Thanks for your reply.
From the code below:
(https://github.com/postgres/postgres/blob/REL_10_7/src/backend/executor/execParallel.c)
#######################################################################################
443 /*
444 * Give parallel-aware nodes a chance to add to the estimates,
and get a
445 * count of how many PlanState nodes there are.
446 */
447 e.pcxt = pcxt;
448 e.nnodes = 0;
449 ExecParallelEstimate(planstate, &e);
450
451 /* Estimate space for instrumentation, if required. */
452 if (estate->es_instrument)
453 {
454 instrumentation_len =
455 offsetof(SharedExecutorInstrumentation,
plan_node_id) +
456 sizeof(int) * e.nnodes;
457 instrumentation_len = MAXALIGN(instrumentation_len);
458 instrument_offset = instrumentation_len;
459 instrumentation_len +=
460 mul_size(sizeof(Instrumentation),
461 mul_size(e.nnodes, nworkers));
462 shm_toc_estimate_chunk(&pcxt->estimator,
instrumentation_len);
463 shm_toc_estimate_keys(&pcxt->estimator, 1);
#######################################################################################
It seems that e.nnodes which returns from ExecParallelEstimate(planstate, &e) ,
determines how much instrumentation structures in DSM(line459~line461).
And e.nnodes also determines the length of SharedExecutorInstrumentation->
plan_node_id(line454~line456).
So, I think here it refers to instrumentation.
SharedExecutorInstrumentation is just likes a master that hold the metadata:
struct SharedExecutorInstrumentation
{
int instrument_options;
int instrument_offset;
int num_workers;
int num_plan_nodes; // this equals to e.nnodes
from the source code
int plan_node_id[FLEXIBLE_ARRAY_MEMBER];
/* array of num_plan_nodes * num_workers Instrumentation objects
follows */
};
What do you think?
With Regards,
Wu Fei
-----Original Message-----
From: Amit Kapila [mailto:[email protected]]
Sent: Wednesday, June 05, 2019 12:20 PM
To: Wu, Fei/吴 非 <[email protected]>
Cc: [email protected]
Subject: Re: Confusing comment for function ExecParallelEstimate
On Wed, Jun 5, 2019 at 9:24 AM Wu, Fei <[email protected]> wrote:
>
> Hi, all
>
> Lately I was researching Parallelism of Postgres 10.7(and it is same in all
> version), and I was confused when reading the comment of function
> ExecParallelEstimate :
>
> (in src/backend/executor/execParallel.c)
>
> ----------------------------------------------
>
>
>
> * While we're at it, count the number of PlanState nodes in the tree,
> so
>
> * we know how many SharedPlanStateInstrumentation structures we need.
>
> static bool
>
> ExecParallelEstimate(PlanState *planstate, ExecParallelEstimateContext
> *e)
>
> ----------------------------------------------
>
>
>
> The structure SharedPlanStateInstrumentation is not exists at all. And I
> noticed that the so called “SharedPlanStateInstrumentation”
>
> maybe is the structure instrumentation now, which is used for storing
> information of planstate in parallelism. The function count the
> number
>
> of planState nodes and stored it in ExecParallelEstimateContext->
> nnodes ,then use it to Estimate space for instrumentation structure in
>
> function ExecInitParallelPlan.
>
I think here it refers to SharedExecutorInstrumentation. This structure is
used for accumulating per-PlanState instrumentation. So, it is not totally
wrong, but I guess we can change it to SharedExecutorInstrumentation to avoid
confusion? What do you think?
--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com
