On Mon, Jan 04, 2016 at 12:07:15AM +0100, Michael Niedermayer wrote: > On Fri, Jan 01, 2016 at 08:07:39AM -0800, Ganesh Ajjanagadde wrote: > > Hi all, > > > > Motivated by a remark by Ronald: > > https://ffmpeg.org/pipermail/ffmpeg-devel/2016-January/186200.html, > > this is a request for comment on disabling compile time tablegen for > > cbrt if the total cycle count < 200000. Note that cbrt tables are only > > used in aacdec. > > Its very hard to state a hard number as the threshold, this is quite > subjective > and different viewpoints would lead to different results > > but lets try anyway > for the hardcoded table case more data needs to be read from disk > ATM this is about 2mb for all tables > > for dynamic tables the used tables need to be generated > theres a difference in binary size (table size vs code to generate it), > that translates into a cost value (bytes * cycles_per_byte) > > and the dynamic init needs some time to build the table, thats > conveniently already in cycles > > (above ignores alot, like HDDs, SDDs disk cache have different speed > hardcoded tables can be shared between processes but then rarely would > a process use all tables while the OS might end up loading all anyway > also we assume a load from cache while a OS might actually map tables > from the disk cache directly) > > now if the dynamic init is faster than loading the table from > the disk cache then dynamic init is likely the better choice >
> using cat into /dev/null i get about 2bytes per cycle to make this more complex if the whole binary is read on init then all tables would need to be loaded while for a use case of repeated execution of ffmpeg only the tables needed for a single file would need to be build (the repeated execution is where the init time matters most) [...] -- Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB Good people do not need laws to tell them to act responsibly, while bad people will find a way around the laws. -- Plato
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