Re: 2.6.11 vs 2.6.10 slowdown on i686

[Nick Piggin]

Ian Pratt wrote:
> Folks, 
>
> When we upgraded arch xen/x86 to kernel 2.6.11, we noticed a slowdown
> on a number of micro-benchmarks. In order to investigate, I built
> native (non Xen) i686 uniprocessor kernels for 2.6.10 and 2.6.11 with
> the same configuration and ran lmbench-3.0-a3 on them. The test
> machine was a 2.4GHz Xeon box, gcc 3.3.3 (FC3 default) was used to
> compile the kernels, NOHIGHMEM=y (2-level only).
>
> On the i686 fork and exec benchmarks I found that there's been a
> significant slowdown between 2.6.10 and 2.6.11. Some of the other
> numbers a bit ugly too (see attached).
>
> fork: 166 -> 235  (40% slowdown)
> exec: 857 -> 1003 (17% slowdown)
>
> I'm guessing this is down to the 4 level pagetables. This is rather a
> surprise as I thought the compiler would optimise most of these
> changes away. Apparently not. 

There are some changes in the current -bk tree (which are a
bit in-flux at the moment) which introduce some optimisations.

They should bring 2-level performance close to par with 2.6.10.
If not, complain again :)

Thanks,
Nick

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