Memory exhausted in AllocSetAlloc

Hey folks,

I'm running 7.3.5 on an OpenBSD 3.5-STABLE machine, with 512M of RAM.
I'm running VACUUM ANALYZE every hour, with a VACUUM FULL once per night
before backups.

I'm trying to speed up the VACCUMs, so I tried bumping up vacuum_mem
in postgresql.conf from the default to 64M. This resulted in:

ERROR: Memory exhausted in AllocSetAlloc(somenumber)

(where somenumber was not written down by me, sorry)

I understand what happened, but not exactly why - this machine is
running with over 300M of RAM unused. Bumping this value down to 16M
works fine. But why does it hit a memory exhausted error at 64M?

Please let me know if this belongs on pgsql-performance - I'm getting
an actual error, so I'm hoping this is the right place. If not, I'll
join the other and post there.

Thanks much!

Benny

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