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 From : Sergey Lentsov                       2:4615/71.10   04 Oct 2001  17:56:58
 To : All
 Subject : URL: http://www.lwn.net/2001/1004/kernel.php3
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    See also: [14]last week's Kernel page.
    
 Kernel development
 
    The current kernel release is 2.4.10. The latest prepatch from Linus
    is [15]2.4.11-pre2, released on October 1. It includes a number of
    tweaks, including the unpleasant oops that afflicted some -pre1 users,
    and a bunch of merging from the "ac" series. Among other things, the
    new license tags (see the [16]September 6 kernel page) are going into
    the standard kernel.
    
    The latest prepatch from Alan Cox is [17]2.4.10-ac4. It includes most
    of the 2.4.10 changes, but has explicitly left out the massive virtual
    memory changes and other "seriously unsafe stuff." As Alan [18]says:
    "I actually use the trees I release and I want to keep my machines
    working."
    
    For those who find current kernels to be a bit too much on the
    bleeding edge, Mikulas Patocka has [19]released a patch to the 0.01
    kernel fixing a bug in the disk request sorting algorithm. Linus
    [20]responded by offering to make Mikulas the official maintainer of
    the 0.01 series. Time to plug that 386 back in and help out...
    
    On the licensing of security modules. A compromise wording has been
    worked out for the Linux Security Module interface; the include file
    will now carry this statement:
    
      This file is GPL. See the Linux Kernel's COPYING file for details.
      There is controversy over whether this permits you to write a
      module that #includes this file without placing your module under
      the GPL. Consult your lawyer for advice.
      
    Meanwhile, it appears that the EXPORT_SYMBOL macro, which makes kernel
    functions and data structures available to modules, will be augmented
    with a new EXPORT_SYMBOL_GPL variant. The new tag, clearly, will make
    a symbol available only to GPL-licensed code; the new license tags
    should make it possible to enforce that restriction automatically.
    Once EXPORT_SYMBOL_GPL is in place, the security module code may
    switch over to using it. Maybe.
    
    There is, however, no plan to switch any existing symbols over to a
    GPL-only mode. Alan [21]says:
    
      Linus has made it absolutely (as in he'll send out the killer
      penguin with chainsaw if need be) clear that existing symbols wont
      mysteriously turn GPL only.
      
    So authors of existing, proprietary modules need not worry that they
    will lose access to kernel symbols in the future.
    
    Dealing with high network loads. The 2.4 networking stack works quite
    well, for the most part. It does have one issue, however: dealing with
    extremely high load. When very large numbers of packets are coming
    into a system, interrupt processing tends to push all other work
    aside. In the best case, no user-space work gets done. High loads can
    also bring a 2.4.10 system down entirely.
    
    Ingo Molnar decided to address this problem, and some related ones
    having to do with the processing of software interrupts. [22]His patch
    implements a technique called "soft mitigation." Essentially, if the
    hardware interrupt rate exceeds a given threshold, the kernel simply
    disables that interrupt for a timer tick interval (10ms on most
    systems). The system thus gets a break in which it can catch up.
    
    There are, however, some problems with this approach. The constant
    threshold can not be set in a way that works for all situations; the
    maximum tolerable interrupt rate depends on a great many things,
    including the CPU speed, the cost of servicing the interrupt, and what
    else is happening on the system. Simply disabling the offending
    interrupt is easy (no cooperation from the driver or hardware is
    required), but it is hard on the performance of any other device that
    may be sharing the same interrupt line. Simply shutting down
    interrupts on a network interface for 10ms can cause it to start
    dropping packets in a big way, creating serious network performance
    problems.
    
    The biggest problem, however, may be that another solution exists and
    has been in testing for some time. The NAPI ("New API") code,
    developed by Jamal Hadi Salim, Robert Olsson, and Alexey Kuznetsov,
    deals with interrupt load problems and much more. The NAPI work is
    based on the techniques discussed at the [23]Kernel Summit last March,
    but the work has progressed since then. It has not, perhaps, received
    the degree of attention that it should have, though this discussion
    has raised its profile somewhat. Now, if only the project had a proper
    web site, it might become truly widely known...
    
    NAPI works with modern network adaptors which implement a "ring" of
    DMA buffers; each packet, as it is received, is placed into the next
    buffer in the ring. Normally, the processor is interrupted for each
    packet, and the system is expected to empty the packet from the ring.
    The NAPI patch responds to the first interrupt by telling the adaptor
    to stop interrupting; it will then check the ring occasionally as it
    processes packets and pull new ones without the need for further
    interrupts.
    
    People who have been on the net for a long time might appreciate this
    analogy: back in the 1980's, many of us had our systems configured to
    beep (interrupt) at us ever time an email message arrived. In 2001,
    beeping mail notifiers are far less common. There's almost always new
    mail, there's no need for the system to be obnoxious about it.
    Similarly, on a loaded system, there will always be new packets to
    process, so there is no need for all those interrupts.
    
    When the networking code checks an interface and finds that no more
    packets have arrived, interrupts are reenabled and polling stops.
    
    NAPI takes things a little farther by eliminating the packet backlog
    queue currently maintained in the 2.4 network stack. Instead, the
    adaptor's DMA ring becomes that queue. In this way, system memory is
    conserved, packets are less likely to be reordered, and, if the load
    requires that packets be dropped, they will be disposed of before ever
    being copied into the kernel.
    
