Re: Large writable variables

Andres Freund <andres(at)anarazel(dot)de> writes:
> So we have 500kb of not-initialized memory mapped into every
> process. That's, uh, not nothing.

Bleah.

> 0000000008585088 0000000000131104 b hist_entries
> 0000000008716192 0000000000016384 b hist_start

I'm unsure what fraction of processes would have use for these.

> 0000000008435040 0000000000085280 b DCHCache
> 0000000008391168 0000000000043840 b NUMCache

We could surely allocate these on first use.

> 0000000008560224 0000000000023440 b tzdefrules_s
> 0000000008536704 0000000000023440 b gmtmem.7009

I think that tzdefrules_s is not used in common cases (though I could be
wrong about that), so we could win by alloc-on-first-use. The same might
be true for gmtmem, but there's a sticking point: there is no provision
for failure there, so I'm unsure how we avoid crashing on OOM.

> 0000000008238336 0000000000008192 b PqRecvBuffer
> 0000000008734208 0000000000005136 B BackendWritebackContext
> 0000000008386368 0000000000003200 b held_lwlocks

These are below my personal threshold of pain.

> fmgr_builtins isn't readonly even though declared a const - I assume
> because it's full of addresses that will be mapped differently from
> execution to execution.

Check.

> I'm unclear as to why ScanKeywords, DCH_keywords aren't in a readonly
> section.

I think it's the same problem: pointers can't be truly const because
they have to be changed if one relocates the executable.

We could possibly fix these by changing the data structure so that
what's in a ScanKeywords entry is an offset into some giant string
constant somewhere. No idea how that would affect performance, but
I do notice that we could reduce the sizeof(ScanKeyword), which can't
hurt.

regards, tom lane