duke8253 commented on pull request #7719:
URL: https://github.com/apache/trafficserver/pull/7719#issuecomment-824984717


   > > std::unordered_map always takes up around 4% cpu whenever it's paired 
with other data structures, but by itself it's not even showing up in the perf 
top list.
   > 
   > Are you saying erasing a session by key (a session id obtained from the 
other data structure) in removeOldestSession was the heavy part? It implies 
find operation to get a cached session is heavy too, but you don't see it on 
the perf top list?
   
   No, I'm saying that whenever `std::unordered_map` is used with something 
else, such as `map` or `queue` or `list` or `CountQueue`, the utilization goes 
up unexpectedly. And the reason for needing a `queue` is because in order to 
implement `removeOldestSession` we need to keep track the insertion order.


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