Re: Dynamic gathering the values for seq_page_cost/xxx_cost

From: Julien Rouhaud <rjuju123(at)gmail(dot)com>
To: Andy Fan <zhihui(dot)fan1213(at)gmail(dot)com>
Cc: Ashutosh Bapat <ashutosh(dot)bapat(dot)oss(at)gmail(dot)com>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: Dynamic gathering the values for seq_page_cost/xxx_cost
Date: 2020-09-26 05:51:37
Message-ID: CAOBaU_YWB-j9e0Y=dLpPJhxMTM44yZgN3TJ_VZE1U5n3JAsSMw@mail.gmail.com
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On Sat, Sep 26, 2020 at 8:17 AM Andy Fan <zhihui(dot)fan1213(at)gmail(dot)com> wrote:
>
> As for the testing with cache considered, I found how to estimate cache hit
> ratio is hard or how to control a hit ratio to test is hard. Recently I am thinking
> a method that we can get a page_reads, shared_buffer_hit from pg_kernel
> and the real io (without the file system cache hit) at os level (just as what
> iotop/pidstat do). then we can know the shared_buffer hit ratio and file system
> cache hit ratio (assume it will be stable after a long run). and then do a testing.
> However this would be another branch of manual work and I still have not got
> it done until now.

FWIW pg_stat_kcache [1] extension accumulates per (database, user,
queryid) physical reads and writes, so you can easily compute a
shared_buffers / IO cache / disk hit ratio.

[1] https://github.com/powa-team/pg_stat_kcache

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