[postgis-devel] lwgeom_*_in_place()

Darafei "Komяpa" Praliaskouski me at komzpa.net
Mon Oct 2 17:19:23 PDT 2017


Can you please share the way you are using profiler on postgis?
I'd like to profile sorting by geometry. For now I've found for myself

objdump -S --disassemble /usr/lib/liblwgeom-2.5.so.0 |less

... but that doesn't scale well enough. Still allows to see that
geometry/geography if's reading bits of header with something hidden three
files away under #define aren't a simple thing.

For _in_place suggestion, I think it's enough to make sure it's done for
the critical path of MVT and codified somewhere as a guideline.

вт, 3 окт. 2017 г. в 0:53, Paul Ramsey <pramsey at cleverelephant.ca>:

> Hey devs,
> So, in pursuit of spare CPU cycles for the good folks at Carto, I've been
> putting various workloads through a profiler. Some of the results you've
> seen in recent tweaks committed.
> One thing I found as a general rule was that as the number of simple
> objects goes up, the more noticeable the overhead of things like memory
> allocation / deallocation becomes. So the overhead of just constructing an
> LWPOINT on a point, when you're slamming through 1M of them, is not nothing.
> For some workloads, like generating MVT (there it is) we call numerous
> geometry processing functions in a row. For some functions, that expect
> const LWGEOM inputs, that can result in a lot of cloning of objects, and
> hence, memory management. For other functions, that expect to modify things
> in place, the coordinates are just altered right there.
> This has always been a little ugliness in our API, and for a while we
> settled on defaulting to making copies, so the API was at least getting
> cleaner. But also getting slower.
> I built a prototype for generating MVT that does all the coordinate
> processing in place, and for bulk simple objects it ran about 30% faster
> than the existing code. There's a benefit to be had to in working in place.
> I propose that we... cut the baby in half.
> * All liblwgeom functions that do in place coordinate modification should
> be re-named to *_in_place() and a bare version that takes a const LWGEOM
> and returns a copy as necessary be added.
> * All liblwgeom functions that already take const LWGEOM be supplemented
> with *_in_place() variants so that it's possible to build processing chains
> that are capable of doing processing with a minimal allocation/deallocation
> footprint.
> Functions on my list immediately:
>   * remove_repeated_points
>   * simplify
>   * affine
>   * grid
>   * transform
> I will, however, do a full review and attempt to ensure all non-GEOS
> functions have in_place variants available in liblwgeom.
> Thoughts?
> P.
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