Re: On-demand running query plans using auto_explain and signals

On Mon, Sep 28, 2015 at 12:05 PM, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
wrote:

>
> 2015-09-28 12:01 GMT+02:00 Shulgin, Oleksandr <
> oleksandr(dot)shulgin(at)zalando(dot)de>:
>
>> On Sun, Sep 27, 2015 at 8:05 AM, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
>> wrote:
>>
>> the preparing of content before execution is interesting idea, that can
>>> be used more. The almost queries and plans are not too big, so when the
>>> size of content is not too big - less than 1MB, then can be used one DSM
>>> for all backends.
>>>
>>
>>
>>> When size of content is bigger than limit, then DSM will be allocated
>>> specially for this content. The pointer to DSM and offset can be stored in
>>> requested process slot. The reading and writing to requested slot should be
>>> protected by spinlock, but it should block only two related processes for
>>> short time (copy memory).
>>>
>>
>> Sorry, I don't think this will fly.
>>
>> The whole idea is that a backend publishes the plan of a query just
>> before running it and it doesn't care which other backend(s) might be
>> reading it, how many times and in which order. The only required locking
>> (implicit) is contained in the code for dsm_attach/detach().
>>
>
> I didn't propose too different solution. There is only one difference -
> sharing DSM for smaller data. It is similar to using usual shared memory.
>

Does this mean implementing some sort of allocator on top of the shared
memory segment? If so, how are you going to prevent fragmentation?

--
Alex