Re: sinval synchronization considered harmful

On Thu, Jul 21, 2011 at 2:50 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> ... On these machines, you need to issue an explicit memory barrier
>> instruction at each sequence point, or just acquire and release a
>> spinlock.
>
> Right, and the reason that a spinlock fixes it is that we have memory
> barrier instructions built into the spinlock code sequences on machines
> where it matters.
>
> To get to the point where we could do the sort of optimization Robert
> is talking about, someone will have to build suitable primitives for
> all the platforms we support.  In the cases where we use gcc ASM in
> s_lock.h, it shouldn't be too hard to pull out the barrier
> instruction(s) ... but on platforms where we rely on OS-supplied
> functions, some research is going to be needed.

Yeah, although falling back to SpinLockAcquire() and SpinLockRelease()
on a backend-private slock_t should work anywhere that PostgreSQL
works at all[1]. That will probably be slower than a memory fence
instruction and certainly slower than a compiler barrier, but the
point is that - right now - we're doing it the slow way everywhere.

I think the real challenge is going to be testing. If anyone has a
machine with weak memory ordering they can give me access to, that
would be really helpful for flushing the bugs out of this stuff.
Getting it to work on x86 is not the hard part.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

[1] This was a suggestion from Noah Misch. I wasn't quite convinced
when he initially made it, but having studied the issue a lot more, I
now am. The CPU doesn't know how many processes have the memory
mapped into their address space.