Re: [HACKERS] WIP aPatch: Pgbench Serialization and deadlock errors

From: Tatsuo Ishii <ishii(at)sraoss(dot)co(dot)jp>
To: nagata(at)sraoss(dot)co(dot)jp
Cc: coelho(at)cri(dot)ensmp(dot)fr, thomas(dot)munro(at)gmail(dot)com, m(dot)polyakova(at)postgrespro(dot)ru, alvherre(at)2ndquadrant(dot)com, pgsql-hackers(at)postgresql(dot)org, teodor(at)sigaev(dot)ru
Subject: Re: [HACKERS] WIP aPatch: Pgbench Serialization and deadlock errors
Date: 2022-03-20 07:11:43
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> Hi Yugo,
> I tested with serialization error scenario by setting:
> default_transaction_isolation = 'repeatable read'
> The result was:
> $ pgbench -t 10 -c 10 --max-tries=10 test
> transaction type: <builtin: TPC-B (sort of)>
> scaling factor: 10
> query mode: simple
> number of clients: 10
> number of threads: 1
> maximum number of tries: 10
> number of transactions per client: 10
> number of transactions actually processed: 100/100
> number of failed transactions: 0 (0.000%)
> number of transactions retried: 35 (35.000%)
> total number of retries: 74
> latency average = 5.306 ms
> initial connection time = 15.575 ms
> tps = 1884.516810 (without initial connection time)
> I had hard time to understand what those numbers mean:
> number of transactions retried: 35 (35.000%)
> total number of retries: 74
> It seems "total number of retries" matches with the number of ERRORs
> reported in PostgreSQL. Good. What I am not sure is "number of
> transactions retried". What does this mean?

Oh, ok. I see it now. It turned out that "number of transactions
retried" does not actually means the number of transactions
rtried. Suppose pgbench exectutes following in a session:

BEGIN; -- transaction A starts
ROLLBACK; -- transaction A aborts


BEGIN; -- transaction B starts
ROLLBACK; -- transaction B aborts


BEGIN; -- transaction C starts
END; -- finally succeeds

In this case "total number of retries:" = 2 and "number of
transactions retried:" = 1. In this patch transactions A, B and C are
regarded as "same" transaction, so the retried transaction count
becomes 1. But it's confusing to use the language "transaction" here
because A, B and C are different transactions. I would think it's
better to use different language instead of "transaction", something
like "cycle"? i.e.

number of cycles retried: 35 (35.000%)

Best reagards,
Tatsuo Ishii
SRA OSS, Inc. Japan

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