On Thu, Jun 7, 2018 at 4:19 AM, Frits Jalvingh <jal@etc.to> wrote:
>>work_mem has always been like that.
> I know. My observation is that this behavior has more of a bad effect with
> newer postgresses: because of the increased parallelism (and the apparent OK
> to every node to grab work_mem when they see fit) newer version make way
> less good use of memory than older versions because you have to decrease the
> parameter. That 2GB value I had worked fine on 10, and helped a lot with
> speeding up my workload. Now for the same workload I have to put it on
> 512MB, so all queries that just do one sort are slower - and memory is used
> less well. It means that in all the system might perform less well despite
> parallelism because you have to prevent aborting queries.
It's a problem alright, and many people think we should address it.
It's not exactly obvious how though... Here's a recent thread on the
topic:
https://www.postgresql.org/message-id/flat/CAH2-WzmNwV%3DLfDRXPsmCqgmm91mp%3D2b4FvXNF%3DcCvMrb8YFLfQ%40mail.gmail.com
> ERROR: invalid DSA memory alloc request size 1073741824
The problem is that I failed to constrain nbuckets to fit in
MaxAllocSize when increasing it due to load factor, and for now
Parallel Hash doesn't use DSA_ALLOC_HUGE (so essentially any attempt
to allocate 1GB+ is assumed to be crazy and rejected by dsa.c, just
like the equivalent non-parallel code). Here is a proposed fix that
constrains it.
Longer term, I don't see why we should limit large memory systems to <
1GB of hash buckets, but that doesn't seem like a change that belongs
in a pgsql-bugs thread. I'll put that on a list of hash join
ideas/improvements for discussion on the pgsql-hackers list. In your
case I'd doubt the limit really hurts since work_mem was clearly set
too high for the system and "64 million buckets ought to be enough"
for a smaller setting on your machine. Clearly a 1TB box could make
good use of more buckets than that though.
--
Thomas Munro
http://www.enterprisedb.com
