Messages in this thread | | | From | John Stultz <> | Date | Wed, 16 Nov 2016 12:29:45 -0800 | Subject | Re: [PATCH v2] tile: avoid using clocksource_cyc2ns with absolute cycle count |
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On Wed, Nov 16, 2016 at 12:16 PM, Chris Metcalf <cmetcalf@mellanox.com> wrote: > On 11/16/2016 2:59 PM, John Stultz wrote: >> >> In your earlier patch, you mentioned this was similar to 4cecf6d401a0 >> ("sched, x86: Avoid unnecessary overflow in >> sched_clock"). It might be better to actually try to use similar logic >> there, to make sure the performance impact is minimal. > > > This was the first thing I looked at when I saw the mult_frac() > implementation. The modulus operations are indeed converted to > bitmasks and the divides to shifts. We do have to do two multiplies > instead of one, but that's basically the worst of the cost. > > Change 4cecf6d401a0 results in essentially identical code for x86 as > this proposed change does for tile. In fact a follow-on change by > Salman introduced mult_frac() and switched to using it, so it was > identical at that point. > > PeterZ (cc'ed) then improved it to use __int128 math via > mul_u64_u32_shr(), but that doesn't help tile; we only do one multiply > instead of two, but the multiply is handled by an out-of-line call to > __multi3, and the sched_clock() function ends up about 2.5x slower as > a result. > > Thanks for thinking about this!
Heh. Thanks for the history lesson and apologies for my forgetfulness. :)
thanks -john
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