Tom Lane wrote:
> That guy doesn't actually have the foggiest idea what he's doing.
> The reason there is no built-in capability to do that is that it *does
> not work well*. Search the list archives for "priority inversion" to
> find out why not.
I agree that that particular author seems clueless, but better
researched papers do show benefits as well:
The CMU paper
"Priority Mechanisms for OLTP and Transactional Web Applications" [1]
studied both TPC-C and TPC-W workloads on postgresql (as well as DB2).
For PostgreSQL they found that without priority inheritance they
had factor-of-2 benefits for high-priority transactions;
and with priority inheritance they had factor-of-6 benefits
for high priority transactions -- both with negligible harm
to the low priority transactions.
Unless there's something wrong with that paper (and at first glance
it looks like their methodologies apply at least to many workloads)
it seems that "it *does not work well*" is a bit of a generalization;
and that databases with TPC-C and TPC-W like workloads may indeed
be cases where this feature would be useful.
[1] http://www.cs.cmu.edu/~harchol/Papers/actual-icde-submission.pdf
"
...This paper analyzes and proposes prioritization for
transactional workloads in conventional DBMS...This paper
provides a detailed resource utilization breakdown for
OLTP workloads executing on a range of database platforms
including IBM DB2[14], Shore[16], and PostgreSQL[17]....
...
For DBMS using MVCC (with TPC-C or TPC-W workloads) and
for TPC-W workloads (with any concurrency control mechanism),
we find that lock scheduling is largely ineffective (even
preemptive lock scheduling) and CPU scheduling is highly
effective. For example, we find that for PostgreSQL
running under TPC-C, the simplest CPU scheduling
algorithm CPU-Prio provides a factor of 2 improvement
for the high-priority transactions, and adding priority
inheritance (CPU-Prio-Inherit) brings this up to a factor
of near 6 improvement under high loads, while hardly
penalizing low-priority transactions.
"
Or am I missing something?
Ron
