On 12.01.2012 14:31, Simon Riggs wrote:
In order to simulate real-world clog contention, we need to use
benchmarks that deal with real world situations.
Currently, pgbench pre-loads data using COPY and executes a VACUUM so
that all hint bits are set on every row of every page of every table.
Thus, as pgbench runs it sees zero clog accesses from historical data.
As a result, clog access is minimised and the effects of clog
contention in the real world go unnoticed.
The following patch adds a pgbench option -I to load data using
INSERTs, so that we can begin benchmark testing with rows that have
large numbers of distinct un-hinted transaction ids. With a database
pre-created using this we will be better able to simulate and thus
more easily measure clog contention. Note that current clog has space
for 1 million xids, so a scale factor of greater than 10 is required
to really stress the clog.
No doubt this is handy for testing this particular area, but overall I
feel this is too much of a one-trick pony to include in pgbench.
Alternatively, you could do something like this:
do $$
declare
i int4;
naccounts int4;
begin
select count(*) into naccounts from pgbench_accounts;
for i in 1..naccounts loop
-- use a begin-exception block to create a new subtransaction
begin
update pgbench_accounts set abalance = abalance where aid = i;
exception
when division_by_zero then raise 'unexpected division by zero
error'; end;
end loop;
end;
$$;
after initializing the pgbench database, to assign distinct xmins to all
rows. Another idea would be to run pg_dump in --inserts mode, edit the
dump to remove BEGIN/COMMIT from it, and restore it back.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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