Re: Automatically setting work_mem

"Qingqing Zhou" <zhouqq(at)cs(dot)toronto(dot)edu> writes:
> So what's the difference between these two strategy?
> (1) Running time: do they use the same amount of memory? Why option 2 is
> better than 1?
> (2) Idle time: after sort done, option 1 will return all 1024 to the OS and
> 2 will still keep 512?

Point 2 is actually a serious flaw in Simon's proposal, because there
is no portable way to make malloc return freed memory to the OS. Some
mallocs will do that ... in some cases ... but many simply don't ever
move the brk address down. It's not an easy thing to do when the arena
gets cluttered with a lot of different alloc chunks and only some of
them get freed.

So the semantics we'd have to adopt is that once a backend claims some
"shared work mem", it keeps it until process exit. I don't think that
makes the idea worthless, because there's usually a clear distinction
between processes doing expensive stuff and processes doing cheap
stuff. But it's definitely a limitation. Also, if you've got a process
doing expensive stuff, it's certainly possible to expect the user to
just increase work_mem locally.

(BTW, given that work_mem is locally increasable, I'm not sure what's
the point of considering that shared_work_mem has to be SUSET. It's
not to prevent users from blowing out memory.)

My own thoughts about the problems with our work_mem arrangement are
that the real problem is the rule that we can allocate work_mem per sort
or hash operation; this makes the actual total memory use per backend
pretty unpredictable for nontrivial queries. I don't know how to fix
this though. The planner needs to know the work_mem that will be used
for any one of these operations in order to estimate costs, so simply
trying to divide up work_mem among the operations of a completed plan
tree is not going to improve matters.

regards, tom lane