[vdr] VDR-1.3.41: speedup for cVideoRepacker

Laz laz at club-burniston.co.uk
Wed Feb 1 22:54:10 CET 2006


On Wednesday 01 February 2006 00:33, Jon Burgess wrote:
> Reinhard Nissl wrote:
> > I don't think that it is worth a try as it tests every byte while the
> > above code tests most of the time only every third byte.
>
> I agree that your algorithm is clever and does greatly cut down the
> number of comparisons as compared to the old code.
>
> The glibc memchr() implementation does the comparisons 4 bytes at a time
> using a clever algorithm. It also has assembler optimised variants for
> some CPU's. I don't think that only doing a comparison of every 3rd byte
> wins you anything over memchr().
>
> I believe the bulk of the time taken by the routine is transferring all
> the data from memory into the CPU. Every byte of the data will have to
> be read into the CPU caches due to cacheline effects. I believe that the
> asm optimisations will take into account the possibilities of
> speculative readahead etc. I've not looked into the assembler to see
> whether it actually exploits this.
>
> I've atached the quickly hacked up test program that I wrote. The output
>   is the time taken for many iterations of the 2 different algorithms.
> For me the difference is within the measurement noise. It certainly
> isn't any slower. I'd be interested to know whether it makes any
> difference on your EPIA, both in the test program and in VDR.
>
> $ ./search /video0/%Click_Online/2005-04-10.04\:28.99.99.rec/001.vdr
> Found 10585344 matches in 12.5873 seconds
> Found 10585344 matches in 12.6235 seconds

Just done a quick test on my Epia MII-12000 system (whilst under heavy load 
running softdevice, etc. so the numbers are probably crap!):

laz at vdr-tng 2006-02-01.18.28.50.50.del $ ~/stmp 001.vdr
Found 12721024 matches in 276.0596 seconds
Found 12721024 matches in 201.9245 seconds

I'll try to test it again later when it has finished recording so it is more 
idle! With the above numbers, it looks like the memchr() is quite a bit 
faster, at least under these conditions!

Cheers,

Laz

Cheers,

Laz



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