Re: Memory Size of 1 Mio Point Polyline table?

[Rolf Kalbermatter <rolf.kalbermatter@...>]

You shouldn’t forget for alignment. If the data is an 8 byte element and always followed by a data type integer it really won’t matter if that data type value is a byte, word, long or quad (int64). Unless you change the default alignment too, the compiler will always align the 8 byte data value on an address that is a multiple of 8 bytes, wasting the unused bytes in between.

And just blindly enabling byte packing usually has some serious implications if the code is not completely written with that in mind and on non-Intel x86 platforms potentially huge performance implications for unaligned access of memory addresses.

Sent from my iPhone

> On 23 Oct 2022, at 07:25, bil til <biltil52@gmail.com> wrote:
> 
> Thank you for the PDF version of Roberto's "Lua performance tricks"
> article from Lua Gems.
> 
> After this extremely nicely presented tragic-philosophical start about
> modern software programming in the first paragraph, it gives me really
> very interesting insights into the storage of Lua. As I especially am
> just interested in tables, it is especially this Polyline
> construction:
> 
> polyline = { x = { 10.3, 10.3, 15.0, ...},
> y = { 98.5, 18.3, 98.5, ...}
> }
> 
> It is stated that this needs 24MBytes for a 1 Mio-Point Polyline (see
> errata, there is a misprint of "kBytes" in the original article).
> 
> I assume, this depends strongly on the C compiler used?
> 
> So 24MByte presumably for a C compiler for Lua64 using 'int' fo  32
> bits to facilitate to "uplift 32bits software to 64bit world" (as it
> does MS Visual Studion C++ e. g. for the creation of Win64 bit
> software)?
> 
> And I assume this was before Lua started with 64bits integers / lua
> integers, so possible in Lua5.1 times? (8bytes per double, plus 4
> bytes per type, so 12 bytes per Lua number?).
> 
> And after Lua integers appearing (I think Lua 5.2) this would possibly
> even have increased to 32 MByte? (8 bytes per double, plus 8 bytes per
> type info, so 16 bytes per Lua number?).
> 
> And after Lua 5.4, where the type info shrinked to 1 byte, if I
> understand this correctly, this shrinked now to 18 MByte (8bytes per
> double, plus 1 byte per type info)?
> 
> And for Lua32 the numbers would be quite identical, except there of
> course 4 byte per float, and int also remains with 4 bytes - so
> approximately halfed memory usage?
> 
> Could some "Lua internals specialst" tell me, whether this is correct
> understanding from my side?