Re: [FFmpeg-devel] [PATCH 10/39] lavc/ffv1dec: move the bitreader to stack

[Anton Khirnov <anton@khirnov.net>]

Quoting Michael Niedermayer (2024-07-18 16:48:06)
> On Thu, Jul 18, 2024 at 11:08:59AM +0200, Anton Khirnov wrote:
> > Quoting Michael Niedermayer (2024-07-18 00:42:05)
> > > all the stuff should be put together close so its efficiently
> > > using CPU caches
> > 
> > Which is why it shares its cacheline with PutBitContext, because the
> > code benefits from having the both in the cache, right? And the 4-byte
> > hole in PutBitContext is there presumably to aerate the cache for
> > smoother data streaming.
> 
> thanks for spoting these, can you fix these ?

I have no interest in optimizing the performance of this code. My
primary goal here is to remove FFV1-specific hacks from the frame
threading code for patch 33/39, which is in turn needed for 38/39.

As a public service, I also spent some effort on making the ffv1 code
easier to understand, but if you insist on keeping the code as it is I
can also just drop its non-compliant frame threading implementation.

> > 
> > More seriously, this is not how caches work. Being close together
> > matters mainly so long as your data fits in a cacheline, beyond that
> > physical proximity matters little. On stack, the bitreader is likely to
> > share the cacheline with other data that is currently needed, thus
> > improving cache utilization.
> 
> caches are complex, and being close does matter.
> having things in seperate allocations risks hitting aliassing cases
> (that is things that cannot be in the cache at the same time)
> so when you have the bitstream, the frame buffer, the context already
> in 3 independant locations adding a few more increases the risk for hitting
> these.
> Also sequential memory access is faster than non sequential, it does
> make sense to put things together in few places than to scatter them
> 
> Its years since ive done hardcore optimization stuff but i dont think
> the principles have changed that much that random access is faster than
> sequential and that caches work fundamentally differently

I don't see how any of these arguments are relevant - I am not moving
the bitreader to a new allocation, but to stack, which is already highly
likely to be in cache.

> > 
> > Another factor that matters in efficient cache use is e.g. not having
> > multiple copies of the same constant data scattered around, which you're
> > objecting to in my other patches.
> 
> copying the actually used small data together per slice
> where its accessed per pixel should improve teh speed per pixel while
> making the per slice code a little slower. now we have 4 slices maybe
> and millions of pixels. Thats why this can give an overall gain

This all sounds like premature optimization, AKA the root of all evil.
As I said above, I intended to make this code more readable, not faster.
Yet somehow it became faster anyway, which suggests this code is not
very optimized. So then arguing whether this or that specific change
adds or removes a few cycles per frame seems like a waste time to me.

-- 
Anton Khirnov
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