On Tue, 12 Feb 2013 19:07:20 +0200 Imre Deak <imre.d...@intel.com> 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++ = iter.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 utilisation. 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% hit. > So at least for now I'm ok with just uninlining all the helpers. OK. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
