Tom Lane <tgl@sss.pgh.pa.us> writes:
> In principle you should be able to adjust the constant so that vmstat
> shows about 50% CPU busy, and then enabling fadvise should improve
> matters significantly.
I think in practice individual queries don't interleave much cpu with i/o
work. A single random page fetch is 5ms which is an awful lot of cpu cycles to
be sinking somewhere. In practice I think this is going to be single-digit
percentages.
Aside from big sorts and such which tend not to be interleaved with i/o the
main time I've seen queries have a significant cpu load is when the data is
mostly in cache. In which case prefetching would be hard pressed to help at
all.
We could construct a synthetic case but the main point of this feature is to
make use of raid arrays that are currently going idle, not to pick up a few
percentage points for single spindle systems.
-- Gregory Stark EnterpriseDB http://www.enterprisedb.com Ask me about EnterpriseDB's On-Demand Production
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