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Summary: In previous discussions, it has been pointed out that `folly::RequestContext::setContext` consumes considerable amount of CPU cycles in production environments. After some investigation, we thought that might be caused by looping over all `RequestData` instances can calling the virtual `onSet` and `onUnset` callback of them. Both the iteration and invoking virtual methods are not cheap. As you can see from this change, most of the derived classes of `RequestData` don't override the `onSet` and `onUnset` methods. Mostly likely they are only used for per-Request tracking. So the natural idea is to skip those instances when iterating and avoid the unnecessary virtual method invoke. I have explored the solution to dynamically examine if the `RequestData` instance added has `onSet` and `onUnset` method overridden. That is possible with GCC's PMF extension, but not [[ http://lists.llvm.org/pipermail/llvm-bugs/2015-July/041164.html | for Clang ]] and probably many other compilers. This definitely won't be very good for `folly`'s probability, even if we gate it by compiler flags. Therefore, this Diff adds the `hasCallback` method to `RequestData` class indicating whether the instance would have `onSet` and `onUnset` overridden. To make it clear to users that they need to correctly override it in order for their `onSet` and `onUnset` callback to work, making it abstract so that user must override it to something and would aware of that. Also made some improvements on documentation. Reviewed By: myreg Differential Revision: D6144049 fbshipit-source-id: 4c9fd72e9efaeb6763d55f63760eaf582ee4839e |
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