On Sun, May 20, 2007 at 01:46:47AM -0700, William Lee Irwin III wrote: > On Sat, May 19, 2007 at 11:15:01AM -0700, William Lee Irwin III wrote: > >> The cache cost argument is specious. Even misaligned, smaller is > >> smaller. > > On Sun, May 20, 2007 at 07:22:29AM +0200, Nick Piggin wrote: > > Of course smaller is smaller ;) Why would that make the cache cost > > argument specious? > > It's not possible to ignore aggregation. For instance, for a subset > of mem_map whose size ignoring alignment would otherwise fit in the > cache to completely avoid sharing any cachelines between page > structures requires page structures to be separated by at least one > mem_map index. This is highly unlikely in uniform distributions.
But that wasn't my argument. I _know_ there are cases where the smaller struct would be better, and I'm sure they would even arise in a running kernel. > On Sat, May 19, 2007 at 11:15:01AM -0700, William Lee Irwin III wrote: > >> The cache footprint reduction is merely amortized, > >> probabilistic, etc. > > On Sun, May 20, 2007 at 07:22:29AM +0200, Nick Piggin wrote: > > I don't really know what you mean by this, or what part of my cache cost > > argument you disagree with... > > I think it is that you could construct mem_map access patterns, without > > specifically looking at alignment, where a 56 byte struct page would suffer > > about 75% more cache misses than a 64 byte aligned one (and you could also > > get about 12% fewer cache misses with other access patterns). > > I also think the kernel's mem_map access patterns would be more on the > > random side, so overall would result in significantly fewer cache misses > > with 64 byte aligned pages. > > Which part do you disagree with? > > The lack of consideration of the average case. I'll see what I can smoke > out there. I _am_ considering the average case, and I consider the aligned structure is likely to win on average :) I just don't have numbers for it yet. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/