On 09/18/2016 06:08 AM, Amit Kapila wrote:
On Sat, Sep 17, 2016 at 11:25 PM, Tomas Vondra
<tomas.von...@2ndquadrant.com> wrote:
On 09/17/2016 07:05 AM, Amit Kapila wrote:


On Sat, Sep 17, 2016 at 9:17 AM, Tomas Vondra
<tomas.von...@2ndquadrant.com> wrote:

On 09/14/2016 05:29 PM, Robert Haas wrote:

...

Sure, but you're testing at *really* high client counts here.
Almost nobody is going to benefit from a 5% improvement at 256
clients. You need to test 64 clients and 32 clients and 16
clients and 8 clients and see what happens there. Those cases are
a lot more likely than these stratospheric client counts.


Right. My impression from the discussion so far is that the patches
only improve performance with very many concurrent clients - but as
Robert points out, almost no one is running with 256 active
clients, unless they have 128 cores or so. At least not if they
value latency more than throughput.


See, I am also not in favor of going with any of these patches, if
they doesn't help in reduction of contention. However, I think it is
important to understand, under what kind of workload and which
environment it can show the benefit or regression whichever is
applicable.


Sure. Which is why I initially asked what type of workload should I be
testing, and then done the testing with multiple savepoints as that's what
you suggested. But apparently that's not a workload that could benefit from
this patch, so I'm a bit confused.

Just FYI, couple of days back one of EDB's partner who was doing the
performance tests by using HammerDB (which is again OLTP focussed
workload) on 9.5 based code has found that CLogControlLock has the
significantly high contention. They were using synchronous_commit=off
in their settings. Now, it is quite possible that with improvements
done in 9.6, the contention they are seeing will be eliminated, but
we have yet to figure that out. I just shared this information to you
with the intention that this seems to be a real problem and we should
try to work on it unless we are able to convince ourselves that this
is not a problem.


So, can we approach the problem from this direction instead? That is,
instead of looking for workloads that might benefit from the patches, look
at real-world examples of CLOG lock contention and then evaluate the impact
on those?


Sure, we can go that way as well, but I thought instead of testing
with a new benchmark kit (HammerDB), it is better to first get with
some simple statements.


IMHO in the ideal case the first message in this thread would provide a test case, demonstrating the effect of the patch. Then we wouldn't have the issue of looking for a good workload two years later.

But now that I look at the first post, I see it apparently used a plain tpc-b pgbench (with synchronous_commit=on) to show the benefits, which is the workload I'm running right now (results sometime tomorrow).

That workload clearly uses no savepoints at all, so I'm wondering why you suggested to use several of them - I know you said that it's to show differences between the approaches, but why should that matter to any of the patches (and if it matters, why I got almost no differences in the benchmarks)?

Pardon my ignorance, CLOG is not my area of expertise ...

Extracting the workload from benchmarks probably is not ideal, but
it's still better than constructing the workload on our own to fit
the patch.

FWIW I'll do a simple pgbench test - first with
synchronous_commit=on and then with synchronous_commit=off.
Probably the workloads we should have started with anyway, I
guess.


Here, synchronous_commit = off case could be interesting. Do you see
any problem with first trying a workload where Dilip is seeing
benefit? I am not suggesting we should not do any other testing, but
just first lets try to reproduce the performance gain which is seen
in Dilip's tests.


I plan to run Dilip's workload once the current benchmarks complete.


regard

--
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services


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