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From Philippe <>
Subject Re: Meaning of values in tpstats
Date Mon, 12 Dec 2011 09:15:48 GMT
> Took me a while to figure out that // == "parallel" :)
Sorry, that's left over from Math classes :)

> I'm pretty sure (but not entirely, I'd have to check the code) that
> the request is forwarded as one request to the necessary node(s); what
Humm... hadn't even thought of that forwarding aspect.
So can someone knowledgeable confirm that a multigetslice on counter super
columns (assume CL=QUORUM):

   - gets sent in whole to the coordinator
   - the coordinator resends it as a whole to RF replicas and waits for
   QUORUM identical responses (and how does that work in a multiget ? Does it
   hash the whole multiget or key by key ?). Does this use one thread ? What
   JMX counter does it show up in ?
   - each replica runs the multiget in parallel so a multiget with a lot of
   keys will saturate the ReadStage.Active counter
   - the coordinator sends the result back.Does this use one thread ? What
   JMX counter does it show up in ?

> I was saying rather was that the individual gets get queued up as
> individual tasks to be executed internally in the different stages.
> That does lead to parallelism locally on the node (subject to the
> concurrent reader setting.
Ok. So in my case, average ping is 0.2ms and stable while average
multigetslice is 40ms for 512 keys at a time. So network time is accounting
for 0.5% of query time : I can really lower the batch size without it
hurting too much. And given the better results I've seen on my workload
with smaller batches, I'm going to do just that.


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