Re: [PATCH] Add GUCs for predicate lock promotion thresholds

On Mon, Jan 23, 2017 at 9:50 AM, Dagfinn Ilmari Mannsåker
<ilmari(at)ilmari(dot)org> wrote:
> Kevin Grittner <kgrittn(at)gmail(dot)com> writes:
>
>> (1) The new GUCs are named max_pred_locks_per_*, but the
>> implementation passes them unmodified from a function specifying at
>> what count the lock should be promoted. We either need to change
>> the names or (probably better, only because I can't think of a good
>> name for the other way) return the GUC value + 1 from the function.
>> Or maybe change the function name and how the returned number is
>> used, if we care about eliminating the overhead of the increment
>> instruction. That actually seems least confusing.
>
> I've done the latter, and renamed the function MaxPredicateChildLocks.
> I also changed the default for max_pred_locks_per_page from 3 to 4 and
> the minimum to 0, to keep the effective thresholds the same.
>
>> (2) The new GUCs are declared and documented to be set only on
>> startup. This was probably just copied from
>> max_pred_locks_per_transaction, but that sets an allocation size
>> while these don't. I think these new GUCs should be PGC_SIGHUP,
>> and documented to change on reload.
>
> Fixed.
>
>> (3) The documentation for max_pred_locks_per_relation needs a fix.
>> Both page locks and tuple locks for the relation are counted toward
>> the limit.
>
> Fixed.
>
>> In releases prior to this patch, max_pred_locks_per_relation was
>> calculated as "max_pred_locks_per_transaction / 2". I know that
>> people have sometimes increased max_pred_locks_per_transaction
>> specifically to raise the limit on locks within a relation before
>> the promotion to relation granularity occurs. It seems kinda
>> anti-social not to support a special value for continuing that
>> behavior or, if we don't want to do that, at least modifying
>> pg_upgrade to set max_pred_locks_per_relation to the value that was
>> in effect in the old version.
>
> This is exactly what we've been doing at my workplace, and I mentioned
> this in my original email:
>
>>> ilmari(at)ilmari(dot)org (Dagfinn Ilmari Mannsåker) writes:
>>>> One thing I don't like about this patch is that if a user has
>>>> increased max_pred_locks_per_transaction, they need to set
>>>> max_pred_locks_per_relation to half of that to retain the current
>>>> behaviour, or they'll suddenly find themselves with a lot more
>>>> relation locks. If it's possible to make a GUCs default value
>>>> dependent on the value of another, that could be a solution.
>>>> Otherwise, the page lock threshold GUC could be changed to be
>>>> expressed as a fraction of max_pred_locks_per_transaction, to keep
>>>> the current behaviour.
>>
>>> Another option would be to have a special sentinel (e.g. -1) which is
>>> the default, and keeps the current behaviour.
>
> The attached updated patch combines the two behaviours. Positive values
> mean an absolute number of locks, while negative values mean
> max_predicate_locks_per_transaction / -max_predicate_locks_per_relation.
> Making the default value -2 keeps backwards compatibility.
>
> Alternatively we could have a separate setting for the fraction (which
> could then be a floating-point number, for finer control), and use the
> absolute value if that is zero.
>
> Regards,
>
> Ilmari
>
> --
> "A disappointingly low fraction of the human race is,
> at any given time, on fire." - Stig Sandbeck Mathisen
>

--
Kevin Grittner
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