linux-kernel - Re: [PATCH 1/4] stringbuf: A string buffer implementation

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Date:	Sun, 28 Oct 2007 22:03:52 -0500
From:	Matt Mackall <mpm@...enic.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Matthew Wilcox <matthew@....cx>,
	Linus Torvalds <torvalds@...l.org>,
	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org,
	Matthew Wilcox <willy@...ux.intel.com>
Subject: Re: [PATCH 1/4] stringbuf: A string buffer implementation

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.

-- 
Mathematics is the supreme nostalgia of our time.
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