In-Reply-To: <[email protected]>
On Mon, 24 Jul 2006 18:57:46 +0300, Al Boldi wrote:
>
> > With your changes on:
> >
> > stock kernel, randomize_va_space=0, gcc.322 -Os tstExec.c,
> > while :; do ./a.out; done
> > &x = 0xbffff874, &y = 0xbffff86c 28 28
> > &x = 0xbffff874, &y = 0xbffff86c 27 27
> > &x = 0xbffff874, &y = 0xbffff86c 27 27
> > &x = 0xbffff874, &y = 0xbffff86c 28 27
> > &x = 0xbffff874, &y = 0xbffff86c 27 30
> > &x = 0xbffff874, &y = 0xbffff86c 27 29
> >
> > stock kernel, randomize_va_space=1, gcc.322 -Os tstExec.c,
> > while :; do ./a.out; done
> > &x = 0xbfe2e614, &y = 0xbfe2e60c 29 28
> > &x = 0xbfd6a104, &y = 0xbfd6a0fc 55 56
> > &x = 0xbf91d454, &y = 0xbf91d44c 27 27
> > &x = 0xbf941e84, &y = 0xbf941e7c 55 56
> > &x = 0xbfa75834, &y = 0xbfa7582c 28 27
> > &x = 0xbfb58634, &y = 0xbfb5862c 27 30
>
> After closer inspection, it looks like addresses ending with 3c,7c,bc,fc
> cause a slowdown on P4, while addresses ending with 1c,3c,5c,7c,9c,bc,dc,fc
> cause a slowdown on P2.
>
Those addresses cause 'y' to span a cacheline (P4 = 64 bytes, P2 = 32.)
Even when the kernel aligns to 128 bytes this could happen depending
on how deeply you nest functions.
> Any easy way to instruct the kernel to skip those addresses?
First, I think you need to define locals in order of decreasing size.
IOW 'x' and 'y' need to be first inside fn(), but that may not work
when things get inlined. So using the '-malign-double' GCC option,
or forcing alignment with '__attribute__ ((aligned(8)))' for each variable
might be better.
Then you have to make sure the stack is aligned. See
'-mpreferred-stack-boundary'.
I still think the kernel should be aligning the stack to 128 bytes anyway.
--
Chuck
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