Re: rados bench single instance vs. multiple instances

[Mark Nelson <mnelson@redhat.com>]

On 05/11/2015 09:18 AM, Deneau, Tom wrote:
> I have noticed the following while running rados bench seq read tests with a 40M object size
>
>      single rados bench, 4 concurrent ops,                     bandwidth = 190 MB/s
>      4 copies of rados bench, 1 concurrent op each,  aggregate bandwidth = 310 MB/s
>
> and in fact the single rados bench seems limited to 190, no matter how many concurrent ops.

What's your CPU utilization like?

>
> I don't see this kind of behavior with a 4M object size.
>
> (The above are with caches dropped on the osd targets)
>
> It doesn't seem to be related to the total number of bytes being processed by the single
> because if I don't drop the caches, both the single rados bench and the 4-copy rados bench
> get much higher numbers (600 vs. 900) but still the single rados bench appears limited, no matter
> how many concurrent ops are used.
>
> Is there kind of throttling going on by design here?

Nope, probably just client side inefficiency, especially if you are 
running on slower cores.  It's one of the reasons we tooled support for 
multiple concurrent copies of rados bench into cbt.

Mark

>
> -- Tom Deneau, AMD
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