Re: Proposition - latency histogram

["Bartłomiej Święcki" <bartlomiej.swiecki@corp.ovh.com>]

Hi,

I've made a simple implementation of performance histograms. 
Implementation is not very sophisticated
but I think it could be a good start for more detailed discussion.

Here's the PR: https://github.com/ceph/ceph/pull/12829


Regards,
Bartek


On 11/28/2016 05:22 PM, Bartłomiej Święcki wrote:
> Hi,
>
>
> Currently we can query OSD for op latency but it's given as an 
> average. Average may not give
> the bets information in this case - i.e. spikes can easily get hidden 
> there.
>
> Instead of an average we could easily do a simple histogram - quantize 
> the latency into
> predefined set of time intervals, for each of them have a simple 
> performance counter,
> at each op increase one of them. Since those are per OSD, we could 
> have pretty high resolution
> with fractional memory usage, performance impact should be negligible 
> since only one (two if split
> into read and write) of those counters would be incremented per one 
> osd op.
>
> In addition we could also do this in 2D - each counter matching given 
> latency range and op size range.
> having such 2D table would show both latency histogram, request size 
> histogram and combinations of those
> (i.e. latency histogram of ~4k ops only).
>
> What do you think about this idea? I can prepare some code - a simple 
> proof of concept looks really
> straightforward to implement.
>
>
> Bartek
>
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