Thomas, On Wed, Jun 4, 2014 at 11:05 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote: > On Wed, 4 Jun 2014, Doug Anderson wrote: > >> As we saw in (clocksource: exynos_mct: cache mct upper count), the >> time spent reading the MCT shows up fairly high in real-world >> profiles. That means that it's worth some optimization. >> >> We get a roughly 10% speedup in userspace gettimeofday() by using an >> ldmia to read the two halfs of the MCT. That seems like a worthwhile >> thing to do. >> >> Before: 1173084 us for 1000000 gettimeofday in userspace >> After: 1045674 us for 1000000 gettimeofday in userspace >> >> NOTE: we could actually do better than this if we really wanted to. >> Technically we could register the clocksource as a 32-bit timer and >> only use the "lower" half. Doing so brings us down to 1014429 us for >> 1000000 gettimeofday in userspace (and doesn't even require assembly >> code). That would be an alternative to this change. > > I was about to ask exactly that question: What's the advantage of the > 64 bit dance there? AFAICT nothing. I debated whether to send out the 32-bit version, since I'd implemented both. I'm happy to send out the 32-bit version too and people can pick which they like better. Let me know. The final thing that pushed me over the edge to send the 64-bit version was that I didn't know enough about how MCT was used with respect to low power modes (we don't use AFTR / LPA modes on our system). I could actually believe that we might want to set a timer for more than 178 seconds into the future for these low power modes. If that's the case, we still need to keep around the 64-bit read code for that case. ...and once we have the 64-bit code then we might as well use it for the rest of the timers. Perhaps Tomasz will comment on this. -Doug -- To unsubscribe from this list: send the line "unsubscribe linux-samsung-soc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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