On 02/03/2016 06:46 AM, Noah Misch wrote:
On Mon, Feb 01, 2016 at 07:03:45PM +0100, Tomas Vondra wrote:
On 12/22/2015 03:49 PM, Noah Misch wrote:
If the collector entered pgstat_write_statsfile() with more
inquiries waiting in its socket receive buffer, it would ignore
them as being too old once it finished the write and resumed
message processing. Commit 187492b converted last_statrequest to
a "last_statrequests" list that we wipe after each write.

So essentially we remove the list of requests, and thus on the next
round we don't know the timestamp of the last request and write the
file again unnecessarily. Do I get that right?

Essentially right. Specifically, for each database, we must remember
the globalStats.stats_timestamp of the most recent write. It could be
okay to forget the last request timestamp. (I now doubt I picked the
best lines to quote, above.)

What if we instead kept the list but marked the requests as
'invalid' so that we know the timestamp? In that case we'd be able
to do pretty much exactly what the original code did (but at per-db

The most natural translation of the old code would be to add a
write_time field to struct DBWriteRequest. One can infer "invalid"
from write_time and request_time. There are other reasonable designs,

OK, makes sense. I'll look into that.

We'd have to cleanup the list once in a while not to grow
excessively large, but something like removing entries older than
PGSTAT_STAT_INTERVAL should be enough.

Specifically, if you assume the socket delivers messages in the order
sent, you may as well discard entries having write_time at least
PGSTAT_STAT_INTERVAL older than the most recent cutoff_time seen in a
PgStat_MsgInquiry. That delivery order assumption does not hold in
general, but I expect it's close enough for this purpose.

Agreed. If I get that right, it might result in some false negatives (in the sense that we'll remove a record too early, forcing us to write the database file again). But I expect that to be a rare case.


Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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