Re: spinlocks on HP-UX

On Sun, Aug 28, 2011 at 11:35 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> First, I did this:
>
>> -#define TAS(lock) _Asm_xchg(_SZ_W, lock, 1, _LDHINT_NONE)
>> +#define TAS(lock) (*(lock) ? 1 : _Asm_xchg(_SZ_W, lock, 1, _LDHINT_NONE))
>
> Seems reasonable, and similar to x86 logic.
>
>> Then, I did this:
>
>> -       while (TAS(lock))
>> +       while (*lock ? 1 : TAS(lock))
>
> Er, what?  That sure looks like a manual application of what you'd
> already done in the TAS macro.

Sorry, I blew through that a little too blithely. If you change TAS()
itself, then even the very first attempt to acquire the lock will try
the unlocked instruction first, whereas changing s_lock() allows you
to do something different in the contended case than you do in the
uncontended case. We COULD just change the TAS() macro since, in this
case, it seems to make only a minor difference, but what I was
thinking is that we could change s_lock.h to define two macros, TAS()
and TAS_SPIN(). If a particular architecture defines TAS() but not
TAS_SPIN(), then we define TAS_SPIN(x) to be TAS(x). Then, S_LOCK()
can stay as-is - calling TAS() - but s_lock() can call TAS_SPIN(),
which will normally be the same as TAS() but can be made different on
any architecture where the retry loop should do something different
than the initial attempt.

> Please clarify: when you say "this architecture", are you talking about
> IA64 or PA-RISC?  Is there any reason to think that this is specific to
> HP-UX rather than any other system on the same architecture?  (I'm sure
> I can get access to some IA64 clusters at Red Hat, though maybe not
> 64-core ones.)

I tested on IA64; I don't currently have access to a PA-RISC box. The
documentation I'm looking at implies that the same approach would be
desirable there, but that's just an unsubstantiated rumor at this
point....

--
Robert Haas
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