Re: Scaling shared buffer eviction

On 2014-09-12 12:38:48 +0300, Ants Aasma wrote:
> On Thu, Sep 11, 2014 at 4:22 PM, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
> >> > Hm. Perhaps we should do a bufHdr->refcount != zero check without
> >> > locking here? The atomic op will transfer the cacheline exclusively to
> >> > the reclaimer's CPU. Even though it very shortly afterwards will be
> >> > touched afterwards by the pinning backend.
> >>
> >> Meh. I'm not in favor of adding more funny games with locking unless
> >> we can prove they're necessary for performance.
> >
> > Well, this in theory increases the number of processes touching buffer
> > headers regularly. Currently, if you have one read IO intensive backend,
> > there's pretty much only process touching the cachelines. This will make
> > it two. I don't think it's unreasonable to try to reduce the cacheline
> > pingpong caused by that...
>
> I don't think it will help much. A pinned buffer is pretty likely to
> be in modified state in the cache of the cpu of the pinning backend.

Right. Unless you're on a MOESI platforms. I'd really like to know why
that's not more widely used.

> Even taking a look at the refcount will trigger a writeback and
> demotion to shared state. When time comes to unpin the buffer the
> cacheline must again be promoted to exclusive state introducing
> coherency traffic. Not locking the buffer only saves transfering the
> cacheline back to the pinning backend, not a huge amount of savings.

Yes. But: In many, if not most, cases the cacheline will be read a
couple times before modifying it via the spinlock.

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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