Re: memory leak in trigger handling (since PG12)

On 5/23/23 19:14, Andres Freund wrote:
> Hi,
>
> On 2023-05-23 18:23:00 +0200, Tomas Vondra wrote:
>> This means that for an UPDATE with triggers, we may end up calling this
>> for each row, possibly multiple bitmaps. And those bitmaps are allocated
>> in ExecutorState, so won't be freed until the end of the query :-(
>
> Ugh.
>
>
> I've wondered about some form of instrumentation to detect such issues
> before. It's obviously a problem that we can have fairly large leaks, like the
> one you just discovered, without detecting it for a couple years. It's kinda
> made worse by the memory context infrastructure, because it hides such issues.
>
> Could it help to have a mode where the executor shutdown hook checks how much
> memory is allocated in ExecutorState and warns if its too much? There's IIRC a
> few places that allocate large things directly in it, but most of those
> probably should be dedicated contexts anyway. Something similar could be
> useful for some other long-lived contexts.
>

Not sure such simple instrumentation would help much, unfortunately :-(

We only discovered this because the user reported OOM crashes, which
means the executor didn't get to the shutdown hook at all. Yeah, maybe
if we had such instrumentation it'd get triggered for milder cases, but
I'd bet the amount of noise is going to be significant.

For example, there's a nearby thread about hashjoin allocating buffiles
etc. in ExecutorState - we ended up moving that to a separate context,
but surely there are more such cases (not just for ExecutorState).

The really hard thing was determining what causes the memory leak - the
simple instrumentation doesn't help with that at all. It tells you there
might be a leak, but you don't know where did the allocations came from.

What I ended up doing is a simple gdb script that sets breakpoints on
all palloc/pfree variants, and prints info (including the backtrace) for
each call on ExecutorState. And then a script that aggregate those to
identify which backtraces allocated most chunks that were not freed.

This was super slow (definitely useless outside development environment)
but it made it super obvious where the root cause is.

I experimented with instrumenting the code a bit (as alternative to gdb
breakpoints) - still slow, but much faster than gdb. But perhaps useful
for special (valgrind-like) testing mode ...

Would require testing with more data, though. I doubt we'd find much
with our regression tests, which have tiny data sets.

> ...
>
>> Attached is a patch, restoring the pre-12 behavior for me.
>
> Hm. Somehow this doesn't seem quite right. Shouldn't we try to use a shorter
> lived memory context instead? Otherwise we'll just end up with the same
> problem in a few years.
>

I agree using a shorter lived memory context would be more elegant, and
more in line with how we do things. But it's not clear to me why we'd
end up with the same problem in a few years with what the patch does.

regards

--
Tomas Vondra
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