On Tue, Jan 28, 2020 at 10:55:11AM +0900, Kohei KaiGai wrote:
Hello,I noticed MemoryContextIsValid() called by various kinds of memory context routines checks its node-tag as follows: #define MemoryContextIsValid(context) \ ((context) != NULL && \ (IsA((context), AllocSetContext) || \ IsA((context), SlabContext) || \ IsA((context), GenerationContext))) It allows only "known" memory context methods, even though the memory context mechanism enables to implement custom memory allocator by extensions. Here is a node tag nobody used: T_MemoryContext. It looks to me T_MemoryContext is a neutral naming for custom memory context, and here is no reason why memory context functions prevents custom methods.
Good question. I don't think there's an explicit reason not to allow extensions to define custom memory contexts, and using T_MemoryContext seems like a possible solution. It's a bit weird though, because all the actual contexts are kinda "subclasses" of MemoryContext. So maybe adding T_CustomMemoryContext would be a better choice, but that only works in master, of course. Also, it won't work if we need to add memory contexts to equalfuncs.c etc. but maybe won't need that - it's more a theoretical issue.
https://github.com/heterodb/pg-strom/blob/master/src/shmbuf.c#L1243 I recently implemented a custom memory context for shared memory allocation with portable pointers. It shall be used for cache of pre-built gpu binary code and metadata cache of apache arrow files. However, the assertion check above requires extension to set a fake node-tag to avoid backend crash. Right now, it is harmless to set T_AllocSetContext, but feel a bit bad.
Interesting. Does that mean the hared memory contexts are part of the same hierarchy as "normal" contexts? That would be a bit confusing, I think. regards -- Tomas Vondra http://www.2ndQuadrant.com PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
