Re: TODO : Allow parallel cores to be used by vacuumdb [ WIP ]

On Tue, Oct 28, 2014 at 9:27 AM, Dilip kumar <dilip(dot)kumar(at)huawei(dot)com> wrote:
> On 28 October 2014 09:18, Amit Kapila Wrote,
>
> >I am worried about the case if after setting the inAbort flag,
>
> >PQCancel() fails (returns error).
>
> >
>
> >> If select(maxFd + 1, workerset, NULL, NULL, &tv); come out, we can
know whether it came out because of cancel query and handle it accordingly.
>
> >>
>
> >
>
> >Yeah, it is fine for the case when PQCancel() is successful, what
>
> >if it fails?
>
> >I think even if select comes out due to any other reason, it will behave
>
> >as if it came out due to Cancel, even though actually Cancel is failed,
>
> >how are planning to handle that case?
>
>
>
> I think If PQcancel fails then also there is no problem, because we are
setting inAbort flag in handle_sigint handler, it means user have tried to
terminate.
>

Yeah, user has tried to terminate, however utility will emit the
message: "Could not send cancel request" in such a case and still
silently tries to cancel and disconnect all connections.

One other related point is that I think still cancel handling mechanism
is not completely right, code is doing that when there are not enough
number of freeslots, but otherwise it won't handle the cancel request,
basically I am referring to below part of code:

run_parallel_vacuum()
{
..
for (cell = tables->head; cell; cell = cell->next)
{
/*
* This will give the free connection slot, if no slot is free it will
* wait for atleast one slot to get free.
*/

free_slot = GetIdleSlot(connSlot, max_slot, dbname, progname,
completedb);

if (free_slot == NO_SLOT)
{
error = true;
goto fail;
}

prepare_command(connSlot[free_slot].connection, full, verbose,
and_analyze, analyze_only, freeze, &sql);

appendPQExpBuffer(&sql, " %s", cell->val);

connSlot[free_slot].isFree = false;

slotconn = connSlot[free_slot].connection;

PQsendQuery(slotconn, sql.data);

resetPQExpBuffer(&sql);
}
..
}

I am wondering if it would be better to setcancelconn in above loop.

With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com