use SkTaskGroup for multidraw

use SkTaskGroup for multidraw

BUG=skia:

[reed2, 2014-10-25 19:47:14]

reed@chromium.org changed reviewers:
+ mtklein@google.com, robertphillips@google.com

[reed2, 2014-10-25 19:47:15]

Just a stab at using threads.

Rob,

How can we know if/when to use threads for this API? Do we want to support a mix
of canvas types (e.g. some gpu some raster). I presume this threaded technique
would only work with raster (or non-gpu anyway).

[reed2, 2014-10-25 19:48:05]

reed@chromium.org changed reviewers:
+ reed@google.com

[mtklein, 2014-10-25 20:24:38]

This impl looks good to me.  I would be interested to see how this looks under
Instruments.

If we are using, say, nanobench to measure this, we'll probably want to add
something like

  SkTaskGroup::Enabler enabled;
or
  SkTaskGroup::Enabler enabled(FLAGS_threads);

somewhere early on in main() like we do in DM.  Otherwise this amounts to a
convoluted way to write a single-threaded for-loop.

[reed2, 2014-10-25 21:12:19]

reed@chromium.org changed reviewers:
+ bsalomon@google.com

[reed2, 2014-10-25 21:12:20]

enabled in sample app.

On my 2-core mac-mini, I get about 200% perf when I draw via pictures than pure
raster! Instruments doesn't show any of the overhead (granted, my samples were
blit-heavy).

Lets figure out the GPU thing, and make this sort of patch real.

[robertphillips, 2014-10-27 12:34:06]

The MPD draw does allow mixed GPU/other canvases but that seems like a pretty
rare use case. 

W.r.t. enabling threaded rendering it seems like a flag to draw might be the
answer.

https://codereview.chromium.org/641943005/diff/20001/samplecode/SampleApp.cpp
File samplecode/SampleApp.cpp (right):

https://codereview.chromium.org/641943005/diff/20001/samplecode/SampleApp.cpp...
samplecode/SampleApp.cpp:1345: size_t rowBytes;
:( No GPU :(

https://codereview.chromium.org/641943005/diff/20001/src/core/SkMultiPictureD...
File src/core/SkMultiPictureDraw.cpp (right):

https://codereview.chromium.org/641943005/diff/20001/src/core/SkMultiPictureD...
src/core/SkMultiPictureDraw.cpp:62: 
a struct?

https://codereview.chromium.org/641943005/diff/20001/src/core/SkMultiPictureD...
src/core/SkMultiPictureDraw.cpp:88: for (int i = 0; i < count; ++i) {
It seems like this portion should go with the drawPicture call below.

[bsalomon, 2014-10-27 13:48:18]

On 2014/10/27 12:34:06, robertphillips wrote:
> The MPD draw does allow mixed GPU/other canvases but that seems like a pretty
> rare use case. 

Could SkMPD stick sw and gpu canvas/picture pairs into two different buckets as
they're accumulated and then issue then in different ways in draw()?

[reed1, 2014-10-28 17:43:15]

On 2014/10/27 13:48:18, bsalomon wrote:
> On 2014/10/27 12:34:06, robertphillips wrote:
> > The MPD draw does allow mixed GPU/other canvases but that seems like a
pretty
> > rare use case. 
> 
> Could SkMPD stick sw and gpu canvas/picture pairs into two different buckets
as
> they're accumulated and then issue then in different ways in draw()?

good idea

[reed1, 2014-10-28 17:43:24]

closing in favor of https://codereview.chromium.org/684923002