[russellw]

It's not load related as I've resolved those issues however we are clearing the SQL cache every couple of hours and that will result in long queries for a few minutes while the cache rebuilds.

However, that is the lesser of evils as what was happening was that the system would run low on memory when getting a lot of queries and then it starts doing what are known as low memory prunes of the cache.

In simple terms, these are a bad thing as they cause all the memory addresses used by the query cache to be rebuilt and performance suffers - flushing the cache every couple of hours allows the cache to rebuild gracefully and has less of a performance impact.

We will also be doubling the primary memory in the server so that some of those thresholds can be raised in order to avoid this issue - or at least flush the cache less often.

When I first started the tuning exercise the low memory prunes were going up every hour so that by the early evening they were in the half million bracket whereas now they are in the low thousands and not increasing until late at night.

Cheers
Russ