On 08/26/2014 12:03 PM, Jeff Davis wrote:
On Fri, 2014-06-13 at 13:24 +0300, Heikki Linnakangas wrote:
Attached is a new patch. It now keeps the current pg_clog unchanged, but
adds a new pg_csnlog besides it. pg_csnlog is more similar to
pg_subtrans than pg_clog: it's not WAL-logged, is reset at startup, and
segments older than GlobalXmin can be truncated.
It appears that this patch weakens the idea of hint bits. Even if
HEAP_XMIN_COMMITTED is set, it still needs to find out if it's in the
snapshot.
That's fast if the xid is less than snap->xmin, but otherwise it needs
to do a fetch from the csnlog, which is exactly what the hint bits are
designed to avoid. And we can't get around this, because the whole point
of this patch is to remove the xip array from the snapshot.
Yeah. This patch in the current state is likely much much slower than
unpatched master, except in extreme cases where you have thousands of
connections and short transactions so that without the patch, you spend
most of the time acquiring snapshots.
My current thinking is that access to the csnlog will need to be made
faster. Currently, it's just another SLRU, but I'm sure we can do better
than that. Or add a backend-private cache on top of it; that might
already alleviate it enough..
- Heikki
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