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From "Stu Hood (JIRA)" <>
Subject [jira] Commented: (CASSANDRA-1972) Default concurrency values are improperly proportioned
Date Fri, 14 Jan 2011 04:08:45 GMT


Stu Hood commented on CASSANDRA-1972:

Based on IO/seek bound tests on two machines with 4 and 12 drives in RAID-0 and RAID-10, we
saw the best performance with around 20 threads per drive. Not everyone will be completely
IO bound though (hopefully _something_ is in cache), so perhaps a better recommended value
is 16 threads per drive. I'll write a new blurb for the config file.

> Default concurrency values are improperly proportioned
> ------------------------------------------------------
>                 Key: CASSANDRA-1972
>                 URL:
>             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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