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https://issues.apache.org/jira/browse/CASSANDRA-3852?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Benedict reassigned CASSANDRA-3852:
-----------------------------------
Assignee: Benedict
> use LIFO queueing policy when queue size exceeds thresholds
> -----------------------------------------------------------
>
> Key: CASSANDRA-3852
> URL: https://issues.apache.org/jira/browse/CASSANDRA-3852
> Project: Cassandra
> Issue Type: Improvement
> Reporter: Peter Schuller
> Assignee: Benedict
> Labels: performance
> Fix For: 3.1
>
>
> A strict FIFO policy for queueing (between stages) is detrimental to latency
> and forward progress. Whenever a node is saturated beyond incoming request
> rate, *all* requests become slow. If it is consistently saturated, you start
> effectively timing out on *all* requests.
> A much better strategy from the point of view of latency is to serve a subset
> requests quickly, and letting some time out, rather than letting all either
> time out or be slow.
> Care must be taken such that:
> * We still guarantee that requests are processed reasonably timely (we
> couldn't go strict LIFO for example as that would result in requests getting
> stuck potentially forever on a loaded node).
> * Maybe, depending on the previous point's solution, ensure that some
> requests bypass the policy and get prioritized (e.g., schema migrations, or
> anything "internal" to a node).
> A possible implementation is to go LIFO whenever there are requests in the
> queue that are older than N milliseconds (or a certain queue size, etc).
> Benefits:
> * All cases where the client is directly, or is indirectly affecting through
> other layers, a system which has limited concurrency (e.g., thread pool size
> of X to serve some incoming request rate), it is *much* better for a few
> requests to time out while most are serviced quickly, than for all requests
> to become slow, as it doesn't explode concurrency. Think any random
> non-super-advanced php app, ruby web app, java servlet based app, etc.
> Essentially, it optimizes very heavily for improved average latencies.
> * Systems with strict p95/p99/p999 requirements on latencies should greatly
> benefit from such a policy. For example, suppose you have a system at 85% of
> capacity, and it takes a write spike (or has a hiccup like GC pause, blocking
> on a commit log write, etc). Suppose the hiccup racks up 500 ms worth of
> requests. At 15% margin at steady state, that takes 500ms * 100/15 = 3.2
> seconds to recover. Instead of *all* requests for an entire 3.2 second window
> being slow, we'd serve requests quickly for 2.7 of those seconds, with the
> incoming requests during that 500 ms interval being the ones primarily
> affected. The flip side though is that once you're at the point where more
> than N percent of requests end up having to wait for others to take LIFO
> priority, the p(100-N) latencies will actually be *worse* than without this
> change (but at this point you have to consider what the root reason for those
> pXX requirements are).
> * In the case of complete saturation, it allows forward progress. Suppose
> you're taking 25% more traffic than you are able to handle. Instead of
> getting backed up and ending up essentially timing out *every single
> request*, you will succeed in processing up to 75% of them (I say "up to"
> because it depends; for example on a {{QUORUM}} request you need at least two
> of the requests from the co-ordinator to succeed so the percentage is brought
> down) and allowing clients to make forward progress and get work done, rather
> than being stuck.
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