On 3/5/07 8:53 AM, "Tom Lane" <[EMAIL PROTECTED]> wrote:

> Hm, that seems to blow the "it's an L2 cache effect" theory out of the
> water.  If it were a cache effect then there should be a performance
> cliff at the point where the cache size is exceeded.  I see no such
> cliff, in fact the middle part of the curve is darn near a straight
> line on a log scale ...

Here's that cliff you were looking for:

Size of Orders table: 7178MB
Blocksize: 8KB

Shared_buffers  Select Count    Vacuum
(KB)            (s)             (s)
248             5.52            2.46
368             4.77            2.40
552             5.82            2.40
824             6.20            2.43
1232            5.60            3.59
1848            6.02            3.14
2768            5.53            4.56

All of these were run three times and the *lowest* time reported.  Also, the
behavior of "fast VACUUM after SELECT" begins abruptly at 1232KB of

These are Opterons with 2MB of L2 cache shared between two cores.

- Luke

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