Custom cache implemented in a postgresql C function

Are you sure you cache needs to grow endlessly? Otherwise you could use
RequestAddinShmemSpace and manage you're map within that space, perhaps
"overwriting" chunks on an LRU basis or a rollover. i.e. Grab it all and
do your own management within that single block of shmem.
Caches are best for thing revisited often, so old/unused ought to be
expendable with little performance loss, at least compared with the
heavy traffic.

Gabi Julien <gabi.julien@broadsign.com> writes:

In my case, I do not know how big my cache will be.

That makes it awfully hard to use shared memory.

If shared memory turns out too difficult to use, I could create
separate caches for each postgresql processes.

That's what I'd recommend. A big advantage of private caches is that
you don't have any need to manage concurrent access, which simplifies
the code and avoids contention. All the caches that the core Postgres
code maintains are per-backend.

This would be a waste
of space but it might be better then nothing. In this case, do I need
to make my code thread safe? In other words, is postgresql using more
then one thread per processes?

No.

regards, tom lane

On 21 Oct 2010, at 1:44, Gabi Julien wrote:

Hi,

Here is my problem: I have a postgresql C function that looks like this:

Datum filter(PG_FUNCTION_ARGS);

It takes identifiers and queries a bunch of tables and ends up returning true or false. So far nothing difficult except that we want better performance. The function was already optimized to the best of my abilities and changing the structure of the database would not help. However, having a

That sounds like your function would classify as a STABLE function within Postgres, did you define it as such? Postgres will cache the results of STABLE (and IMMUTABLE) functions all by itself, in which case you may not need your custom cache. The default is to classify a function as VOLATILE, meaning the results aren't suitable for caching.

Another possible solution is to store the results of your function (or of the queries it performs) in a separate table[1]A TEMP TABLE wouldn't work, as it isn't visible to other sessions, although you could create one per session of course. that would function as a cache of sorts. The benefit is that the table gets managed by Postgres, so you won't have to worry about stuff like spilling to disk if the cache grows too large to fit in (available) memory.

[1]: A TEMP TABLE wouldn't work, as it isn't visible to other sessions, although you could create one per session of course.

Of course, with a custom cache you have more control over how it behaves, so that may still be your best solution.

Alban Hertroys

--
Screwing up is an excellent way to attach something to the ceiling.

!DSPAM:737,4cc01f6410281645420170!

Alban Hertroys <dalroi@solfertje.student.utwente.nl> writes:

That sounds like your function would classify as a STABLE function
within Postgres, did you define it as such? Postgres will cache the
results of STABLE (and IMMUTABLE) functions all by itself, in which
case you may not need your custom cache.

Uh, no it won't. It will pre-evaluate immutable functions that are
called with constant arguments, which is not the same thing at all.

regards, tom lane

On Oct 20, 2010, at 7:44 PM, Gabi Julien wrote:

Hi,

Here is my problem: I have a postgresql C function that looks like this:

Datum filter(PG_FUNCTION_ARGS);

It takes identifiers and queries a bunch of tables and ends up returning true or false. So far nothing difficult except that we want better performance. The function was already optimized to the best of my abilities and changing the structure of the database would not help. However, having a cache would be the perfect solution. I could implement this cache outside of postgresql if need be but nothing could beat implementing this directly in a postgresql C function.

So this is what I want, a custom cache built into a postgresql C function. Since postgresql uses different processes, it would be best to use the shared memory. Can this be done safely? At its core, the cache could be considered as simple as a map protected by a mutex. With postgresql, I first need to initialized some shared memory. This is explained at the end of this link:

http://www.postgresql.org/docs/8.2/static/xfunc-c.html

However, it sounds like I need to reserve the shared memory in advance using:

void RequestAddinShmemSpace(int size)

In my case, I do not know how big my cache will be. I would preferably allocate the memory dynamically. Is this possible? In any case, am I trying to reinvent the wheel here? Is there already a shared map or a shared hash structure available in postgresql?

If shared memory turns out too difficult to use, I could create separate caches for each postgresql processes. This would be a waste of space but it might be better then nothing. In this case, do I need to make my code thread safe? In other words, is postgresql using more then one thread per processes?

Apart from the other suggestions made, another option could be to use your own shared memory which you allocate and manage yourself (without postgresql managing it). You could implement a simple least-recently-used cache to purge old entries as the cache grows.

Cheers,
M

Thanks to all of you. This was very good feedback. I'll use the one cache per process suggestion of Tom Lane. This will be the easiest to implement.