Horton

Re: Fix performance of generic atomics

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Sokolov Yura <funny.falcon@postgrespro.ru>, Robert Haas <robertmhaas@gmail.com>, Simon Riggs <simon@2ndquadrant.com>, Jeff Janes <jeff.janes@gmail.com>, Jesper Pedersen <jesper.pedersen@redhat.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2017-09-06T19:19:05Z
Lists: pgsql-hackers 
On 2017-09-06 15:12:13 -0400, Tom Lane wrote:
> Andres Freund <andres@anarazel.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

Commits

  1. Further marginal hacking on generic atomic ops.

  2. Use more of gcc's __sync_fetch_and_xxx builtin functions for atomic ops.

  3. Remove duplicate reads from the inner loops in generic atomic ops.