On 19 October 2013 19:22, Peter Geoghegan <pg@heroku.com> wrote:
> I won't repeat the rationale for the patch here.
I can't see the problem that this patch is trying to solve. I'm having
trouble understanding when I would use this.
VACUUM uses 6 bytes per dead tuple. And autovacuum regularly removes
dead tuples, limiting their numbers.
In what circumstances will the memory usage from multiple concurrent
VACUUMs become a problem? In those circumstances, reducing
autovacuum_work_mem will cause more passes through indexes, dirtying
more pages and elongating the problem workload.
I agree that multiple concurrent VACUUMs could be a problem but this
doesn't solve that, it just makes things worse.
Freezing doesn't require any memory at all, so wraparound vacuums
won't be controlled by this parameter.
Can we re-state what problem actually is here and discuss how to solve
it. (The reference [2] didn't provide a detailed explanation of the
problem, only the reason why we want a separate parameter).
-- Simon Riggs http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services
