Update: Debox sendfile modifications

[Alan Cox]

On Wed, Nov 05, 2003 at 01:25:43AM -0500, Vivek Pai wrote:
> Mike Silbersack wrote:
> >On Tue, 4 Nov 2003, Vivek Pai wrote:
> >>The one other aspect of this is that sf_bufs mappings are maintained for
> >>a configurable amount of time, reducing the number of TLB ops. You can
> >>specify the parameter for how long, ranging from -1 (no coalescing at
> >>all), 0 (coalesce, but free immediately after last holder release), to
> >>any other time. Obviously, any value above 0 will increase the amount of
> >>wired memory at any given point in time, but it's configurable.
> >
> >Ah, I missed that point.  Did your testing show the caching part of the
> >functionality to be significant?
> 
> I think it buys us a small gain (a few percent) under static-content
> workloads, and a little less under SpecWeb99, where more time is spent
> in dynamic content. However, it's almost free - the additional
> complexity beyond just coalescing is hooking into the timer to free
> unused mappings.

I think it's reasonable to expect a more pronounced effect on i386
SMP.  In order to maintain TLB coherence, we issue two interprocessor
interrupts _per_page_ transmitted by sendfile(2).

Alan