Re: dynamically allocating chunks from shared memory

Hi,

On 07/20/2010 08:23 PM, Robert Haas wrote:
> Well, you can't really fix that problem with this infrastructure,

No, but it would allow you to better use the existing amount of shared
memory. Possibly avoiding the problem is certain scenarios.

> The failure might
> manifest itself in a totally different subsystem though, since the
> allocation that failed wouldn't necessarily be a heavyweight lock
> allocation, but some other allocation that failed as a result of space
> used by the heavyweight locks.

Yeah, that's a valid concern. Maybe it could be addressed by keeping
track of usage of dynshmem per module, and somehow inform the user about
the usage pattern in case of OOM.

> It would be more interesting

Sure, but then you'd definitely need a dynamic allocator, no?

> With respect to imessages specifically, what is the motivation for
> using shared memory rather than something like an SLRU? The new
> LISTEN implementation uses an SLRU and handles variable-size messages,
> so it seems like it might be well-suited to this task.

Well, imessages predates the new LISTEN implementation by some moons.
They are intended to replace (unix-ish) pipes between processes. I fail
to see the immediate link between (S)LRU and inter-process message
passing. It might be more useful for multiple LISTENers, but I bet it
has slightly different semantics than imessages.

But to be honest, I don't know too much about the new LISTEN
implementation. Do you think a loss-less
(single)-process-to-(single)-process message passing system could be
built on top of it?

> Incidentally, the link for the imessages patch on the CommitFest page
> points to http://archives.postgresql.org/message-id/ab0cd52a64e788f4ecb4515d1e6e4691@localhost
> - which is the dynamic shmem patch. So I'm not sure where to find the
> latest imessages patch.

The archive doesn't display attachments very well. But the imessages
patch is part of that mail. Maybe you still find it in your local mailbox?

In the archive view, it starts at the line that says:
*** src/backend/storage/ipc/imsg.c dc149eef487eafb43409a78b8a33c70e7d3c2bfa

(and, well, the dynshmem stuff ends just before that line. Those were
two .diff files attached, IIRC).

Regards

Markus Wanner