Commits on Feb 24, 2023

1. *: enables elastic CPU limiter for all users of ExportRequest …

   Previously, there was a strange coupling between the elastic CPU
   limiter and the `header.TargetBytes` DistSender limit set on each
   ExportRequest. Even if a request was preempted on exhausting its
   allotted CPU tokens, it would only return from kvserver by virtue
   of its `header.TargetBytes` being set to a non-zero value. Out of the
   four users of ExportRequest, only backup set this field to a sentinel
   value of 1 to limit the number of SSTs we send back in an ExportResponse.
   The remaining callers of ExportRequest would not return from the kvserver.
   Instead they would evaluate the request from the resume key immediately,
   not giving the scheduler a chance to take the goroutine off CPU.
   
   This change breaks this coupling by introducing a `resumeInfo` object that
   indicates whether the resumption was because we were over our CPU limit. If
   it was, we return an ExportResponse with our progress so far. This change
   shifts the burden of handling pagination to the client. This seems better than
   having the server sleep or wait around until its CPU tokens are replenished
   as the client would be left wondering why a request is taking so long.
   
   To that effect this change adds pagination support to the other callers of
   ExportRequest. Note, we do not set `SplitMidKey` at these other callsites
   yet. Thus, all pagination will happen at key boundaries in the ExportRequest.
   A follow-up will add support for `SplitMidKey` to these callers.
   
   Informs: cockroachdb#96684
   
   Release note: None

   

   adityamaru committed Feb 24, 2023
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