Messages in this thread | | | From | Vineeth Remanan Pillai <> | Date | Thu, 11 May 2023 14:34:38 -0400 | Subject | Re: [PATCH 1/2] sched/deadline: accurate reclaim bandwidth for GRUB |
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Hi Luca,
On Thu, May 11, 2023 at 3:37 AM luca abeni <luca.abeni@santannapisa.it> wrote: > > I've just seen v2, and (unless I misunderstand something) I see you > removed the max{u_i/u_max, 1 - (u_inact + u_extra}} thing? > > I fear this might break the real-time guarantees provided by the > algorithm... > I am sorry I missed sending more details before sending out v2. So, I think there is another bug in the existing implementation. Let me try to explain the details.
SMP GRUB paper has the equation for depreciating runtime as: dq_i = -max{u_i, 1 - (extra_bw + Uinact)} dt
Since we are caping at Umax, the equation would be dq_i = -(max{u_i, Umax - (extra_bw + Uinact)} / Umax) dt (1)
But existing implementation is: dq_i = -max{u_i/Umax, 1 - (extra_bw + Uinact)} dt (2)
Here in (2), we factored Umax only to the first term "u_i" and the second term in max{} was as left as it is. What do you think?
Now with normal DL and SCHED_FLAG_RECLAIM tasks, equation (1) can be re-written as: dq_i = -(max{u_i, Ureclaim_max - (extra_bw + Uinact)}/Ureclaim_max)dt (3)
I tested this equation (3) and it works as expected. What do you think about the correctness of equation (3)?
I felt that, since we are using sequential reclaim mentioned in the paper and we isolate all parameters per-cpu(except for extra_bw) we could use the "-dq = -(U/Umax) dt" equation as it was simpler than equation (3). Sorry that I missed discussing this. I shall send out v3 with equation (3), if you think it's the right way to go to enforce deadline guarantees.
Thanks, Vineeth
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