Re: [PATCH] [RFT] drm/i915: Don't waste our paultry cache on a context object

[Ben Widawsky <ben@bwidawsk.net>]

On Fri, Sep 20, 2013 at 04:12:37PM +0100, Chris Wilson wrote:
> On Fri, Sep 20, 2013 at 08:05:02AM -0700, Ben Widawsky wrote:
> > Context save and restore is by definition a slow process, however it is
> > also an infrequent process. Don't try to optimize the save restore at
> > the cost of any of our precious cache space. Contexts begin to get quite
> > large on HSW and beyond.
> 
> Infrequent?

Relative to operations which use the cache.

>  
> > At least for benchmarks people seem to care about, there is almost
> > always only 1 context running, which means I don't expect this to do any
> > harm. For benchmarks with many contexts, there could be performance
> > degradation - but I have a sneaking suspicion the HW will do some fancy
> > magic to speak up context save & restores anyway.
> 
> There are at least 2 contexts in every benchmark QA cares about. It
> wasn't like making them L3 objects in the first place was motivated by
> benchmark results...
> 

No, I've no doubt it was motivated by benchmarks, but I think making it
L3 (which I still have a hard time believe would do at all what it's
intended to do) would only prove further that not wasting LLC space is a
good thing. The theory follows that not wasting L3 space is also a good
thing.

> Anyway the idea was to see if QA still notice a difference...
> -Chris
> 

I'll try to be an optimist for once.

-- 
Ben Widawsky, Intel Open Source Technology Center