Re: Online enabling of checksums

On 2018-04-06 11:25:59 +0200, Magnus Hagander wrote:
> Since you know a lot more about that type of interlocks than I do :) We
> already wait for all running transactions to finish before we start doing
> anything. Obviously transactions != buffer writes (and we have things like
> the checkpointer/bgwriter to consider). Is there something else that we
> could safely just *wait* for? I have no problem whatsoever if this is a
> long wait (given the total time). I mean to the point of "what if we just
> stick a sleep(10) in there" level waiting.

I don't think anything just related to "time" is resonable in any sort
of way. On a overloaded system you can see long long stalls of processes
that have done a lot of work. Locking protocols should be correct, and
that's that.

> Or can that somehow be cleanly solved using some of the new atomic
> operators? Or is that likely to cause the same kind of overhead as throwing
> a barrier in there?

Worse.

I wonder if we could introduce "MegaExpensiveRareMemoryBarrier()" that
goes through pgproc and signals every process with a signal that
requiers the other side to do an operation implying a memory barrier.

That's actually not hard to do (e.g. every latch operation qualifies),
the problem is that signal delivery isn't synchronous. So you need some
acknowledgement protocol.

I think you could introduce a procsignal message that does a memory
barrier and then sets PGPROC->barrierGeneration to
ProcArrayStruct->barrierGeneration. MegaExpensiveRareMemoryBarrier()
increments ProcArrayStruct->barrierGeneration, signals everyone, and
then waits till every PGPROC->barrierGeneration has surpassed
ProcArrayStruct->barrierGeneration.

Greetings,

Andres Freund