On 11/6/23 19:54, Matthias van de Meent wrote: > > ... > > Tangent: Do we have specific notes on worst-case memory usage of > memory contexts with various allocation patterns? This new bump > allocator seems to be quite efficient, but in a worst-case allocation > pattern it can still waste about 1/3 of its allocated memory due to > never using free space on previous blocks after an allocation didn't > fit on that block. > It probably isn't going to be a huge problem in general, but this > seems like something that could be documented as a potential problem > when you're looking for which allocator to use and compare it with > other allocators that handle different allocation sizes more > gracefully. >
I don't think it's documented anywhere, but I agree it might be an interesting piece of information. It probably did not matter too much when we had just AllocSet, but now we have 3 very different allocators, so maybe we should explain this. When implementing these allocators, it didn't feel that important, because the new allocators started as intended for a very specific part of the code (as in "This piece of code has a very unique allocation pattern, let's develop a custom allocator for it."), but if we feel we want to make it simpler to use the allocators elsewhere ... I think there are two obvious places where to document this - either in the header of each memory context .c file, or a README in the mmgr directory. Or some combination of it. At some point I was thinking about writing a "proper paper" comparing these allocators in a more scientific / thorough way, but I never got to do it. I wonder if that'd be interesting for enough people. regards -- Tomas Vondra EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company
