On Tue, Mar 15, 2016 at 7:43 PM, Alexander Korotkov < a.korot...@postgrespro.ru> wrote:
> On Tue, Mar 15, 2016 at 12:57 AM, Robert Haas <robertmh...@gmail.com> > wrote: > >> On Mon, Mar 14, 2016 at 4:42 PM, Andres Freund <and...@anarazel.de> >> wrote: >> > On 2016-03-14 16:16:43 -0400, Robert Haas wrote: >> >> > I have already shown [0, 1] the overhead of measuring timings in >> linux on >> >> > representative workload. AFAIK, these tests were the only one that >> showed >> >> > any numbers. All other statements about terrible performance have >> been and >> >> > remain unconfirmed. >> >> >> >> Of course, those numbers are substantial regressions which would >> >> likely make it impractical to turn this on on a heavily-loaded >> >> production system. >> > >> > A lot of people operating production systems are fine with trading a <= >> > 10% impact for more insight into the system; especially if that >> > configuration can be changed without a restart. I know a lot of systems >> > that use pg_stat_statements, track_io_timing = on, etc; just to get >> > that. In fact there's people running perf more or less continuously in >> > production environments; just to get more insight. >> > >> > I think it's important to get as much information out there without >> > performance overhead, so it can be enabled by default. But I don't think >> > it makes sense to not allow features in that cannot be enabled by >> > default, *if* we tried to make them cheap enough beforehand. >> >> Hmm, OK. I would have expected you to be on the other side of this >> question, so maybe I'm all wet. One point I am concerned about is >> that, right now, we have only a handful of types of wait events. I'm >> very interested in seeing us add more, like I/O and client wait. So >> any overhead we pay here is likely to eventually be paid in a lot of >> places - thus it had better be extremely small. >> > > OK. Let's start to produce light, not heat. > > As I get we have two features which we suspect to introduce overhead: > 1) Recording parameters of wait events which requires some kind of > synchronization protocol. > 2) Recording time of wait events because time measurements might be > expensive on some platforms. > > Simultaneously there are machines and workloads where both of these > features doesn't produce measurable overhead. And, we're talking not about > toy databases. Vladimir is DBA from Yandex which is in TOP-20 (by traffic) > internet companies in the world. They do run both of this features in > production highload database without noticing any overhead of them. > > It would be great progress, if we decide that we could add both of these > features controlled by GUC (off by default). > enable_waits_statistics ? > > If we decide so, then let's start working on this. At first, we should > construct list of machines and workloads for testing. Each list of machines > and workloads would be not comprehensive. But let's find something that > would be enough for testing of GUC controlled, off by default features. > Then we can turn our conversation from theoretical thoughts to particular > benchmarks which would be objective and convincing to everybody. > Vladimir, could you provide a test suite, so other people could measure overhead on their machines ? > > Otherwise, let's just add these features to the list of unwanted > functionality and close this question. > > ------ > Alexander Korotkov > Postgres Professional: http://www.postgrespro.com > The Russian Postgres Company >
