On 09/24/2015 05:09 PM, Robert Haas wrote:
> On Thu, Sep 24, 2015 at 9:49 AM, Tomas Vondra
> <tomas.vondra@2ndquadrant.com> wrote:
>> So while it does not introduce behavior change in this particular
>> case (because it fails, as you point out), it introduces a behavior
>> change in general - it simply triggers behavior that does not
>> happen below the limit. Would we accept the change if the proposed
>> limit was 256MB, for example?
>
> So, I'm a huge fan of arbitrary limits.
>
> That's probably the single thing I'll say this year that sounds most
> like a troll, but it isn't. I really, honestly believe that.
> Doubling things is very sensible when they are small, but at some
> point it ceases to be sensible. The fact that we can't set a
> black-and-white threshold as to when we've crossed over that line
> doesn't mean that there is no line. We can't imagine that the
> occasional 32GB allocation when 4GB would have been optimal is no
> more problematic than the occasional 32MB allocation when 4MB would
> have been optimal. Where exactly to put the divider is subjective,
> but "what palloc will take" is not an obviously unreasonable
> barometer.
There are two machines - one with 32GB of RAM and work_mem=2GB, the
other one with 256GB of RAM and work_mem=16GB. The machines are hosting
about the same data, just scaled accordingly (~8x more data on the large
machine).
Let's assume there's a significant over-estimate - we expect to get
about 10x the actual number of tuples, and the hash table is expected to
almost exactly fill work_mem. Using the 1:3 ratio (as in the query at
the beginning of this thread) we'll use ~512MB and ~4GB for the buckets,
and the rest is for entries.
Thanks to the 10x over-estimate, ~64MB and 512MB would be enough for the
buckets, so we're wasting ~448MB (13% of RAM) on the small machine and
~3.5GB (~1.3%) on the large machine.
How does it make any sense to address the 1.3% and not the 13%?
> Of course, if we can postpone sizing the hash table until after the
> input size is known, as you suggest, then that would be better still
> (but not back-patch material).
This dynamic resize is 9.5-only anyway.
regards
--
Tomas Vondra http://www.2ndQuadrant.com
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