[jira] Commented: (CASSANDRA-1972) Default concurrency values are improperly proportioned

["Peter Schuller (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CASSANDRA-1972?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12980841#action_12980841 ] 

Peter Schuller commented on CASSANDRA-1972:
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I agree in general, unless the intent is to make out-of-the-box stress testing on memory-bound workloads optimized.

In particular I agree about being queued in the kernel, and even further down on devices or RAID controllers. Propagating outstanding I/O requests all the way down, within reason, is key to saturating the capacity of underlying spindles.

> Default concurrency values are improperly proportioned
> ------------------------------------------------------
>
>                 Key: CASSANDRA-1972
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-1972
>             Project: Cassandra
>          Issue Type: Bug
>            Reporter: Stu Hood
>            Assignee: Stu Hood
>            Priority: Minor
>             Fix For: 0.7.1
>
>
> The "default"/"suggested" {{concurrent_reads}} value is much too low. It assumes that CPU will be the bottleneck, rather than IO, and for most deployments, this will not be the case. Additionally it is better to be queued for IO in the kernel or on your device than in user space, because the former work to optimize queue order.
> Additionally, reads are much cheaper than writes in terms of CPU time (since writes can experience contention due to retries), so while {{concurrent_writes}} should probably factor in the number of cores on the machine, {{concurrent_reads}} should probably be calculated purely by number of spindles.

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