Re: WIP Patch: Pgbench Serialization and deadlock errors

From: Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr>
To: Marina Polyakova <m(dot)polyakova(at)postgrespro(dot)ru>
Cc: PostgreSQL Developers <pgsql-hackers(at)postgresql(dot)org>, Kevin Grittner <kgrittn(at)gmail(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>
Subject: Re: WIP Patch: Pgbench Serialization and deadlock errors
Date: 2018-01-12 12:47:08
Message-ID: alpine.DEB.2.20.1801121309300.10810@lancre
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Hello Marina,

A partial answer, to focus on how the feature may be simplified.

I apologise as it seems that I overlooked one of your mail. Changing the
thread has not helped.

>> I'm wondering whether the whole feature could be simplified by
>> considering that one script is one "transaction" (it is from the
>> report point of view at least), and that any retry is for the full
>> script only, from its beginning. That would remove the trying to guess
>> at transactions begin or end, avoid scanning manually for subcommands,
>> and so on.
>> - Would it make sense?
>> - Would it be ok for your use case?
> I'm not sure if this makes sense: if we retry the script from the very
> beginning including successful transactions, there may be errors..

What I'm suggesting is to simplify the problem by adding some restrictions
to what kind of case which is handled, so as to simplify the code a lot.

I'd start by stating (i.e. documenting) that the features assumes that one
script is just *one* transaction.

Note that pgbench somehow already assumes that one script is one
transaction when it reports performance anyway.

If you want 2 transactions, then you have to put them in two scripts,
which looks fine with me. Different transactions are expected to be
independent, otherwise they should be merged into one transaction.

Under these restrictions, ISTM that a retry is something like:

if (we want to retry) {
// do necessary stats
// reset the initial state (random, vars, current command)
state = START_TX; // loop
else {
// count as failed...
state = FINISHED; // or done.

Once this works, maybe it could go a step further by restarting at
savepoints. I'd put restrictions there to ease detecting a savepoint so
that it cannot occur in a compound command for instance. This would
probably require a new state. Fine.

A detail:

>> For file contents, maybe the << 'EOF' here-document syntax would help
>> instead of using concatenated backslashed strings everywhere.
> Thanks, but I'm not sure that it is better to open file handlers for
> printing in variables..

Perl here-document stuff is just a multiline string, no file is involved,
it is different from the shell.


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