Erik Jones <erik 'at' myemma.com> writes: >> vacuum_cost_delay/vacuum_cost_limit (deactivated) 20/200 >> 40/200 100/1000 150/1000 200/1000 300/1000 >> >> VACUUM ANALYZE time 54 s 112 s 188 >> s 109 s 152 s 190 s 274 s >> SELECT time 50 s 28 s 26 >> s 24 s 22 s 20 s 19 s > > While you do mention that the table you're running your select on is > too big to fit in the shared_buffers, the drop in time between the > first run and the rest most likely still reflects the fact that when These figures don't show a difference between first run and subsequent runs. For each parameter tuning, a couple of runs are fired after database restart, and once the value is approximately constant, it's picked and put in this table. The "deactivated" shows the (stable, from subsequent runs) figure when vacuum delay is disabled (vacuum_cost_delay parameter quoted), not the first run, if that's where the confusion came from. > running those tests successively a good portion of the table will > already be in shared_buffers as well as being in the filesystem > cache, i.e. very little of the runs after the first will have to hit A dd sized at the total RAM size is run between each test (not between each parameter tuning, between each *query test*), to remove the OS disk cache effect. Of course, the PostgreSQL caching effect cannot be removed (maybe, it shouldn't, as after all this caching is here to improve performance), but the query is selected to generate a lot of disk activity even between each run (that's why I said "a query which cannot fit entirely in buffers"). > the disk much. I have of course checked that the subsequent runs mean essentially disk activity, not CPU activity. >> - alternatively, pg could make use of some longer term statistics >> (load average, IO statistics) to dynamically pause the >> vacuuming - this I guess is related to the host OS and probably >> more difficult to have working correctly with multiple disks >> and/or processes running - however, if you want high >> performance from PostgreSQL, you probably won't host other IO >> applications on the same disk(s) > > These ideas have been discussed much. Look in the archives to the > beginning of this year. I think the general consensus was that it Is it on pgsql-hackers? I haven't found much stuff in pgsql-performance while looking for "vacuum_cost_delay tuning". > would be good have multiple autovacuum workers that could be tuned > for different times or workloads. I know Alvarro was going to work Sounds interesting. >> I'm wondering if it would not be possible to dynamically ignore >> (or lower, if it makes more sense?) the Cost-Based Vacuum Delay >> during vacuum full, if a configurable amount of queries are >> waiting for the lock? >> >> (please save yourself from answering "you should never run VACUUM >> FULL if you're vacuuming enough" - as long as VACUUM FULL is >> available in PostgreSQL, there's no reason to not make it as >> practically usable as possible, albeit with low dev priority) > > Ok, I won't say what you said not to say. But, I will say that I > don't agree with you're conjecture that VACUUM FULL should be made > more lightweight, it's like using dynamite to knock a whole in a wall > for a window. Thanks for opening a new kind of trol^Hargument against VACUUM FULL, that one's more fresh (at least to me, who doesn't follow the list too close anyway). Just for the record, I inherited a poorly (actually, "not" would be more appropriate) tuned database, containing more than 90% of dead tuples on large tables, and I witnessed quite some performance improvement while I could fix that. -- Guillaume Cottenceau, MNC Mobile News Channel SA, an Alcatel-Lucent Company Av. de la Gare 10, 1003 Lausanne, Switzerland - direct +41 21 317 50 36 ---------------------------(end of broadcast)--------------------------- TIP 6: explain analyze is your friend
