The Boston Diaries

The ongoing saga of a programmer who doesn't live in Boston, nor does he even like Boston, but yet named his weblog/journal “The Boston Diaries.”

Go figure.

Wednesday, November 28, 2007

Stupid multithreaded benchmarks

Dan Lyke ran a stupid benchmark—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. First, the relevant bit of code (used nasm to compile it under a 2.6GHz 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:

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? Bueller?

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.

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