Re: CPU costs of random_zipfian in pgbench

From: Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Georgios Kokolatos <gkokolatos(at)pm(dot)me>, PostgreSQL Developers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: CPU costs of random_zipfian in pgbench
Date: 2019-03-23 18:11:44
Message-ID: alpine.DEB.2.21.1903231910020.18811@lancre
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Hello again,

>> I started to look through this, and the more I looked the more unhappy
>> I got that we're having this discussion at all. The zipfian support
>> in pgbench is seriously over-engineered and under-documented. As an
>> example, I was flabbergasted to find out that the end-of-run summary
>> statistics now include this:
>> /* Report zipfian cache overflow */
>> for (i = 0; i < nthreads; i++)
>> {
>> totalCacheOverflows += threads[i].zipf_cache.overflowCount;
>> }
>> if (totalCacheOverflows > 0)
>> {
>> printf("zipfian cache array overflowed %d time(s)\n",
>> totalCacheOverflows);
>> }
>> What is the point of that, and if there is a point, why is it nowhere
>> mentioned in pgbench.sgml?

The attached patch simplifies the code by erroring on cache overflow,
instead of the LRU replacement strategy and unhelpful final report. The
above lines are removed.


Attachment Content-Type Size
pgbench-zipf-cache-simple-1.patch text/x-diff 3.6 KB

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