Re: Is custom MemoryContext prohibited?

2020年1月29日(水) 11:06 Thomas Munro <thomas(dot)munro(at)gmail(dot)com>:
>
> On Wed, Jan 29, 2020 at 2:49 PM Kohei KaiGai <kaigai(at)heterodb(dot)com> wrote:
> > 2020年1月29日(水) 4:27 Thomas Munro <thomas(dot)munro(at)gmail(dot)com>:
> > > On Tue, Jan 28, 2020 at 2:55 PM Kohei KaiGai <kaigai(at)heterodb(dot)com> wrote:
> > > FWIW the code in https://commitfest.postgresql.org/26/2325/ ran into
> > > exactly the same problem while making nearly exactly the same kind of
> > > thing (namely, a MemoryContext backed by space in the main shm area,
> > > in this case reusing the dsa.c allocator).
> > >
> > Thanks, I had looked at "Shared Memory Context" title on pgsql-hackers a few
> > months ago, however, missed the thread right now.
> >
> > The main point of the differences from this approach is portability of pointers
> > to shared memory chunks. (If I understand correctly)
> > PG-Strom preserves logical address space, but no physical pages, on startup
> > time, then maps shared memory segment on the fixed address on demand.
> > So, pointers are portable to all the backend processes, thus, suitable to build
> > tree structure on shared memory also.
>
> Very interesting. PostgreSQL's DSM segments could potentially have
> been implemented that way (whereas today they are not mapped with
> MAP_FIXED), but I suppose people were worried about portability
> problems and ASLR. The Windows port struggles with that stuff.
>
Yes. I'm not certain whether Windows can support equivalen behavior
to mmap(..., PROT_NONE) and SIGSEGV/SIGBUS handling.
It is also a reason why PG-Strom (that is only for Linux) wants to have
own shared memory management logic, at least, right now.

Thanks,
--
HeteroDB, Inc / The PG-Strom Project
KaiGai Kohei <kaigai(at)heterodb(dot)com>