On Mon, 8 Aug 2005, Tom Lane wrote:
What that sounds like to me is a machine with inadequate disk I/O bandwidth.
Your earlier comment that checkpoint drives the machine into the ground
fits right into that theory, too.  You said there is "almost no IO-wait"
but are you sure you are measuring that correctly?


Currently there's some iowait caused by "fragmentation" of the comment table. Periodic clustering helps a lot.

Disk configurations looks something like this:
  sda: data (10 spindles, raid10)
  sdb: xlog & clog (2 spindles, raid1)
  sdc: os and other stuff

Usually iostat (2 second interval) says:
  avg-cpu:  %user   %nice    %sys %iowait   %idle
            32.38    0.00   12.88   11.62   43.12

  Device:            tps    kB_read/s    kB_wrtn/s    kB_read    kB_wrtn
  sda             202.00      1720.00         0.00       3440          0
  sdb             152.50         4.00      2724.00          8       5448
  sdc               0.00         0.00         0.00          0          0

And during checkpoint:
  avg-cpu:  %user   %nice    %sys %iowait   %idle
            31.25    0.00   14.75   54.00    0.00

  Device:            tps    kB_read/s    kB_wrtn/s    kB_read    kB_wrtn
  sda            3225.50      1562.00     35144.00       3124      70288
  sdb             104.50        10.00      2348.00         20       4696
  sdc               0.00         0.00         0.00          0          0

I think (insufficiency of) disk IO shouldn't cause those lingering queries because dataset is rather small and it's continuously accessed. It should fit into cache and stay there(?)

400 queries?  Are you launching 400 separate backends to do that?

Well yes. That's the common problem with php and persistent connections.

Some sort of connection pooling seems like a good idea, if you don't
have it in place already.

pg_pool for example? I'm planning to give it a try.

                        regards, tom lane


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