Re: 2048 CPUs [was: Re: New filesystem for Linux]

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Hi!

Hi!

The SGI Altix can have 2048 CPUs.
And does it run one image of Linux? Or more images each
on few cpus?

How do they solve problem with spinlock livelocks?

If time-spent-outside-spinlock/time-spent-in-spinlock <
number-of-cpus, the spinlock livelock may happen ---
this condition is not true normally with 2 or 4 cpus,
but for that high amount of cpus, there is a danger.
Lets say time-spent-outside-spinlock == time-spent-in-spinlock and
number-of-cpus == 2.

1 < 2 , so it should livelock according to you...
There is off-by-one bug in the condition. It should be:
(time_spent_in_spinlock + time_spent_outside_spinlock) / time_spent_in_spinlock < number_of_cpus
... or if you divide it by time_spent_in_spinlock:
time_spent_outside_spinlock / time_spent_in_spinlock + 1 < number_of_cpus

...but afaict this should work okay. Even if spinlocks are very
unfair, as long as time-outside and time-inside comes in big chunks,
it should work.

If you are unlucky, one cpu may stall for a while, but... I see no
livelock.
If some rogue threads (and it may not even be intetional) call the same
syscall stressing the one spinlock all the time, other syscalls needing
the same spinlock may stall.
Maybe there are so few Altices in the world that no one has yet observed
it...
Mikulas
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