From: Nathan Bossart <nathandbossart@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Thomas Munro <thomas.munro@gmail.com>
Cc: Hayato Kuroda (Fujitsu) <kuroda.hayato@fujitsu.com>
Cc: pgsql-hackers@postgresql.org <pgsql-hackers@postgresql.org>
Subject: Re: suppressing useless wakeups in logical/worker.c
Date: Thu, 26 Jan 2023 11:48:12 -0800
Message-ID: <20230126194812.GA1702315@nathanxps13> (raw)
In-Reply-To: <3126727.1674759248@sss.pgh.pa.us>
References: <20221205173523.GA2714833@nathanxps13>
<20230109174217.GA1112698@nathanxps13>
<2557815.1674603908@sss.pgh.pa.us>
<20230125235004.GA1327755@nathanxps13>
<CA+hUKGJ=oFEX=TjeSCnGmqr_KseGLJxy=RnJMwRqPrX7N-Q2sg@mail.gmail.com>
<20230126003319.GA1507251@nathanxps13>
<2878152.1674700077@sss.pgh.pa.us>
<CA+hUKGLpH=y4=Zx+=oLE8WvLqLHWNC-PEDARrrt=x=J_REL9Fg@mail.gmail.com>
<2888102.1674705840@sss.pgh.pa.us>
<3126727.1674759248@sss.pgh.pa.us>
On Thu, Jan 26, 2023 at 01:54:08PM -0500, Tom Lane wrote:
> After looking closer, I see that TimestampDifferenceMilliseconds
> already explicitly states that its output is intended for WaitLatch
> and friends, which makes it perfectly sane for it to clamp the result
> to [0, INT_MAX] rather than depending on the caller to not pass
> out-of-range values.
+1
> * This is typically used to calculate a wait timeout for WaitLatch()
> * or a related function. The choice of "long" as the result type
> - * is to harmonize with that. It is caller's responsibility that the
> - * input timestamps not be so far apart as to risk overflow of "long"
> - * (which'd happen at about 25 days on machines with 32-bit "long").
> + * is to harmonize with that; furthermore, we clamp the result to at most
> + * INT_MAX milliseconds, because that's all that WaitLatch() allows.
> *
> - * Both inputs must be ordinary finite timestamps (in current usage,
> - * they'll be results from GetCurrentTimestamp()).
> + * At least one input must be an ordinary finite timestamp, else the "diff"
> + * calculation might overflow. We do support stop_time == TIMESTAMP_INFINITY,
> + * which will result in INT_MAX wait time.
I wonder if we should explicitly reject negative timestamps to eliminate
any chance of int64 overflow, too. Alternatively, we could detect that the
operation will overflow and return either 0 or INT_MAX, but I assume
there's minimal use of this function with negative timestamps.
--
Nathan Bossart
Amazon Web Services: https://aws.amazon.com
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-hackers@postgresql.org
Cc: nathandbossart@gmail.com, tgl@sss.pgh.pa.us, thomas.munro@gmail.com, kuroda.hayato@fujitsu.com
Subject: Re: suppressing useless wakeups in logical/worker.c
In-Reply-To: <20230126194812.GA1702315@nathanxps13>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox