* * * * *
                                        
                        Stupid multithreaded benchmarks
                                        
Dan Lyke ran a stupid benchmark [1]—just incrementing a variable to a
billion, one just flat out, and one between some pthreads locking primitives.

I wanted to see what stupid results I would get if I used spin locks [2].
First, the relevant bit of code (used nasm [3] to compile it under a 2.6GHz
(gigaHertz) dual-core Pentium running Linux 2.6):

> 		bits	32
> 		global	t1
> 		global	t2
> 
> 		section	.data
> gv		dd	0
> glock		dd	0
> 
> 		section	.text
> 
> 		align	16
> t1:		mov	eax,[gv]
> 		inc	eax
> 		mov	[gv],eax
> 		cmp	eax,1000000000
> 		jl	t1
> 
> 		; ret
> 
> 	; the following implements _exit()
> 	; for the multithreaded version
> 
> 		xor	ebx,ebx
> 		mov	eax,252
> 		int	$80
> 		mov	eax,1
> 		int	$80
> 		hlt
> 
> 		align	16
> t2:		mov	al,1
> t2.wait:	xchg	al,[glock]
> 		or	al,al
> 		jne	t2.wait
> t2.here:	mov	eax,[gv]
> 		inc	eax
> 		mov	[gv],eax
> 		mov	byte [glock],0
> 		cmp	eax,1000000000
> 		jl	t2
> 	
> 		;ret
> 	
> 		xor	ebx,ebx
> 		mov	eax,252
> 		int	$80
> 		mov	eax,1
> 		int	$80
> 		hlt
> 

Straightforward implementations here. t1() is the straight through counting
routine, while t2() is the one with the spin lock. Running single threaded
yielded these results:

Table: Counting, single threaded
routine	time to execute
------------------------------
t1()	2.454s
t2()	39.752s

While I expected the spin lock to be faster than the pthread locking, I
wasn't expecting it to be this slow. But maybe, just maybe, I'll get some of
that speed back by running dual threads. At the very least, it should be a
bit faster than single core, right?

Right?

Bueller? [4] Bueller? [5]

Table: Counting, dual-threaded
routine	time to execute
------------------------------
t1()	0m10.334s
t2()	2m31.307s

Um … 

Wow.

I didn't expect spinlocks to be so expensive.

Ouch.

[1] http://www.flutterby.com/archives/comments/10723.html
[2] gopher://gopher.conman.org/0Phlog:1999/12/08.2
[3] http://nasm.sourceforge.net/
[4] http://www.imdb.com/title/tt0091042/
[5] http://www.idiotsavant.com/ftp/sounds/bueller.wav

Email Sean Conner at sean@conman.org

.