linux-kernel - Re: [PATCH 1/2] lib: more scalable list

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Date:	Thu, 21 Jan 2010 20:54:34 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Artem Bityutskiy <dedekind@...radead.org>
Cc:	Don Mullis <don.mullis@...il.com>, linux-kernel@...r.kernel.org,
	airlied@...hat.com, andi@...stfloor.org
Subject: Re: [PATCH 1/2] lib: more scalable list_sort()

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....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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