Re: Scheduler still seems awful with x264, worse with patches

From: Jason Garrett-Glaser
Date: Wed May 09 2012 - 12:30:19 EST

On Wed, May 9, 2012 at 9:24 AM, Mike Galbraith <efault@xxxxxx> wrote:
> On Wed, 2012-05-09 at 09:00 -0700, Jason Garrett-Glaser wrote:
>> Many months ago, the topic of CFS's inefficiencies with x264 came up
>> and some improvements were made, but BFS and Windows still stayed a
>> little bit in the lead.  This seemed to be because of a mix of two
>> issues.  Firstly, a combination of relatively short-lived jobs (x264
>> uses a thread pool, so the actual threads are long-lived).  Secondly,
>> in frame threads, heavy dependencies between threads, benefiting
>> greatly from a dumb scheduler.  Thirdly, in sliced threads -- the
>> focus of this post -- the best scheduling approach is to simply spread
>> them throughout the cores and do nothing, so again, a dumb scheduler
>> will do the right thing.
> I took x264 for a quick test drive a short while ago, and it looks like
> we slipped a bit.  I didn't have time to futz with it much, but did find
> that SCHED_IDLE kicked SCHED_OTHER's butt.  x264 really really wants RR.

Do remember to separate frame and slice threading in tests; they work
totally differently and, while you might be able to kill two birds
with one stone sometimes, some particular tuning might not affect both
in the same way.

Slice-threading is probably harder in general because the threads last
far less time, and that seems to be the thing that angers CFS.

Note also that my patch slice-threads the lookahead, even if the main
encode is frame-threaded. This is because for various reasons
frame-threading the lookahead may be harder and more difficult, so I
decided to do it this way (and it worked on Windows, so...). Note
also that when using automatic lookahead threads (default in that
patch), x264 currently does:

number of lookahead threads = MIN( sliced threads ? threads : threads / 6, 16 );

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