Re: miserable performance of 2.6.21 under network load

[Andi Kleen]

> # opreport -l
> CPU: AMD64 processors, speed 1994.52 MHz (estimated)
> Counted CPU_CLK_UNHALTED events (Cycles outside of halt state) with a unit mask of 0x00 (No unit mask) count 100000
> samples  %        app name                 symbol name
> 914379   48.8404  vmlinux-2.6.21.5         check_poison_obj
> 341920   18.2632  vmlinux-2.6.21.5         poison_obj

You have slab debugging enabled. That makes everything slow.
It costs you ~66% of your CPU time.  Disable it.

-Andi

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