Invalidating the cache

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Whenever you deal with caching, you have to eventually deal with the challenge of cache invalidation, or content update.

Varnish has different mechanisms to addresses this challenge, but which one to use?

The WBINVD instruction is a privileged instruction.When the processor is running in protected mode, the CPL of a program or procedure must be 0 to execute this instruction.This instruction is also a serializing instruction (see “Serializing Instructions” in Chapter 8 of the ).This might impact CPU usage and thereby performance. The first case is efficient only if you know that the cached objects to be banned are frequently accessed.Otherwise, you might accumulate a lot of ban expressions in the ban-list that are never checked.

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