>> 1) A smart compiler could note the alignment and issue wider copy >> instructions. (Especially on alignment-required architectures.) > Right, that part of it would benefit from stronger alignment, > but does not generally need cacheline alignment. Agreed. The only reason the structure is cacheline aligned is to keep it all in a single cache line, and swapping the order of the elements made the buffer more aligned without hurting the counter. >> 2) The cacheline fetch would get more data faster. The data would >> be transferred in the first 6 beats of the load from RAM (assuming a >> 64-bit data bus) rather than waiting for 7, so you'd finish the copy >> 1 ns sooner or so. Similar 1-cycle win on a 128-bit Ln->L(n-1) cache >> transfer. > That argument worries me. I don't know enough to say whether you are > correct or not. But if you are correct, then it worries me that your > patch will be the first of a trickle growing to a stream to an avalanche > of patches where people align and reorder structures so that the most > commonly accessed fields are at the beginnng of the cacheline, so that > those can then be accessed minutely faster. > > Aargh, and now I am setting off the avalanche with that remark. > Please, someone, save us by discrediting George's argument. It was mostly #1 and #3. The *important* thing is to minimize the number of cache lines touched by common operations, which has already been the subject of a lot of kernel patches. Remember, most hardware does have critical-word-first loads. So alignment to the width of the data bus is enough. "Keep it naturally aligned" is all that's necessary, and most kernel data structures already obey that. I was just extending it, because I wanted to make it *possible* to use wider loads. >> As I said, "infinitesimal". The main reason that I bothered to >> generate a patch was that it appealed to my sense of neatness to >> keep the 3x16-byte buffer 16-byte aligned. > Ah, now you come clean! Yes, it does feel neater to me too; > but I doubt that would be sufficient justification by itself. It took both factors to make it worth it to me. The real reason was: 1) Neater 2) Definitely not slower 3) Maybe a tiny bit faster Conclusion: do it. Sorry to alarm you. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>