[Gluster-devel] Proposal for change regarding latency calculation

[Vipul Nayyar]

Hi

As guided by you, I performed the experiment regarding measurement of the effect of always enabled profiling. I performed two write tests, one with a 20 MB file and the other with a 730 MB file. Each file was written 20 times to the mounted volume after clearing the buffers on every iteration and the time taken measured with the time command. I ran the following bash script for this purpose.

i=1
while [[ $i -lt 21 ]]; do
sync && echo 3 > /proc/sys/vm/drop_caches
path="/mnt/write_test"$i
out=$( time cp /home/vipul/test.avi $path)
i=$((i+1))

done

Since the values at different times for writing the same file are quite varied, I plotted a graph using the obtained values(Y-axis represents seconds) which can be found attached. As you might see in these images, there is no clear pattern found in the variation of values obtained while writing.

So according to me, values in both the conditions are quite near to each other and equally capable of going quite high or low than the mean value and hence, there is no negative effect seen due to the change proposed. I hope someone else can shed more light on whether setting the option(always enabled profiling) really decreased the performance or not.

Regards
Vipul Nayyar 



On Wednesday, 16 July 2014 12:50 PM, Krishnan Parthasarathi <kparthas at redhat.com> wrote:
 


Vipul,


________________________________

Hello,
>
>
>Following is a proposal for modifying the io profiling capability of the io-stats xlator. I recently sent in a patch(review.gluster.org/#/c/8244/) regarding that, which uses the already written latency related functions in io-stats to dump info through meta and added some more data containers which would track some more fops related info each time a request goes through io-stats. Currently, before the io-stats' custom latency functions can run, the measure_latency and count_fop_hits option should be enabled. I propose to remove these two options entirely from io-stats.
>
>
>In order to track io performance, these options should be enabled all the time, or removed entirely, so that a record of io requests can be kept since mount time, else enabling these options only when it is required will not give you the average statistics over the whole period since the start. This is based on the methodology of Linux kernel itself, since it internally maintains the io statistics data structures all the time and presents it via /proc filesystem whenever required. Enabling of any option is not required, and the data available represents statistics since the boot time.
>
>
>I would like to know the views over this, if having io-stats profiling info available all the time would be a good thing?
Could you run the following experiment to measure the effect of profiling being enabled always?

- Fix the I/O workload to be run.

- Setup 1 (control group) : Run the fixed workload on a volume with both the profiling options NOT set.

- Setup 2 : Run the (same) fixed workload on the same volume with the profiling options set.

- In both setup, measure the latencies observed by the said workload. You could use time(1) command

  for a crude measurement.


This should allow us to make an informed decision on whether there is any performance effect

when profiling is enabled on a volume by default.


>
>Apart from this, I was going over latency.c in libglusterfs, which does a fine job of maintaining latency info for every xlator and encountered an anomaly which I thought should be dealt with. The function gf_proc_dump_latency_info which dumps the latency array for the specified xlator consists of a last line which in the end flushes this array through memset after every dump. That means, you get different latency info every time you read the profile file in meta. I think, flushing the data structure after every dump is wrong since, you don't get overall stats since one enabled the option at the top of meta, and more importantly, multiple applications reading this file can get wrong info, since it gets cleared after one read only.
Clearing of the statistics on every request sounds incorrect to me. Could you please send a patch to fix this?


thanks,

Krish


>
>If my reasons seem apt for you, I'll send a patch over for evaluation.
>
>
>Regards
>
>Vipul Nayyar 
>
>
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