  45. Paul Mackerras
  46. Cameron MacKinnon
  47. Joel Marchand
  48. David Maslen
  49. Chris Mauritz
  50. Jean-Francois Micouleau
  51. David Miller
  52. Ingo Molnar
  53. Ulf Nielsen
  54. Jakob Oestergaard
  55. C Polisher
  56. Adrian Portelli
  57. Matt Ranney
  58. Daniel Roesen
  59. Ulf Rompe
  60. Jean-Michel Rouet
  61. Volker Reichelt
  62. Sean Reifschneider
  63. Rik van Riel
  64. Sumit Roy
  65. Thomas Schenk
  66. Matthias Schniedermeyer
  67. Terry Shull
  68. Chris K. Skinner
  69. Hans - Erik Skyttberg
  70. Szakacsits Szabolcs
  71. Jukka Tainio
  72. Stig Telfer
  73. Simen Timian Thoresen
  74. El Warren
  75. Gregory R. Warnes
  76. Gero Wedemann
  77. Christopher Allen Wing
  78. Leonard N. Zubkoff
  79. Mark Hahn
  80. David Haring
  81. David Mentre
  82. Earle Nietzel
  83. Rick Lindsley
  84. Vladimir G. Ivanovic
  85. Daniel Freedman
  86. Matti Aarnio
  87. Maciej W. Rozycki