Commits on Sep 19, 2022

1. runtime: export total GC Assist ns in MemStats and GCStats …

   At a high level, the runtime garbage collector can impact user goroutine
   latency in two ways. The first is that it pauses all goroutines during
   its stop-the-world sweep termination and mark termination phases. The
   second is that it backpressures memory allocations by instructing user
   goroutines to assist with scanning and marking in response to a high
   allocation rate.
   
   There is plenty of observability into the first of these sources of
   user-visible latency. There is significantly less observability into the
   second. As a result, it is often more difficult to diagnose latency
   problems due to over-assist (e.g. golang#14812, golang#27732, golang#40225). To this
   point, the ways to determine that GC assist was a problem were to use
   execution tracing or to use GODEBUG=gctrace=1 tracing, neither of which
   is easy to access programmatically in a running system. CPU profiles
   also give some insight, but are rarely as instructive as one might
   expect because heavy GC assist time is scattered across a profile.
   Notice even in https://tip.golang.org/doc/gc-guide, the guidance on
   recognizing and remedying performance problems due to GC assist is
   sparse.
   
   This commit adds a counter to the MemStats and GCStats structs called
   AssistTotalNs, which tracks the cumulative nanoseconds in GC assist
   since the program started. This provides a new form of observability
   into GC assist delays, and one that can be manipulated programmatically.
   
   There's more work to be done in this area. This feels like a reasonable
   first step.

   

   nvanbenschoten committed Sep 19, 2022
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