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Old 1st October 2013, 18:22   #1  |  Link
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Join Date: Sep 2013
Posts: 38
Too many threads when set to auto

Hi all,

I'm just trying to clear up a little confusion about setting threads in x.264. From all the advice I've read and received here and elsewhere, large numbers of threads is ill-advised, and even has a negative impact on quality. Everyone seems to have a different opinion about how many threads is too many, but most advice is keep them under 20. However, when threads is set to auto (recommended default), x264 seems to set the number of threads to its default formula (1.5*logical processors). In many modern multiprocessor/core servers, this will almost always result in a number of threads considered anecdotally by most x.264 users as "too many". For example...

We are running our x264 encodes on a Dell R710 server with dual 6-core processors for a total of 12 cores (24 processor threads using hyperthreading). When we set threads to "auto" in x264, the number of threads reported by Mediainfo is 36. That makes sense (so long as you consider a processor thread to be equivalent to a "logical processor" (more on that later).

So while x264 seems to be working according to its formula, my question is why does the default "auto" setting permit the threads to rise above a threshold that most everyone seems to be saying will have a negative impact on quality? Or is it the case that the quality impact only comes into play when you set the number of threads higher than the actual number of logical processors on your machine (times 1.5)?

In other words, do people see a reduction in quality with a large number of threads, even if their machine has enough processor threads to support the thread number calculated according to x264s formula?


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