Re: Fw: potential /dev/urandom scalability improvement

[Matt Mackall <mpm@selenic.com>]

On Thu, Mar 25, 2004 at 08:51:54PM -0800, David Mosberger wrote:
> >>>>> On Thu, 25 Mar 2004 22:19:26 -0600, Matt Mackall <mpm@selenic.com> said:
> 
> >  struct entropy_store {
> > +	/* mostly-read data: */
> > +	struct poolinfo poolinfo;
> > +	__u32		*pool;
> > +
> > +	/* read-write data: */
> > +	spinlock_t lock ____cacheline_aligned;
> >  	unsigned	add_ptr;
> >  	int		entropy_count;
> >  	int		input_rotate;
> > -	struct poolinfo poolinfo;
> > -	__u32		*pool;
> > -	spinlock_t lock;
> >  };
> 
>   Matt> Also, I think in general we'd prefer to stick the aligned bit at the
>   Matt> front of the structure rather than at the middle, as we'll avoid extra
>   Matt> padding. The size of cachelines is getting rather obscene on some
>   Matt> modern processors.
> 
> Not sharing the cacheline between the mostly-read data and the
> read-write data is the _point_ of this change.  If you reverse the
> order, the "poolinfo" and "pool" members will also get invalidated
> whenever someone updates the write-intensive data.

Ok, previous observation made no sense; I should really be taking a
nap right now. Hopefully this next one will make more sense: it ought
to be ____cacheline_aligned_in_smp as the zero-byte spinlock struct
still forces alignment.

-- 
Matt Mackall : http://www.selenic.com : Linux development and consulting