From: | Andres Freund <andres(at)anarazel(dot)de> |
---|---|
To: | Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> |
Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Petr Jelinek <petr(dot)jelinek(at)2ndquadrant(dot)com>, Petr Jelinek <petr(at)2ndquadrant(dot)com>, John Gorman <johngorman2(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: PATCH: two slab-like memory allocators |
Date: | 2017-03-06 22:42:00 |
Message-ID: | 20170306224200.6eanapty52itdfj6@alap3.anarazel.de |
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Lists: | pgsql-hackers |
Hi,
On 2017-03-06 23:32:30 +0100, Tomas Vondra wrote:
> > > The question however is whether this won't make the optimization pointless.
> > > I also, wonder how much we save by this optimization and how widely it's
> > > used? Can someone point me to some numbers?
> >
> > I don't recall any recent numbers. I'm more than a bit doubful that it
> > really matters - it's only used for the results of aggregate/window
> > functions, and surely they've a good chunk of their own overhead...
> >
>
> And if the benefit is negligible, trying to keep the optimization might
> easily result in slowdown (compared to non-optimized code).
I doubt the branch is noticeable here, given that we're doing a memory
allocation otherwise. Should also be decently predictable.
> But I'm puzzled why we haven't seen any reports of failures? I mean, doing
> sum(int4) is not particularly extravagant thing, if there really is an
> issue, shouldn't we see a lot of reports? What are we missing?
Reports about what? False positives causing crashes / wrong results? I
think it's quite unlikely to actually trigger this in practice, because
you need a properly aligned pointer, and then the preceding header has
to to point to a bit pattern that's equal to the context - that's
presumably quite unlikely in practice.
Regards,
Andres
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