Subject: Re: PATCH: logical_work_mem and logical streaming of large
 in-progress transactions
To: Peter Eisentraut <peter.eisentraut@2ndquadrant.com>,
 Greg Stark <stark@mit.edu>
Cc: PostgreSQL Hackers <pgsql-hackers@postgresql.org>
References: <688b0b7f-2f6c-d827-c27b-216a8e3ea700@2ndquadrant.com>
 <ec3fbad0-06f6-08cb-7f0e-edd3fb0c2785@2ndquadrant.com>
 <CAM-w4HNWQMnE8j9=Fa1QdwfKYs=XQanx03PL23yULbh1hPYSGQ@mail.gmail.com>
 <5d00346a-b565-7fde-c6da-35f39c72936a@2ndquadrant.com>
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Message-ID: <d72caaf1-a2da-081c-57ea-d30e5f47fd1e@2ndquadrant.com>
Date: Sat, 13 Jan 2018 05:19:27 +0100
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On 01/12/2018 05:35 PM, Peter Eisentraut wrote:
> On 1/11/18 18:23, Greg Stark wrote:
>> AIUI spilling to disk doesn't affect absorbing future updates, we
>> would just keep accumulating them in memory right? We won't need to
>> unspill until it comes time to commit.
> 
> Once a transaction has been serialized, future updates keep accumulating
> in memory, until perhaps it gets serialized again.  But then at commit
> time, if a transaction has been partially serialized at all, all the
> remaining changes are also serialized before the whole thing is read
> back in (see reorderbuffer.c line 855).
> 
> So one optimization would be to specially keep track of all transactions
> that have been serialized already and pick those first for further
> serialization, because it will be done eventually anyway.
> 
> But this is only a secondary optimization, because it doesn't help in
> the extreme cases that either no (or few) transactions have been
> serialized or all (or most) transactions have been serialized.
> 
>> The real aim should be to try to pick the transaction that will be
>> committed furthest in the future. That gives you the most memory to
>> use for live transactions for the longest time and could let you
>> process the maximum amount of transactions without spilling them. So
>> either the oldest transaction (in the expectation that it's been open
>> a while and appears to be a long-lived batch job that will stay open
>> for a long time) or the youngest transaction (in the expectation that
>> all transactions are more or less equally long-lived) might make
>> sense.
> 
> Yes, that makes sense.  We'd still need to keep a separate ordered list
> of transactions somewhere, but that might be easier if we just order
> them in the order we see them.
> 

Wouldn't the 'toplevel_by_lsn' be suitable for this? Subtransactions
don't really commit independently, but as part of the toplevel xact. And
that list is ordered by LSN, which is pretty much exactly the order in
which we see the transactions.

I feel somewhat uncomfortable about evicting oldest (or youngest)
transactions for based on some assumed correlation with the commit
order. I'm pretty sure that will bite us badly for some workloads.

Another somewhat non-intuitive detail is that because ReorderBuffer
switched to Generation allocator for changes (which usually represent
99% of the memory used during decoding), it does not reuse memory the
way AllocSet does. Actually, it does not reuse memory at all, aiming to
eventually give the memory back to libc (which AllocSet can't do).

Because of this evicting the youngest transactions seems like a quite
bad idea, because those chunks will not be reused and there may be other
chunks on the blocks, preventing their release.

Yeah, complicated stuff.

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services