Subject: Re: [PATCH 1/2] lib: more scalable list_sort()
From: Artem Bityutskiy <dedekind@infradead.org>
Reply-To: dedekind@infradead.org
To: Dave Chinner <david@fromorbit.com>
Cc: Don Mullis <don.mullis@gmail.com>, linux-kernel@vger.kernel.org,
       airlied@redhat.com, andi@firstfloor.org
In-Reply-To: <20100121095434.GA25842@discord.disaster>
References: <87fx609i29.fsf@gmail.com> <1264065775.3032.18.camel@localhost>
	 <20100121095434.GA25842@discord.disaster>
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Date: Thu, 21 Jan 2010 13:44:19 +0200
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On Thu, 2010-01-21 at 20:54 +1100, Dave Chinner wrote:
> On Thu, Jan 21, 2010 at 11:22:55AM +0200, Artem Bityutskiy wrote:
> > On Wed, 2010-01-20 at 20:51 -0800, Don Mullis wrote:
> > > The use of list_sort() by UBIFS looks like it could generate long
> > > lists; this alternative implementation scales better, reaching ~3x
> > > performance gain as list length approaches the L2 cache size.
> > > 
> > > Stand-alone program timings were run on a Core 2 duo L1=32KB L2=4MB,
> > > gcc-4.4, with flags extracted from an Ubuntu kernel build.  Object
> > > size is 552 bytes versus 405 for Mark J. Roberts' code.
> > > 
> > > Worst case for either implementation is a list length just over a POT,
> > > and to roughly the same degree, so here are results for a range of
> > > 2^N+1 lengths.  List elements were 16 bytes each including malloc
> > > overhead; random initial order.
> > > 
> > 
> > Could you please add a debugging function which would be compiled-out
> > normally, and which would check that on the output 'list_sort()' gives
> > really sorted list, and number of elements in the list stays the same.
> > You'd call this function before returning from list_sort(). Something
> > like:
> > 
> > #ifdef DEBUG_LIST_SORT
> > static int list_check(void *priv, struct list_head *head,
> >                       int (*cmp)(void *priv, struct list_head *a,
> >                                  struct list_head *b))
> > {
> >    /* Checking */
> > }
> > #else
> > #define list_check(priv, head, cmp) 0
> > #endif
> > 
> > This will provide more confidence in the algorithm correctness for
> > everyone who modifies 'list_sort()'.
> 
> I'd suggest the same method as employed in lib/sort.c - a
> simple userspace program that verifies correct operation is included
> in lib/sort.c....

Yeah, that's also an option.

-- 
Best Regards,
Artem Bityutskiy (Артём Битюцкий)

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