Re: Parallel tuplesort (for parallel B-Tree index creation)

On 09/07/2016 09:17 AM, Peter Geoghegan wrote:
> On Tue, Sep 6, 2016 at 11:09 PM, Heikki Linnakangas <hlinnaka(at)iki(dot)fi> wrote:
>>> The big picture here is that you can't only USEMEM() for tapes as the
>>> need arises for new tapes as new runs are created. You'll just run a
>>> massive availMem deficit, that you have no way of paying back, because
>>> you can't "liquidate assets to pay off your creditors" (e.g., release
>>> a bit of the memtuples memory). The fact is that memtuples growth
>>> doesn't work that way. The memtuples array never shrinks.
>>
>>
>> Hmm. But memtuples is empty, just after we have built the initial runs. Why
>> couldn't we shrink, i.e. free and reallocate, it?
>
> After we've built the initial runs, we do in fact give a FREEMEM()
> refund to those tapes that were not used within beginmerge(), as I
> mentioned just now (with a high workMem, this is often the great
> majority of many thousands of logical tapes -- that's how you get to
> wasting 8% of 5GB of maintenance_work_mem).

I & peter chatted over IM on this. Let me try to summarize the problems,
and my plan:

1. When we start to build the initial runs, we currently reserve memory
for tape buffers, maxTapes * TAPE_BUFFER_OVERHEAD. But we only actually
need the buffers for tapes that are really used. We "refund" the buffers
for the unused tapes after we've built the initial runs, but we're still
wasting that while building the initial runs. We didn't actually
allocate it, but we could've used it for other things. Peter's solution
to this was to put a cap on maxTapes.

2. My observation is that during the build-runs phase, you only actually
need those tape buffers for the one tape you're currently writing to.
When you switch to a different tape, you could flush and free the
buffers for the old tape. So reserving maxTapes * TAPE_BUFFER_OVERHEAD
is excessive, 1 * TAPE_BUFFER_OVERHEAD would be enough. logtape.c
doesn't have an interface for doing that today, but it wouldn't be hard
to add.

3. If we do that, we'll still have to reserve the tape buffers for all
the tapes that we use during merge. So after we've built the initial
runs, we'll need to reserve memory for those buffers. That might require
shrinking memtuples. But that's OK: after building the initial runs,
memtuples is empty, so we can shrink it.

- Heikki