Re: [PATCH v2] lib/scatterlist: add simple page iterator

From: Andrew Morton
Date: Tue Feb 12 2013 - 16:28:33 EST

On Tue, 12 Feb 2013 19:07:20 +0200
Imre Deak <imre.deak@xxxxxxxxx> wrote:

> > So, exactly how big is this thing, and how do we know it's better this
> > way than if we were to uninline some/all of the helpers?
> I admit I only hoped compiler optimization would keep the inlined parts
> at a minimum, but now I actually checked (on Intel CPU). I applied the
> patchset from [1] and uninlined sg_page_iter_start as it's not
> significant for speed:
> size drivers/gpu/drm/i915/i915.ko
> 514855 15996 272 531123 81ab3 drivers/gpu/drm/i915/i915.ko
> Then uninlined all helpers:
> size drivers/gpu/drm/i915/i915.ko
> 513447 15996 272 529715 81533 drivers/gpu/drm/i915/i915.ko
> Since there are 8 invocations of the macro, the overhead for a single
> invocation is about (531123 - 529715) / 8 = 191 bytes.
> For speed, I benchmarked a simple loop which was basically:
> page = vmalloc(sizeof(*page) * 1000, GFP_KERNEL);
> for_each_sg_page(sglist, iter, 0)
> *page++ =;
> where each entry on the sglist contained 16 consecutive pages. This
> takes ~10% more time for the uninlined version to run. This is a rather
> artificial test and I couldn't come up with something more real-life
> using only the i915 driver's ioctl interface that would show a
> significant change in speed.

10% for the function call overhead sounds reasonable. Of course, that
test is biased in one direction. A test which was biased in the other
direction would exercise all eight of the macro's callsites and would
investigate the performance impact of a 1kbyte increase in L1 cache

And I must say, it would need to be a pretty damn carefully crafted
test case to be able to trigger enough cache thrashing to cause a 10%

> So at least for now I'm ok with just uninlining all the helpers.

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