Re: RFC: replace pg_stat_activity.waiting with something more descriptive

On 07/12/2015 06:53 AM, Amit Kapila wrote:

For having duration, I think you need to use gettimeofday or some

similar call to calculate the wait time, now it will be okay for the

cases where wait time is longer, however it could be problematic for

the cases if the waits are very small (which could probably be the

case for LWLocks)

gettimeofday already used in our patch and it gives enough accuracy (in microseconds), especially when lwlock become a problem. Also we tested our realization and it gives overhead less than 1%. (http://www.postgresql.org/message-id/559D4729.9080704@postgrespro.ru, testing part).

We need help here with testing on other platforms. I used gettimeofday because of builtin module "instr_time.h" that already gives cross-platform tested functions for measuring, but I'm planning to make similar implementation for monotonic functions based on clock_gettime for more accuracy.

 
> 2) Accumulate per backend statistics about each wait event type: number of occurrences and total duration. With this statistics user can identify system bottlenecks again without sampling.
>
> Number #2 will be provided as a separate patch.
> Number #1 require different concurrency model. ldus will extract it from "waits monitoring" patch shortly.
>  

Sure, I think those should be evaluated as separate patches,

and I can look into those patches and see if something more

can be exposed as part of this patch which we can be reused in

those patches.

If you agree I'l do some modifications to your patch, so we can later extend it with our other modifications. Main issue is that one variable for all types is not enough. For flexibity in the future we need at least two - class and event, for example class=LWLock, event=ProcArrayLock, or class=Storage, and event=READ.