On Mon, May 4, 2015 at 12:42 PM, Thomas Munro <thomas.munro@enterprisedb.com>
wrote:
>
> On Mon, May 4, 2015 at 6:25 PM, Amit Kapila <amit.kapila16@gmail.com>
wrote:
> > [...]
> > One more thing, I think the current calculation considers members
> > usage, shouldn't we try to consider offset usage as well?
>
> Offsets are indexed by multixact ID:
>
> #define MultiXactIdToOffsetPage(xid) \
> ((xid) / (MultiXactOffset) MULTIXACT_OFFSETS_PER_PAGE)
> #define MultiXactIdToOffsetEntry(xid) \
> ((xid) % (MultiXactOffset) MULTIXACT_OFFSETS_PER_PAGE)
>
> The existing multixact wraparound prevention code is already managing
> the 32 bit multixact ID space. The problem with members comes about
> because each one of those multixact IDs can have arbitrary numbers of
> members, and yet the members are also addressed with a 32 bit index.
> So we are trying to hijack the multixact ID wraparound prevention and
> make it more aggressive if member space appears to be running out.
> (Perhaps in future there should be a 64 bit index for member indexes
> so that this problem disappears?)
>
Okay, that makes sense.
> >> (whereas when we used a scaled down
> >> autovaccum_multixact_freeze_max_age, we usually didn't select any
> >> tables at all until we scaled it down a lot, ie until we got close to
> >> dangerous_member_count). Finally, I wanted a special value like -1
> >> for 'none' so that table_recheck_autovac and ExecVacuum could use a
> >> simple test >= 0 to know that they also need to set
> >> multixact_freeze_min_age to zero in the case of a
> >> member-space-triggered vacuum, so that we get maximum benefit from our
> >> table scans by freezing all relevant tuples, not just some older ones
> >>
> >
> > I think setting multixact_freeze_min_age to zero could be too aggresive
> > for I/O. Yes with this you can get maximum benefit, but at cost of
> > increased I/O. How would you justify setting it to zero as appropriate
> > w.r.t increased I/O?
>
> I assumed that if you were already vacuuming all your tablesto avoid
> running out of member space,
I think here you mean all tables that has relminmxid lesser than the
newly computed age (compute_max_multixact_age_to_avoid_member_wrap)
> you would want to freeze any tuples you
> possibly could to defer the next wraparound scan for as long as
> possible, since wraparound scans are enormously expensive.
>
The point is valid to an extent, but If we go by this logic, then currently
also we should set multixact_freeze_min_age as zero for wraparound
vacuum.
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com