linux-kernel - Re: [PATCH V2 2/2] module: Fix performance regression on modules with large symbol tables

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Message-ID: <87y5vgoqx0.fsf@rustcorp.com.au>
Date:	Wed, 16 Nov 2011 10:54:27 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Jan Beulich <JBeulich@...e.com>,
	Kevin Cernekee <cernekee@...il.com>
Cc:	linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2 2/2] module: Fix performance regression on modules with large symbol tables

On Mon, 14 Nov 2011 08:48:52 +0000, "Jan Beulich" <JBeulich@...e.com> wrote:
> > +		name = &mod->strtab[src->st_name];
> > +		if (unlikely(!test_bit(src->st_name, info->strmap))) {
> > +			/* Symbol name has already been copied; find it. */
> > +			char *dup;
> > +
> > +			for (dup = mod->core_strtab; strcmp(dup, name); dup++)
> > +				BUG_ON(dup > s);
> 
> Aren't you concerned that this again will be rather slow? It would be
> pretty easy to accelerate by comparing only with the tail of each string
> (as nothing else can possibly match), moving from string to string instead
> of from character to character.

Kevin's central thesis is that this is actually really unusual.

I'm not sure how much faster a tail search would be in practice.  We're
still scanning the string.  Perhaps a "dup[0] == name[0] &&" would
optimize it almost as well, if gcc doesn't already?

Kevin, you're probably in an optimal position to get numbers on this?

Thanks,
Rusty.
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