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    SubjectRe: [PATCH v3 2/5] sched/deadline: Fix reclaim inaccuracy with SMP
    On Fri, 19 May 2023 11:56:21 +0200
    luca abeni <luca.abeni@santannapisa.it> wrote:

    > Hi,
    >
    > sorry for returning on this discussion, but there is something I still
    > do not understand:
    >
    > On Tue, 16 May 2023 11:08:18 -0400
    > Vineeth Remanan Pillai <vineeth@bitbyteword.org> wrote:
    > [...]
    > > I had tested this and it was reclaiming much less compared to the
    > > first one. I had 3 tasks with reservation (3,100) and 3 cpus.
    >
    > So, just to confirm: here you have only 3 SCHED_DEADLINE tasks,
    > scheduled on a root domain containing only 3 CPUs (dl_bw_cpus() return
    > 3)... Right?
    > So, the utilization of each task is 3/100 = 0.03 and Uextra is
    > 1 - (0.03 * 3) / 3 = 0.97.

    OK, sorry again... I found my error immediately after sending the email.
    Uextra is computed as "Umax - ...", not "1 - ...".
    So, I now understand where the 35% comes from.

    I now _suspect_ the correct equation should be
    dq = -(max{u_i / Umax, (Umax - Uinact - Uextra)}) * dt
    but I want to test it before wasting your time again; I'll write more
    after performing some more tests.


    Luca

    > And since all the tasks are always active, Uinact = 0...
    > Is this understanding right?
    >
    > If so:
    > > With dq = -(max{u_i, (Umax - Uinact - Uextra)} / Umax) * dt (1)
    > > TID[636]: RECLAIM=1, (r=3ms, d=100ms, p=100ms), Util: 95.08
    > > TID[635]: RECLAIM=1, (r=3ms, d=100ms, p=100ms), Util: 95.07
    > > TID[637]: RECLAIM=1, (r=3ms, d=100ms, p=100ms), Util: 95.06
    > >
    > > With dq = -(max{u_i, (1 - Uinact - Uextra)} / Umax) * dt (2)
    > > TID[601]: RECLAIM=1, (r=3ms, d=100ms, p=100ms), Util: 35.65
    > > TID[600]: RECLAIM=1, (r=3ms, d=100ms, p=100ms), Util: 35.65
    > > TID[602]: RECLAIM=1, (r=3ms, d=100ms, p=100ms), Util: 35.65
    >
    > Here, we should have
    > dq = -(max{0.03, (1 - 0 - 0.97)} / Umax) * dt
    > = -(0.03 / Umax) * dt
    > which reclaims up to Umax... So, the utilization should be 95%
    > Since you measured 35.65%, it means that (1-Uextra) is much larger
    > than 0.97... So, maybe you found some bug in the Uextra computation?
    >
    > Can you try printing the extra_bw value, to check what happened?
    >
    >
    >
    > Thanks,
    > Luca
    >
    > >
    > > As the task bandwidth goes higher, equation (2) reclaims more, but
    > > equation (2) is a constant of 95% as long as number of tasks less
    > > than cpus. If the number of tasks is more than cpus, eq (2) fares
    > > better in reclaiming than eq (1)
    > >
    > > eq (1) with 5 tasks (3,100)
    > > TID[627]: RECLAIM=1, (r=3ms, d=100ms, p=100ms), Util: 28.64
    > > TID[626]: RECLAIM=1, (r=3ms, d=100ms, p=100ms), Util: 28.64
    > > TID[629]: RECLAIM=1, (r=3ms, d=100ms, p=100ms), Util: 28.62
    > > TID[628]: RECLAIM=1, (r=3ms, d=100ms, p=100ms), Util: 29.00
    > > TID[630]: RECLAIM=1, (r=3ms, d=100ms, p=100ms), Util: 28.99
    > >
    > > Here top shows 3 cpus in the range ~45 to 50% util
    > >
    > > eq (2) with 5 tasks (3,100)
    > > TID[667]: RECLAIM=1, (r=3ms, d=100ms, p=100ms), Util: 57.20
    > > TID[670]: RECLAIM=1, (r=3ms, d=100ms, p=100ms), Util: 57.79
    > > TID[668]: RECLAIM=1, (r=3ms, d=100ms, p=100ms), Util: 57.11
    > > TID[666]: RECLAIM=1, (r=3ms, d=100ms, p=100ms), Util: 56.34
    > > TID[669]: RECLAIM=1, (r=3ms, d=100ms, p=100ms), Util: 55.82
    > >
    > > And here top shows all 3 cpus with 95% util
    > >
    > > > I'll write more about this later... And thanks for coping with all
    > > > my comments!
    > > >
    > > Thanks :-)
    > >
    > > Vineeth
    >

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