Re: [PATCH 2.6.12-rc1-mm5 1/3] perfctr: ppc64 arch hooks

[Andrew Morton <akpm@osdl.org>]

Mikael Pettersson <mikpe@csd.uu.se> wrote:
>
> In addition, there is one unresolved issue:
>  - A counter's value is represented by a 64-bit software sum,
>    a 32-bit start value containing the HW counter's value at the
>    start of the current time slice, and the current HW counter's value
>    (now). The actual value is computed as sum + (now - start).
>    This is reflected in the mmap()ed state, which contains a variable-
>    length { u32 map; u32 start; u64 sum; } pmc[] array.
>    This layout is very cache-efficient on current 32 and 64-bit CPUs,
>    but there is a _possible_ concern that it won't do on 10+ GHz CPUs.
>    So the question is, should we change it to use 64-bit start values
>    already now (and take more cache misses), or should that wait a few
>    years until it becomes a necessity (causing ABI change issues)?

I'd be inclined to make the change now, personally.  ABI changes are a pain
for everyone.

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