Skip to main content
[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [List Home]
Re: [tracecompass-dev] [diamon-discuss] My experience on perf, CTF and TraceCompass, and some suggection. 
Hi Wang,
First of all, thank you very much for posting this use case. This is exactly the type of user feedback that will help make the toolchain better and more useful for users! 

Some comments and questions below,


On 01/23/2015 04:35 AM, Wang Nan wrote:

[...]

Then I need to convert perf.data to ctf. It tooks 140.57s to convert
2598513 samples, which are collected during only 1 second execution. My
working server has 64 2.0GHz Intel Xeon cores, but perf conversion
utilizes only 1 of them. I think this is another thing can be improved.
Out of curiosity, approximately how big (in bytes) is the generated CTF trace directory? 



The next step is visualization. Output ctf trace can be opened with
TraceCompass without problem. The most important views for me should be
resources view (I use them to check CPU usage) and control flow view (I
use them to check thread activities).

The first uncomfortable thing is TraceCompass' slow response time. For
the trace I mentioned above, on resource view, after I click on CPU
idle area, I have to wait more than 10 seconds for event list updating
to get the previous event before the idle area.
Interesting. It is expected that opening a very large trace would take a long time to load the first time, as everything gets indexed. But once that step is done, seeking within the trace should be relatively quick ((log n) wrt to the trace size). In theory ;)
The perf-to-CTF conversion brings a completely new type of CTF traces that was not seen before. It is possible that the CTF parser in Trace Compass has some inefficiencies that were not exposed by other trace types. Are you able to share that trace publicly? Or a trace taken in the same environment, with no sensible information in it? It could be very helpful in finding such problem. 


Then I found through resources view that perf itself tooks lots of CPU
time. In my case 33.5% samples are generated by perf itself. One core is
dedicated to perf and never idle or taken by others. I think this should
be another thing needs to be improved: perf should give a way to
blacklist itself when tracing all CPUs.
I don't want to start a tracer-war here :) but have you investigated using LTTng for recording syscall/sched events ? Compared to perf, LTTng is only about "getting trace events", and is a bit more involved to set up, but it is more focused on performance and minimizing the impact on the traced applications. And it outputs in CTF format too.
I remember when testing the perf-CTF patches, comparing a perf trace to an LTTng one, perf would be doing system calls continuously on one of the CPUs for the whole duration of the trace. Whereas in LTTng traces, the session daemon would be a bit active at the beginning and at then end, but otherwise completely invisible from the trace. 


TraceCompass doesn't recognize syscall:* tracepoints as CPU status
changing point. I have to also catch raw_syscall:*, and which doubles
the number of samples.
This is a gap in the definition of the analysis it seems. I don't remember implementing two types of "syscall" events in the perf analysis, so it should just be a matter of getting the exact event name and adding it to the list. I will take a look and keep you posted! 


Finally I found the syscall which cause idle. However I need to write a
script to do statistics. TraceCompass itself is lack a mean to count
different events in my way.
Could you elaborate on this please? I agree the "Statistics" view in TC is severely lacking, we could be gathering and displaying much more information. The only question is what information would actually be useful. 

What exactly would you have liked to be able to see in the tool?


[...]


  5. Ad-Hoc visualization and statistics. Currently TraceCompass only
     support dwaring pre-defined events and processes. When I try to
     capture syscalls:*, I won't get benefit from TraceCompass because it
     doesn't know them. I believe that during system tuning we will
     finally get somewhere unable to be pre-defined by TraceCompass
     designer. Therefore give users abilities to define their own events
     and model should be much helpful.
As I mentioned earlier, the pre-defined "perf analysis" in Trace Compass should be fixed to handle the syscall events.
But it's interesting that you mention wanting to add your own events and model. I completely agree with you, we will never be able to predict every and all use cases the users will want to use the tool for, so there should be a way for the user to add their own.
Well good news, it *is* possible for the user to define their own analysis and views! This is still undergoing a lot of development, and there is no nice UI yet, which is why it is not really advertized. But starting from any supported trace type, a user today can define a time graph view (like the Resource View for example) or a XY chart, using a data-driven XML syntax.
If you are curious, you can take a look at a full example of doing such a thing on this page: 
https://github.com/alexmonthy/ust-tc-example
(the example uses an LTTng UST trace as a source, but it could work with any supported trace type, even a custom text trace defined in the UI). 



Thank you.


Thanks again for taking the time to write about your experience!

Cheers,
Alexandre

Back to the top