On Saturday 27 October 2007 06:57:14 Matt Mackall wrote: > Well I expect once you start letting people easily build strings by > concatenation, you'll very shortly afterwards have people using them > in loops. And having hidden O(n^2) behavior in there is a little sad, > even though n will tend to be small and well-bounded. If we can do > something simple to avoid it, we should.
Hi Matt, I avoid typing even a single character of optimization until it's justified. This is partially a reaction against the machoptimization tendencies of many kernel programmers, but it's mainly a concern at the kernel's complexity creep. Meanwhile, of course, I've now spent far too long analyzing this :) Building a 1000 byte string 1 byte at a time involves 6 reallocs (SLAB) or 10 reallocs (SLUB). Frankly, that's good enough without an explicit alloc length field (better in some ways). As to keeping an explicit length vs strlen(): those 1000 calls on my test machine take 1491ns per call with an explicit length vs 1496ns per call with strlen(). That's not worth 4 bytes, let alone a single line of code, O(n^2) or no. As the nail in the coffin, callers only use ->buf, so are insulated from any such optimizations if we decided to do them in future. Hope that helps, Rusty. PS. I don't think we should switch this to a simple char ** tho, as the "struct stringbuf" gives us some type safety and reminds people not to simply kfree it. - 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/