| From: | Andres Freund <andres(at)anarazel(dot)de> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Sokolov Yura <funny(dot)falcon(at)postgrespro(dot)ru>, Robert Haas <robertmhaas(at)gmail(dot)com>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Jeff Janes <jeff(dot)janes(at)gmail(dot)com>, Jesper Pedersen <jesper(dot)pedersen(at)redhat(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Fix performance of generic atomics |
| Date: | 2017-09-06 19:19:05 |
| Message-ID: | 20170906191905.cmg5g6rbzwfxlanb@alap3.anarazel.de |
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| Lists: | pgsql-hackers |
On 2017-09-06 15:12:13 -0400, Tom Lane wrote:
> Andres Freund <andres(at)anarazel(dot)de> writes:
> > On 2017-09-06 14:31:26 -0400, Tom Lane wrote:
> >> However, if that's the reasoning, why don't we make all of these
> >> use simple reads? It seems unlikely that a locked read is free.
>
> > We don't really use locked reads? All the _atomic_ wrapper forces is an
> > actual read from memory rather than a register.
>
> It looks to me like two of the three implementations promise no such
> thing.
They're volatile vars, so why not?
> Even if they somehow do, it hardly matters given that the cmpxchg loop
> would be self-correcting.
Well, in this one instance maybe, hardly in others.
> Mostly, though, I'm looking at the fallback pg_atomic_read_u64_impl
> implementation (with a CAS), which seems far more expensive than can
> be justified for this.
What are you suggesting as an alternative?
- Andres
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