| From: | Andres Freund <andres(at)anarazel(dot)de> |
|---|---|
| To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
| Cc: | Dilip Kumar <dilipbalaut(at)gmail(dot)com>, Heikki Linnakangas <hlinnaka(at)iki(dot)fi>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: New WAL record to detect the checkpoint redo location |
| Date: | 2023-10-09 23:20:34 |
| Message-ID: | 20231009232034.6yk552kmlner4pxt@awork3.anarazel.de |
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| Lists: | pgsql-hackers |
Hi,
On 2023-10-09 18:31:11 -0400, Robert Haas wrote:
> On Mon, Oct 9, 2023 at 4:47 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> > I think we might be able to speed some of this up by pre-compute values so we
> > can implement things like bytesleft / UsableBytesInPage with shifts. IIRC we
> > already insist on power-of-two segment sizes, so instead of needing to divide
> > by a runtime value, we should be able to shift by a runtime value (and the
> > modulo should be a mask).
>
> Huh, is there a general technique for this when dividing by a
> non-power-of-two?
There is, but I was just having a brainfart, forgetting that UsableBytesInPage
isn't itself a power of two. The general technique is used by compilers, but
doesn't iirc lend itself well to be done at runtime.
Greetings,
Andres Freund
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