On 2012-11-16 22:31:51 -0500, Noah Misch wrote:
> On Fri, Nov 16, 2012 at 05:31:12PM +0100, Andres Freund wrote:
> > On 2012-11-16 13:17:47 -0300, Alvaro Herrera wrote:
> > > Andres is on the verge of convincing me that we need to support
> > > singleton FOR SHARE without multixacts due to performance concerns.
> >
> > I don't really see any arguments against doing so. We aren't in a that
> > big shortage of flags and if we need more than available I think we can
> > free some (e.g. XMAX/XMIN_INVALID).
>
> The patch currently leaves two unallocated bits. Reclaiming currently-used
> bits means a binary compatibility break. Until we plan out such a thing,
> reclaimable bits are not as handy as never-allocated bits. I don't think we
> should lightly expend one of the final two.
Not sure what you mean with a binary compatibility break?
pg_upgrade'ability?
What I previously suggested somewhere was:
#define HEAP_XMAX_SHR_LOCK 0x0010
#define HEAP_XMAX_EXCL_LOCK 0x0040
#define HEAP_XMAX_KEYSHR_LOCK (HEAP_XMAX_SHR_LOCK|HEAP_XMAX_EXCL_LOCK)
/** Different from _LOCK_BITS because it doesn't include LOCK_ONLY*/
#define HEAP_LOCK_MASK (HEAP_XMAX_SHR_LOCK|HEAP_XMAX_EXCL_LOCK)
#define HEAP_XMAX_IS_SHR_LOCKED(tup) \ (((tup)->t_infomask & HEAP_LOCK_BITS) == HEAP_XMAX_SHR_LOCK)
#define HEAP_XMAX_IS_EXCL_LOCKED(tup) \ (((tup)->t_infomask & HEAP_LOCK_BITS) == HEAP_XMAX_EXCL_LOCK)
#define HEAP_XMAX_IS_KEYSHR_LOCKED(tup) \ (((tup)->t_infomask & HEAP_LOCK_BITS) == HEAP_XMAX_KEYSHR_LOCK)
It makes checking for locks sightly more more complicated, but its not
too bad...
> > > It
> > > would be useful for more people to chime in here: is FOR SHARE an
> > > important case to cater for? I wonder if using FOR KEY SHARE (keep
> > > performance characteristics, but would need to revise application code)
> > > would satisfy Andres' users, for example.
> >
> > It definitely wouldn't help in the cases I have seen because the point
> > is to protect against actual content changes of the rows, not just the
> > keys.
> > Note that you actually need to use explicit FOR SHARE/UPDATE for
> > correctness purposes in many scenarios unless youre running in 9.1+
> > serializable mode. And that cannot be used in some cases (try queuing
> > for example) because the rollback rates would be excessive.
>
> I agree that tripling FOR SHARE cost is risky. Where is the added cost
> concentrated? Perchance that multiple belies optimization opportunities.
Good question, let me play a bit.
Greetings,
Andres Freund
--Andres Freund http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
Services
