Messages in this thread | | | Date | Sun, 28 Oct 2007 22:03:52 -0500 | From | Matt Mackall <> | Subject | Re: [PATCH 1/4] stringbuf: A string buffer implementation |
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On Sat, Oct 27, 2007 at 08:09:30PM +1000, Rusty Russell wrote: > 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).
And on SLOB, which doesn't have those bloaty power-of-2 constraints? Looks like ~500 reallocs, including 250000 bytes of memcpy. Ouch!
SLAB and SLUB (accidentally) internalize the optimization that I'm proposing: grow the string by a constant factor at each step. Failing to do that makes the behavior dismal.
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