Re: Large writable variables

Hi,

On 2018-10-16 01:59:00 -0400, Tom Lane wrote:
> I wrote:
> > Andres Freund <andres(at)anarazel(dot)de> writes:
> >> top unitialized allocations:
> >> 0000000008435040 0000000000085280 b DCHCache
> >> 0000000008391168 0000000000043840 b NUMCache
>
> > Here's a patch to improve that situation.
>
> Hm, looking at that more closely, there's a problem with the idea of
> allocating the cache slots one-at-a-time. Currently,
> sizeof(DCHCacheEntry) and sizeof(NUMCacheEntry) are each just a bit more
> than a power of 2, which would cause palloc to waste nearly 50% of the
> allocation it makes for them.

Hm, that's a bit annoying...

> We could forget the one-at-a-time idea and just allocate the whole
> array on first use, but I feel like that's probably not a good answer.

I suspect it'd be fine, but obviously we can do better.

> Also, I noticed that the biggest part of those structs are arrays of
> FormatNode, which has been designed with complete lack of thought about
> size or padding issues. We can very easily cut it in half on 64-bit
> machines.

Heh, neat. I feel like we've paid very little attention to that in a
myriad of places :(

Greetings,

Andres Freund