Gavin Sherry wrote:
On Mon, 5 Mar 2007, Mark Kirkwood wrote:

To add a little to this - forgetting the scan resistant point for the
moment... cranking down shared_buffers to be smaller than the L2 cache
seems to help *any* sequential scan immensely, even on quite modest HW:

When I've profiled this activity, I've seen a lot of time spent
searching for/allocating a new buffer for each page being fetched.
Obviously having less of them to search through will help, but having
less than the L2 cache-size worth of 'em seems to help a whole lot!

Could you demonstrate that point by showing us timings for shared_buffers
sizes from 512K up to, say, 2 MB? The two numbers you give there might
just have to do with managing a large buffer.

Yeah - good point:

PIII 1.26 Ghz 512Kb L2 cache 2G RAM

Test is elapsed time for: SELECT count(*) FROM lineitem

lineitem has 1535724 pages (11997 MB)

Shared Buffers  Elapsed  IO rate (from vmstat)
--------------  -------  ---------------------
400MB           101 s    122 MB/s

2MB             100 s
1MB              97 s
768KB            93 s
512KB            86 s
256KB            77 s
128KB            74 s    166 MB/s

I've added the observed IO rate for the two extreme cases (the rest can be pretty much deduced via interpolation).

Note that the system will do about 220 MB/s with the now (in)famous dd test, so we have a bit of headroom (not too bad for a PIII).



---------------------------(end of broadcast)---------------------------
TIP 9: In versions below 8.0, the planner will ignore your desire to
      choose an index scan if your joining column's datatypes do not

Reply via email to