On 01/09/2014 11:48 PM, Claudio Freire wrote:
On Thu, Jan 9, 2014 at 4:39 PM, knizhnik <knizh...@garret.ru> wrote:
At fork time I only wrote about reserving the address space. After
reserving it, all you have to do is implement an allocator that works
in shared memory (protected by a lwlock of course).

In essence, a hypothetical pg_dsm_alloc(region_name) would use regular
shared memory to coordinate returning an already mapped region (same
address which is guaranteed to work since we reserved that region), or
allocate one (within the reserved address space).
Why do we need named segments? There is ShmemAlloc function in PostgreSQL
If RequestAddinShmemSpace can be used without requirement to place module in
preloaded list, then isn't it enough for most extensions?
And ShmemInitHash can be used to maintain named regions if it is needed...
If you want to dynamically create the segments, you need some way to
identify them. That is, the name. Otherwise, RequestWhateverShmemSpace
won't know when to return an already-mapped region or not.

Mind you, the name can be a number. No need to make it a string.

So if we have some reserved address space, do we actually need some special
allocator for this space to allocate new segments in it?
Why existed API to shared memory is not enough?
I don't know this existing API you mention. But I think this is quite
a specific case very unlikely to be serviced from existing APIs. You
need a data structure that can map names to regions, any hash map will
do, or even an array since one wouldn't expect it to be too big, or
require it to be too fast, and then you need to unmap the "reserve"
mapping and put a shared region there instead, before returning the
pointer to this shared region.

So, the special thing is, the book-keeping region sits in regular
shared memory, whereas the allocated regions sit in newly-created
segments. And segments are referenced by pointers (since the address
space is fixed and shared). Is there something like that already?
By existed API I mostly mean 6 functions:


If it will be possible to use this function without requirement for module to be included in "shared_preload_libraries" list, then do we really need DSM?
And it can be achieved by
1. Preserving address space (as you suggested)
2. Preserving some fixed number of free LWLocks (not very large < 100).

I do not have something against creation of own allocator of named shared memory segments within preserved address space. I just not sure if it is actually needed. In some sense RequestAddinShmemSpace() can be such allocator.

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