On 2016-09-13 15:37:22 -0400, Tom Lane wrote:
> Andres Freund <andres@anarazel.de> writes:
> > On 2016-09-13 14:47:08 -0400, Tom Lane wrote:
> >> Also I notice that the WaitEventSet thread started with a simple
> >> pgbench test, so I don't really buy the claim that that's not a
> >> way that will reach the problem.
>
> > You can reach it, but not when using 1 core:one pgbench thread:one
> > client connection, there need to be more connections than that. At least
> > that was my observation on x86 / linux.
>
> Well, that original test was
>
> >> I tried to run pgbench -s 1000 -j 48 -c 48 -S -M prepared on 70 CPU-core
> >> machine:
>
> so no, not 1 client ;-)
What I meant wasn't one client, but less than one client per cpu, and
using a pgbench thread per backend. That way usually, at least on linux,
there'll be a relatively small amount of poll/epoll/whatever, because
the recvmsg()s will always have data available.
> Anyway, I decided to put my money where my mouth was and run my own
> benchmark.
Cool.
> (It's a 4-core CPU so I saw little point in pressing harder than
> that.)
I think in reality most busy machines, were performance and scalability
matter, are overcommitted in the number of connections vs. cores. And
if you look at throughput graphs that makes sense; they tend to increase
considerably after reaching #hardware-threads, even if all connections
are full throttle busy. It might not make sense if you just run large
analytics queries, or if you want the lowest latency possible, but in
everything else, the reality is that machines are often overcommitted
for good reason.
> So at this point I'm wondering why Thomas and Heikki could not measure
> any win. Based on my results it should be easy. Is it possible that
> OS X is better tuned for multi-CPU hardware than FreeBSD?
Hah!
Greetings,
Andres Freund