Re: [PATCH] Futexes IV (Fast Lightweight Userspace Semaphores)

["H. Peter Anvin" <hpa@zytor.com>]

Followup to:  <Pine.LNX.4.33.0203081252450.1412-100000@penguin.transmeta.com>
By author:    Linus Torvalds <torvalds@transmeta.com>
In newsgroup: linux.dev.kernel
> 
> On Fri, 8 Mar 2002, Alan Cox wrote:
> > 
> > Can we go to cache line alignment - for an array of locks thats clearly
> > advantageous
> 
> I disagree about the "clearly". Firstly, the cacheline alignment is CPU 
> dependent, so on some CPU's it's 32 bytes (or even 16), on others it is 
> 128 bytes. 
> 
> Secondly, a lot of locking is actually done inside a single thread, and
> false sharing doesn't happen much - so keeping the locks dense can be
> quite advantageous.
> 
> The cases where false sharing _does_ happen and are a problem should be 
> for the application writer to worry about, not for the kernel to force.
> 
> So I think 8 bytes is plenty fine enough - with 16 bytes a remote 
> possibility (I don't think it is needed, but it gives you som epadding for 
> future expansion). And people who have arrays and find false sharing to be 
> a problem can fix it themselves.
> 
> I personally don't find arrays of locks very common. It's much more common
> to have arrays of data structures that _contain_ locks (eg things like
> having hash tables etc with a per-hashchain lock) and then those container 
> structures may want to be cacheline aligned, but the locks themselves 
> should not need to be.
> 

Also, on UP this is all a waste.

	-hpa
-- 
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