Re: Parallel threads in query

On 11/01/2018 07:40 PM, Andres Freund wrote:
> Hi,
>
> On 2018-11-01 19:33:39 +0100, Tomas Vondra wrote:
>> In theory, simulating such global limit should be possible using a bit
>> of shared memory for the current total, per-process counter and probably
>> some simple abort handling (say, just like contrib/openssl does using
>> ResourceOwner).
>
> Right. I don't think you even need something resowner like, given that
> anything using threads better make it absolutely absolutely impossible
> that an error can escape.
>

True. Still, I wonder if the process can die in a way that would fail to
update the counter.

>
>> A better solution might be to start a bgworker managing a connection
>> pool and forward the requests to it using IPC (and enforce the thread
>> count limit there).
>
> That doesn't really seem feasible for cases like this - after all, you'd
> only use threads to work on individual rows if you wanted to parallelize
> relatively granular per-row work or such. Adding cross-process IPC seems
> like it'd make that perform badly.
>

I think that very much depends on how expensive the tasks handled by the
threads are. It may still be cheaper than a reasonable IPC, and if you
don't create/destroy threads, that also saves quite a bit of time.

regards

--
Tomas Vondra http://www.2ndQuadrant.com
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