Re: Large writable variables

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.

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'm inclined to instead reduce DCH_CACHE_SIZE and NUM_CACHE_SIZE enough
to make the structs just less than powers of 2 instead of just more ---
AFAICS, both of those numbers were picked out of the air, and so there's
no reason not to pick a different number out of the air.

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.

The attached patch does both of those things. It's independent of
the other one but is important to make that one efficient.

regards, tom lane