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Re: shared memory release following failed lock acquirement.

From: "Simon Riggs" <simon(at)2ndquadrant(dot)com>
To: "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>,"Merlin Moncure" <merlin(dot)moncure(at)rcsonline(dot)com>
Cc: <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: shared memory release following failed lock acquirement.
Date: 2004-09-29 21:48:47
Message-ID: NOEFLCFHBPDAFHEIPGBOKEEHCFAA.simon@2ndquadrant.com (view raw or flat)
Thread:
Lists: pgsql-hackers
>Tom Lane
> "Merlin Moncure" <merlin(dot)moncure(at)rcsonline(dot)com> writes:
> > According to postgresql.conf, using these settings the lock table eats
> > 64*260*100 bytes = < 2M.  Well, if it's running my server out of shared
> > memory, it's eating much, much more shmem than previously thought.
>
> Hmm, the 260 is out of date I think.  I was seeing about 184 bytes/lock
> in my tests just now.
>
> > Also, I was able to acquire around 10k locks before the server borked.
> > This is obviously a lot more than than 64*100.
>
> Sure, because there's about 100K of deliberate slop in the shared memory
> size allocation, and you are probably also testing a scenario where the
> buffer and FSM hash tables haven't ramped to full size yet, so the lock
> table is able to eat more than the nominal amount of space.
>
> > As I see it, this means the user-locks (and perhaps all
> > locks...?) eat around ~ 6k bytes memory each.
>
> They're allocated in groups of 32, which would work out to close to 6k;
> maybe you were measuring the incremental cost of allocating the first one?
>
> I did some digging, and as far as I can see the only shared memory
> allocations that occur after postmaster startup are for the four shmem
> hash tables: buffers, FSM relations, locks, and proclocks.  Of these,
> the buffer and FSM hashtables have predetermined maximum sizes.  So
> arranging for the space in those tables to be fully preallocated should
> prevent any continuing problems from lock table overflow.  I've
> committed a fix that does this.  I verified that after running the thing
> out of shared memory via creating a lot of user locks and then releasing
> same, I could run the regression tests.
>

Few questions:

Is that fix in 8.0?

Does this mean that the parameter max_locks_per_transaction isn't honoured
at all, it is just used to size the lock table - which itself can expand
beyond that max limit in various circumstances? (Though with the bug fix,
not THAT much more than the max limit)
Should we rename and redocument the parameter? If that is so, the current
name is so far away from its real meaning as to constitute a bug in
itself....

Best Regards, Simon Riggs



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