FAQ
Ants,

the more I think about this, the more I start to like it.
On 06/07/2013 02:50 PM, Ants Aasma wrote:
On Fri, Jun 7, 2013 at 2:59 PM, Markus Wanner wrote:
Agreed. Postgres-R uses a CommitOrderId, which is very similar in
concept, for example.
Do you think having this snapshot scheme would be helpful for Postgres-R?
Yeah, it could help to reduce patch size, after a rewrite to use such a CSN.
Or why do you need to tell apart aborted from in-progress transactions
by CSN?
I need to detect aborted transactions so they can be discared during
the eviction process, otherwise the sparse array will fill up. They
could also be filtered out by cross-referencing uncommitted slots with
the procarray. Having the abort case do some additional work to make
xid assigment cheaper looks like a good tradeoff.
I see.
Sparse buffer needs to be at least big enough to fit CSN slots for the
xids of all active transactions and non-overflowed subtransactions. At
the current level PGPROC_MAX_CACHED_SUBXIDS=64, the minimum comes out
at 16 bytes * (64 + 1) slots * 100 = backends = 101.6KB per buffer,
or 203KB total in the default configuration.
A CSN is 8 bytes, the XID 4, resulting in 12 bytes per slot. So I guess
the given 16 bytes includes alignment to 8 byte boundaries. Sounds good.
8 byte alignment for CSNs is needed for atomic if not something else.
Oh, right, atomic writes.
I think the size could be cut in half by using a base value for CSNs
if we assume that no xid is active for longer than 2B transactions as
is currently the case. I didn't want to include the complication in
the first iteration, so I didn't verify if that would have any
gotchas.
In Postgres-R, I effectively used a 32-bit order id which wraps around.

In this case, I guess adjusting the base value will get tricky. Wrapping
could probably be used as well, instead.
The number of times each cache line can be invalidated is
bounded by 8.
Hm.. good point.

Regards

Markus Wanner

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