So I'm doing my normal profiling ("empty kernel build" is my favorite one), and link_path_walk() and __d_lookup_rcu() remain some of the hottest kernel functions due to their per-character loops. I can improve __d_lookup_rcu() on my machine by what appears to be around 15% by doing things a "unsigned long" at a time (it would be an option that only works on little-endian and with cheap unaligned accesses, although the big-endian modifications should be pretty trivial). Sure, that optimization would have to be disabled if you do DEBUG_PAGEALLOC, because it might opportunistically access bytes past the end of the string, but it does seem to be a very reasonable and easy thing to do apart from that small detail, and the numbers do look good. Basically, dentry_cmp() just becomes /* Little-endian with fast unaligned accesses? */ unsigned long a,b,mask; if (scount != tcount) return 1; for (;;) { a = *(unsigned long *)cs; b = *(unsigned long *)ct; if (tcount < sizeof(unsigned long)) break; if (a != b) return 1; cs += sizeof(unsigned long); ct += sizeof(unsigned long); tcount -= sizeof(unsigned long); if (!tcount) return 0; } mask = ~(~0ul << tcount*8); return !!((a ^ b) & mask); for that case, and gcc generates good code for it. However, doing the same thing for link_path_walk() would require that we actually change the hash function we use internally in the VFS layer, and while I think that shouldn't really be a problem, I worry that some filesystem might actually use the hash we generate and save it somewhere on disk (rather than only use it for the hashed lookup itself). Computing the hash one long-word at a time is trivial if we just change what the hash is. Finding the terminating NUL or '/' characters that involves some big constants (0x2f2f2f2f2f2f2f2f, 0x0101010101010101 and 0x8080808080808080 but seems similarly fairly easy. But if filesystems actually depend on our current hash algorithm, the word-at-a-time model falls apart. Anybody? I think we've kept the namehash unchanged since very early on, so I could imagine that somebody has grown to think that it's "stable". As far as I can tell, the current hash function goes back to 2.4.2 (just over ten years ago). Linus -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html