Re: [PATCH 3/3] clocksource: exynos_mct: Optimize register reads with ldmia

From: Thomas Gleixner
Date: Wed Jun 04 2014 - 14:05:59 EST

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.


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