On Mon, Oct 25, 2004 at 05:53:25PM -0400, Tom Lane wrote: > Greg Stark <[EMAIL PROTECTED]> writes: > > So I would suggest using something like 100us as the threshold for > > determining whether a buffer fetch came from cache. > > I see no reason to hardwire such a number. On any hardware, the > distribution is going to be double-humped, and it will be pretty easy to > determine a cutoff after minimal accumulation of data. The real question > is whether we can afford a pair of gettimeofday() calls per read(). > This isn't a big issue if the read actually results in I/O, but if it > doesn't, the percentage overhead could be significant. > How invasive would reading the "CPU counter" be, if it is available? A read operation should avoid flushing a cache line and we can throw out the obvious outliers since we only need an estimate and not the actual value.
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