Re: __i915_spin_request() sucks

From: Jens Axboe
Date: Thu Nov 12 2015 - 15:40:41 EST

On 11/12/2015 01:36 PM, Jens Axboe wrote:

So a few months ago I got an XPS13 laptop, the one with the high res
screen. GUI performance was never really that great, I attributed it to
coming from a more powerful laptop, and the i915 driving a lot of
pixels. But yesterday I browsed from my wife's macbook, and was blown
away. Wow, scrolling in chrome SUCKS on the xps13. Not just scrolling,
basically anything in chrome. Molasses. So I got sick of it, fired up a
quick perf record, did a bunch of stuff in chrome. No super smoking
guns, but one thing did stick out - the path leading to

So today, I figured I'd try just killing that spin. If it fails, we'll
punt to normal completions, so easy change. And wow, MASSIVE difference.
I can now scroll in chrome and not rage! It's like the laptop is 10x
faster now.

Ran git blame, and found:

commit 2def4ad99befa25775dd2f714fdd4d92faec6e34
Author: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Date: Tue Apr 7 16:20:41 2015 +0100

drm/i915: Optimistically spin for the request completion

and read the commit message. Doesn't sound that impressive. Especially
not for something that screws up interactive performance by a LOT.

What's the deal? Revert?

BTW, this:

"Limit the spinning to a single jiffie (~1us) at most"

is totally wrong. I have HZ=100 on my laptop. That's 10ms. 10ms! Even if I had HZ=1000, that'd still be 1ms of spinning. That's seriously screwed up, guys.

Jens Axboe

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