Re: [HACKERS] Re: [PATCHES] Try again: S_LOCK reduced contention - Mailing list pgsql-hackers

>
> Bruce Momjian writes:
> > David Gould writes:
> > > Most of the original tas() __asm__() implementations are GCC specific. This
> > > includes all the Linux platforms except PPC, all the *BSD platforms, even the
> > > VAX. GCC is also fairly commonly used even on the commercial OSes.
> > >
> > > As far as I can tell, the only C coded platforms that are not GCC specific
> > > are SCO i386 and SunOS/Solaris on Sun3 and Sparc. The other non-GCC platforms
> > > have external tas.s function implementations (HP), or have system specific
> > > calls (AIX, OSF, SGI, Nextstep).
> >
> > That s_lock.h file is a hornets nest of portability problems.  I really
> > don't want to have multiple functions/macros for different CPU's if I
> > can help it.  I don't even want to mix functions/macros for the same
> > function name if I can help it.  I also do not want to start playing
> > around with isGNU/isnotGNU in a file that is already complex.
>
> Actually, my main motivation for this file is to reduce the portability
> problems. If you will look at the next patch (when I submit it, probably
> tonight) I think you will see that it is fairly clear what to do to port to
> a new platform, and how the existing platforms work.
>
> We already implicitly make a isGCC vs notGCC distinction when we use the
> GCC asm() syntax. I am merely intending to make it explict.

Ah, I see.  I wondered how other compilers were understanding the asm()
stuff.  I thought it was gcc-specific, but then other platforms were
using it.  I guess they have gcc.

>
> > Macros work and we already have tons of them, we don't use inline
> > anywhere else, and the actual locks are 80% asm code anyway, so it looks
> > the same whether it is in a macro or an inline function.
> >
> > I have made them macros before for this file, so I can do it again quite
> > easily.
> >
> > As for the benefits, well, when I see lots of calls to a function, and I
> > try and eliminate the calls if it is reasonable.  In many places, the
> > call handling is actually more instructions than the inlining.  I look
> > at the measured performance change vs. the executable size increase and
> > make a decision. With something like s_lock, it just seems normal to
> > make it a macro.
>
> With the old S_LOCK this was a reasonable choice. With the new S_LOCK which
> is quite a bit more complex, the macro expansion generates quite a bit of
> code. See the generated code for the "MacroSLOCK" case in my large post.

Yes, I suspected that may be a problem.  I will apply your patch as soon
as I see it.

>
> > > Finally, the difference between a tas() function implementation and the best
> > > possible inline implementation appears to be only 0.06 microseconds on a P133.
> > > This will add 0.0003 seconds to startup. On SCO only. On Sparc this is a leaf
> > > call and possibly even cheaper. No other platforms are affected.
> > >
> > > Remember also that I am adding two features that previously did not exist,
> > > backoff, and stuck lock detection.
> >
> > Yes, and good improvements.
>
> Again, please have a look at the (forthcoming) patch. It gives up nothing in
> either space or time performance compared to the original, is clearer imho,
> and incorporates the the new features.

Sounds like a plan.

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