Large writable variables

Hi,

while briefly thinking about the per-process overhead of postgres, I
looked at the section sizes of a modern postgres. In particular which
memory areas are *not* shared between processes.

If you look at the section sizes that are mapped read-write:

$ size --format=sysv src/backend/postgres
src/backend/postgres :
section size addr
.rodata 1593617 5730304 (read-only, for reference)
.data.rel.ro 134944 8039904 (read only after start)
.data 53888 8178912 (read-write, initialized)
.bss 510416 8232800 (read-write, uninitialized)
Total 52417197

So we have 500kb of not-initialized memory mapped into every
process. That's, uh, not nothing.

top unitialized allocations:
$ nm -t d --size-sort -r -S src/backend/postgres|grep '\b[bB]\b'|head
0000000008251872 0000000000131144 b LagTracker
0000000008585088 0000000000131104 b hist_entries
0000000008435040 0000000000085280 b DCHCache
0000000008391168 0000000000043840 b NUMCache
0000000008560224 0000000000023440 b tzdefrules_s
0000000008536704 0000000000023440 b gmtmem.7009
0000000008716192 0000000000016384 b hist_start
0000000008238336 0000000000008192 b PqRecvBuffer
0000000008734208 0000000000005136 B BackendWritebackContext
0000000008386368 0000000000003200 b held_lwlocks

So we have a two variables sized 130kb. Yikes.

struct {
XLogRecPtr last_lsn; /* 0 8 */
WalTimeSample buffer[8192]; /* 8 131072 */
/* --- cacheline 2048 boundary (131072 bytes) was 8 bytes ago --- */
int write_head; /* 131080 4 */
int read_heads[3]; /* 131084 12 */
WalTimeSample last_read[3]; /* 131096 48 */
/* --- cacheline 2049 boundary (131136 bytes) was 8 bytes ago --- */

/* size: 131144, cachelines: 2050, members: 5 */
/* last cacheline: 8 bytes */
};

that's not actually used very often, nor in all processes... Thomas?

hist_entries is pg_lzcompress; DCHCache,NUMCache are formatting.c.

top itialized allocations:
$ nm -t d --size-sort -r -S src/backend/postgres|grep '\b[dD]\b'|head
0000000008086944 0000000000087904 D fmgr_builtins
0000000008201120 0000000000017280 d ConfigureNamesInt
0000000008218400 0000000000013680 d ConfigureNamesBool
0000000008189248 0000000000008512 d ConfigureNamesString
0000000008077344 0000000000007040 D ScanKeywords
0000000008184928 0000000000004320 d ConfigureNamesEnum
0000000008197760 0000000000003360 d ConfigureNamesReal
0000000008062976 0000000000002304 d DCH_keywords
0000000008069952 0000000000002016 D pg_wchar_table
0000000008075552 0000000000001776 d encoding_match_list

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.

ConfigureNames* isn't marked as const, because we update them:
/* Rather than requiring vartype to be filled in by hand, do this: */
conf->gen.vartype = PGC_BOOL;

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

Greetings,

Andres Freund