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    SubjectRe: RSDL v0.31
    Xavier Bestel wrote:
    > On Tue, 2007-03-20 at 07:11 +0100, Willy Tarreau wrote:
    >> I don't agree with starting to renice X to get something usable
    >
    > X looks very special to me: it's a big userspace driver, the primary
    > task handling user interaction on the desktop, and on some OS the part
    > responsible for moving the mouse pointer and interacting with windows is
    > even implemented as an interrupt handler, and that for sure provides for
    > smooth user experience even on very low-end hardware. Why not compensate
    > for X design by prioritizing it a bit ?
    > If RSDL + reniced X makes for a better desktop than sotck kernel + X, on
    > all kind of workloads, it's good to know.

    No, running X at a different priority than its clients is not really
    a good idea. If it isn't immediately obvious why try something like
    this:

    mkdir /tmp/tempdir
    cd /tmp/tempdir
    for i in `seq -w 1 10000` ; do touch
    longfilenamexxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx$i
    ; done
    nice --20 xterm &
    xterm &
    nice -20 xterm &

    then do "time ls -l ." in each xterm.

    This is what i get on UP 2.6.20+RSDL.31 w/ X at nice 0:
    -20: 0m0.244s user 0m0.156s system 0m3.113s elapsed 12.84% CPU
    0: 0m0.216s user 0m0.168s system 0m2.801s elapsed 13.70% CPU
    19: 0m0.188s user 0m0.196s system 0m3.268s elapsed 11.75% CPU

    I just made this simple example up and it doesn't show the problem
    too well, but you can already see the ~10% performance drop. It's
    actually worse in practice, because for some apps the increased
    amount of rendering is clearly visible; text areas scroll
    line-by-line, content is incrementally redrawn several times etc.
    This happens because an X server running at a higher priority than a
    client will often get scheduled immediately after some x11 traffic
    arrives; when the process priorities are equal usually the client
    gets a chance to supply some more data. IOW by renicing the server
    you make X almost synchronous.

    This isn't specific to RSDL - it happens w/ any cpu scheduler; and
    while the effects of less extreme prio differences (ie -5 instead of
    -20 etc) may be less visible i also doubt they will help much.

    A better approach to X interactivity might be allowing the server to
    use (part of) the clients timeslice, but it's not trivial -- you'd
    only want to do that when the client is waiting for a reply and you
    almost never want to preempt the client just because the server
    received some data.

    As to RSDL - it seems to work great for desktop use and feels better
    than mainline. However top output under 100% load (eg kernel
    compilation) looks like below -- the %CPU error seems a bit high...

    Tasks: 97 total, 6 running, 91 sleeping, 0 stopped, 0 zombie
    Cpu(s): 81.7% us, 18.3% sy, 0.0% ni, 0.0% id, 0.0% wa, 0.0% hi,
    0.0% si
    PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND

    7566 root 17 0 9196 4108 1188 R 3.0 0.8 0:00.09 cc1

    7499 root 11 0 1952 924 648 S 0.3 0.2 0:00.01 make

    12279 root 1 0 5556 2928 2064 S 0.3 0.6 0:00.83 xterm

    31510 root 1 0 2152 1100 840 R 0.3 0.2 0:00.25 top

    1 root 1 0 1584 88 60 S 0.0 0.0 0:00.30 init



    artur
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