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    SubjectPoor desktop responsiveness with background I/O-operations

    using a recent hard-/software setup, I observed that continuous
    read/write operations severely degrade the overall system responsiveness
    in typical desktop-PC use cases.

    Merely doing write/read operations on a data volume leads to stuck text
    and mouse cursors, seconds-long delays for simple window-context
    switches in X11, dropouts in low-resolution video playback etc.

    Test case:
    - 64-bit dual-core PC, SATA harddrive, plenty of free RAM
    - vanilla Linux 2.6.31, Kubuntu 9.10 packages, all software 64-bit

    How to reproduce:
    - start KDE/GNOME-session
    - open a terminal window and do as a non-root user:
    dd if=/dev/zero of=/home/john-doe/testfile
    (or dd if=/home/john-doe/big-testfile of=/dev/null)

    - a real use scenario would be a daily disk-backup or the
    simple extraction of a tarball containing slightly bigger files

    - The system becomes _really_ slow as described above; unusable for
    any multimedia tasks.

    - Using an encrypted (dm-crypt/LUKS) /home (e.g. on mobile computers)
    compounds the issue to a painful extent.

    Possible culprits (I'm guessing) are the Linux I/O- or CPU scheduler.

    I realize that a single heavyweight transfer slows down I/O for the
    corresponding transactions/processes/volumes etc.

    But there needs to be a fair distribution of I/O or CPU time which
    leaves enough for other basic operations. And this doesn't seem to be
    the case with recent Linux versions.

    On a side note, I've tried the BFS-patches (bfs-221 on 2.6.31): This
    yields significantly higher throughput when using disk encryption (50%
    improvement with dm-crypt/LUKS, 512 bit aes-xts-plain cipher mode). But
    with these patches, the responsiveness was even worse during my quick
    test. Switching to a text-mode console: several /minutes/ delay...

    I'm attaching my .config for linux 2.6.31 (grep ^C and bzip2-ed)

    [unhandled content-type:application/octet-stream]
     \ /
      Last update: 2009-09-20 05:37    [W:0.045 / U:6.164 seconds]
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