On 3/17/23 09:18, Jehan-Guillaume de Rorthais wrote:
> Hi there,
>
> On Fri, 10 Mar 2023 19:51:14 +0100
> Jehan-Guillaume de Rorthais <j...@dalibo.com> wrote:
>
>>> So I guess the best thing would be to go through these threads, see what
>>> the status is, restart the discussion and propose what to do. If you do
>>> that, I'm happy to rebase the patches, and maybe see if I could improve
>>> them in some way.
>>
>> [...]
>>
>>> I was hoping we'd solve this by the BNL, but if we didn't get that in 4
>>> years, maybe we shouldn't stall and get at least an imperfect stop-gap
>>> solution ...
>>
>> Indeed. So, to sum-up:
>>
>> * Patch 1 could be rebased/applied/backpatched
>
> Would it help if I rebase Patch 1 ("move BufFile stuff into separate
> context")?
>
Yeah, I think this is something we'd want to do. It doesn't change the
behavior, but it makes it easier to track the memory consumption etc.
>> * Patch 2 is worth considering to backpatch
>
I'm not quite sure what exactly are the numbered patches, as some of the
threads had a number of different patch ideas, and I'm not sure which
one was/is the most promising one.
IIRC there were two directions:
a) "balancing" i.e. increasing work_mem to minimize the total memory
consumption (best effort, but allowing exceeding work_mem)
b) limiting the number of BufFiles, and combining data from "future"
batches into a single file
I think the spilling is "nicer" in that it actually enforces work_mem
more strictly than (a), but we should probably spend a bit more time on
the exact spilling strategy. I only really tried two trivial ideas, but
maybe we can be smarter about allocating / sizing the files? Maybe
instead of slices of constant size we could/should make them larger,
similarly to what log-structured storage does.
For example we might double the number of batches a file represents, so
the first file would be for batch 1, the next one for batches 2+3, then
4+5+6+7, then 8-15, then 16-31, ...
We should have some model calculating (i) amount of disk space we would
need for the spilled files, and (ii) the amount of I/O we do when
writing the data between temp files.
> Same question.
>
>> * Patch 3 seemed withdrawn in favor of BNLJ
>> * Patch 4 is waiting for some more review and has some TODO
>> * discussion 5 worth few minutes to discuss before jumping on previous topics
>
> These other patches needs more discussions and hacking. They have a low
> priority compare to other discussions and running commitfest. However, how can
> avoid losing them in limbo again?
>
I think focusing on the backpatchability is not particularly good
approach. It's probably be better to fist check if there's any hope of
restarting the work on BNLJ, which seems like a "proper" solution for
the future.
It getting that soon (in PG17) is unlikely, let's revive the rebalance
and/or spilling patches. Imperfect but better than nothing.
And then in the end we can talk about if/what can be backpatched.
FWIW I don't think there's a lot of rush, considering this is clearly a
matter for PG17. So the summer CF at the earliest, people are going to
be busy until then.
regards
--
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