    NAPI requires some changes to the network driver interface, of course.
    The changes have been designed to be incremental, though. Drivers
    which have not been converted will continue to function as always
    (well, at least, as in 2.4.x), but the higher performance enabled by
    NAPI will require modifications.
    
    Linus [24]likes the NAPI approach, but has said nothing about when it
    might be merged. One would normally expect it to go into 2.5, with a
    possible backport to 2.4 later. In the modern world, though, one never
    knows... It is also possible that parts of Ingo's patch may end up
    being used as a last-resort, "save the system" response.
    
    Those interested in NAPI can [25]download the USENIX paper describing
    the techniques used. The actual code is available from [26]Robert
    Olsson's FTP site.
    
    Other patches and updates released this week include:
    
      * Jaroslav Kysela has released [27]version 0.9.0beta8 of the ALSA
        sound driver system.
      * kdb v1.9 has been [28]released by Keith Owens.
      * Also from Keith is [29]modutils 2.4.10.
      * Robert Love has posted [30]a new version of his patch which
        enables network devices to contribute to the random entropy pool.
      * [31]Version 1.0.6 of IBM's Journaling Filesystem was announced by
        Steve Best.
      * Dave Jones has released [32]Powertweak 0.99.4, a tuning and
        hardware configuration tool.
      * A new single-copy pipe implementation was [33]posted by Manfred
        Spraul.
      * Jeremy Elson has [34]announced the first public release of the
        "Framework for User-Space Devices," which allows a user-space
        daemon to handle operations to device files.
      * A [35]preemptible kernel patch was released by Robert Love.
      * The third release candidate of LVM 1.0.1 was [36]announced by
        Heinz Mauelshagen.
      * [37]Loop-AES-v1.4e, a file and swap encryption module, was
        released by Jari Ruusu.
        
    Section Editor: [38]Jonathan Corbet
    October 4, 2001
    
    For other kernel news, see:
      * [39]Kernel traffic
      * [40]Kernel Newsflash
      * [41]Kernel Trap
    
    Other resources:
      * [42]Kernel Source Reference
      * [43]L-K mailing list FAQ
      * [44]Linux-MM
      * [45]Linux Scalability Effort
      * [46]Kernel Newbies
      * [47]Linux Device Drivers
    
    
    
                                                   [48]Next: Distributions
    
    [49]Eklektix, Inc. Linux powered! Copyright Л 2001 [50]Eklektix, Inc.,
    all rights reserved
    Linux (R) is a registered trademark of Linus Torvalds
 
 References
 
    1. http://lwn.net/
    2. http://ads.tucows.com/click.ng/pageid=001-012-132-000-000-003-000-000-012
    3. http://lwn.net/2001/1004/
    4. http://lwn.net/2001/1004/security.php3
    5. http://lwn.net/2001/1004/dists.php3
    6. http://lwn.net/2001/1004/desktop.php3
    7. http://lwn.net/2001/1004/devel.php3
    8. http://lwn.net/2001/1004/commerce.php3
    9. http://lwn.net/2001/1004/press.php3
   10. http://lwn.net/2001/1004/announce.php3
   11. http://lwn.net/2001/1004/history.php3
   12. http://lwn.net/2001/1004/letters.php3
   13. http://lwn.net/2001/1004/bigpage.php3
   14. http://lwn.net/2001/0927/kernel.php3
   15. http://lwn.net/2001/1004/a/2.4.11-pre2.php3
   16. http://lwn.net/2001/0906/kernel.php3
   17. http://lwn.net/2001/1004/a/2.4.10-ac4.php3
   18. http://lwn.net/2001/1004/a/ac-working.php3
   19. http://lwn.net/2001/1004/a/sorting-patch.php3
   20. http://lwn.net/2001/1004/a/lt-01.php3
   21. http://lwn.net/2001/1004/a/killer-penguin.php3
   22. http://lwn.net/2001/1004/a/im-interrupt.php3
   23. http://lwn.net/2001/features/KernelSummit/
   24. http://lwn.net/2001/1004/a/lt-napi.php3
   25. http://www.cyberus.ca/~hadi/usenix-paper.tgz
   26. ftp://robur.slu.se/pub/Linux/net-development/NAPI/
   27. http://lwn.net/2001/1004/a/alsa.php3
   28. http://lwn.net/2001/1004/a/kdb.php3
   29. http://lwn.net/2001/1004/a/modutils.php3
   30. http://lwn.net/2001/1004/a/netdev-random.php3
   31. http://lwn.net/2001/1004/a/jfs.php3
   32. http://lwn.net/2001/1004/a/powertweak.php3
   33. http://lwn.net/2001/1004/a/single-copy-pipe.php3
   34. http://lwn.net/2001/1004/a/fusd.php3
   35. http://lwn.net/2001/1004/a/preemptible-kernel.php3
   36. http://lwn.net/2001/1004/a/lvm.php3
   37. http://lwn.net/2001/1004/a/loop-aes.php3
   38. mailto:lwn@lwn.net
   39. http://kt.zork.net/
   40. http://www.atnf.csiro.au/~rgooch/linux/docs/kernel-newsflash.html
   41. http://www.kerneltrap.com/
   42. http://lksr.org/
   43. http://www.tux.org/lkml/
   44. http://www.linux.eu.org/Linux-MM/
   45. http://lse.sourceforge.net/
   46. http://www.kernelnewbies.org/
   47. http://www.xml.com/ldd/chapter/book/index.html
   48. http://lwn.net/2001/1004/dists.php3
   49. http://www.eklektix.com/
   50. http://www.eklektix.com/
 
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