[lttng-dev] [diamon-discuss] My experience on perf, CTF and TraceCompass, and some suggection.

[Wang Nan]

On 2015/1/27 0:25, Jérémie Galarneau wrote:
> On Fri, Jan 23, 2015 at 4:35 AM, Wang Nan <wangnan0 at huawei.com> wrote:
>> Hi folks,
>>
>> I'd like to share my first tuning experience with perf, ctf and
>> TraceCompass here, and I hope my experience helpful to diamon.org. Most
>> part of this mail is talking about my work. If you don't
>> interest in it, you can directly jump to conclusion part.
>>
>> *My Task*
>>
>> What I'm working on is finding the reason why CPU idle rate is high when
>> we benchmarking a database. I think it should be a very simple task:
>> tracing scheduling and system calls, finding the previous syscall issued
>> before idle, then based on statistics, collecting some user spaces call
>> stack, I can give an answer. I use perf to collect trace,
>> perf-convert-to-ctf to get ctf output and TraceCompass for
>> visualization.
>>
>>
>> *My Experience*
>>
>> First of all I use perf to collect trace:
>>
>>  # perf record -a -e sched:* -e raw_syscalls:* sleep 1
>>
>> then
>>
>>  # perf data convert --to-ctf out.ctf
>>
>> Which is simple. However, raw_syscalls:* tracepoints export less
>> information than syscalls:* tracepoints. Without them I have to manually
>> find syscall name from syscall id. I prefer to use:
>>
>>  # perf record -a -e sched:* -e syscalls:* sleep 1
>>
>> However there are some bugs and I have to make some patches. They are
>> posted and being disscussed currently, those bugs are still exist
>> upstream.
>>
>> 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.
> 
> Thanks for taking the time to do this write-up!
> 
> Would it be possible to profile perf-to-ctf so we can spot the bottleneck?
> 
> Regards,
> Jérémie
> 

We should, but I don't have enough time to do this. In addition, I think perf-to-ctf
conversion is possible to be parallelized without too much work. I suggest to consider
code tuning after we done that at perf side. What do you think?

>>
>> 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.
>>
>> 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.
>>
>> 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.
>>
>> 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.
>>
>> The next thing I should do is to find the calltrace which issue the
>> syscall. This time TraceCompass won't help, mostly because perf
>> convertion now doesn't support converting calltrace.
>>
>> *Conclusion*
>>
>> I suggest perf and TraceCompass to think about following improvements:
>>
>>  1. Reducing the cost of perf recording. There are one third events are
>>     generated by perf itself in my case. Is it possible that perf could
>>     provide an ability that blacklist itself and collect all other
>>     events?
>>
>>  2. Improving perf converting performance. Converting perf.data to CTF is
>>     slow, but it should be offline most of the time. We can utilize the
>>     abilities multi-core server to make it working in parallel.
>>
>>  3. Improving TraceCompass responding performance, especially when
>>     synchronizing different views.
>>
>>  4. Support converting userspace call trace. I think perf side should already
>>     have a plan on it.
>>
>>  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.
>>
>> Thank you.
>>
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> 
> 
>