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 also about START/STOP_TIMER, the decicyles are tens of cycles so one tenths of the displayed value is the amount of cpu cycles iam sure if one looks at this from a different angle a quite different threshold mathod and value could result [...] -- Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB Opposition brings concord. Out of discord comes the fairest harmony. -- Heraclitus
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