Details
Major Description
When the page cache is large the performance gets lower as the page
cache is being filled. As soon as the page cache is filled, the
throughput increases. In the period with low performance, the CPU
usage is high, and when the performance increases the CPU usage is
lower.
This behaviour is caused by the algorithm for finding free slots in
the page cache. If there are invalid pages in the page cache, it will
be scanned to find one of those pages. However, when multiple clients
access the database, the invalid pages are often already taken. This
means that the entire page cache will be scanned, but no free invalid
page is found. Since the scan of the page cache is synchronized on the
cache manager, all other threads that want to access the page cache
have to wait. When the page cache is large, this will kill the
performance.
When the page cache is full, this is not a problem, as there will be
no invalid pages.
