Messages in this thread | ![/](/images/icornerl.gif) | | Date | Fri, 1 Apr 2005 10:25:14 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH 2.6.12-rc1-mm5 1/3] perfctr: ppc64 arch hooks |
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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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