Re: rwsem benchmarks [Re: generic rwsem [Re: Alpha "process table hang"]]

[Andrea Arcangeli <andrea@suse.de>]

On Fri, Apr 20, 2001 at 12:28:09AM +0100, D . W . Howells wrote:
> I benchmarked four different environments:
> 
> 	(1) 2.4.4-pre3 + Andrea's generic rwsem patch
> 	(2) 2.4.4-pre4 using XADD to implement the rwsems
> 	(3) same as (2) but with a tweak to make rwsem_wake() less fair
> 	(4) 2.4.4-pre3 using my generic spinlock code to implement the rwsems
> 
> David
> 
> 
> TEST		NUM READERS	NUM WRITERS	CONTENTION
> ===============	===============	===============	==========
> rwsem-rw	4		2		r-w & w-w
> rwsem-ro	4		0		no
> rwsem-wo	0		4		w-w only
> rwsem-r1	1		0		no
> rwsem-w1	0		1		no
> rwsem-r2	2		0		no
> 
> 
> ENVIRONMENT			TEST	SCHED	READERS		WRITERS
> ===============================	=======	=======	===============	=======
> Linux-2.4.4-pre3 + AA-rwsem	rws-rw	no	 3330281	    1009
> 						 3331972	     994
[..]
> -------------------------------	-------	-------	---------------	-------
> Linux-2.4.4-pre4 [GENERIC-SPIN]	rws-rw	no	  545138	  274002
> 						  545378	  273785
> 					yes	  755343	  187874
> 						  745888	  185562

Some explanation on the above extreme difference. In the misc rw benchmark the
reason in the same amount of time I get a total number of down 3332966 and you
get only 819163 is that I provide recursive down_read and that in turn can
starve the down_write (my first patches weren't implementing fair semaphores).

As you can see in my post of yesterday I made my semaphores fair in my last
patches (from rwsem-generic-5). (you didn't said which patch you used exactly
but obviously it was earlier than the -5 revision)

I'm uncertain if I should drop the list_empty() check from the fast path and if
I should still allow up_* to be called from irq/softirq, if I reduce the max
number of sleepers to 2^16 and I will provide weaker wakeup semantics I won't
be penalizied anymore and then we'll really compare apples to orange making the
comparison more interesting (probably I will do because later on I can probably
re-add that two features without too much pain).

About the benchmark you wrote it looks good measure to me, thanks.

Andrea