On Fri, Jan 23, 2015 at 4:35 AM, Wang Nan <[email protected]> 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 > > 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. > > _______________________________________________ > diamon-discuss mailing list > [email protected] > https://lists.linuxfoundation.org/mailman/listinfo/diamon-discuss -- Jérémie Galarneau EfficiOS Inc. http://www.efficios.com _______________________________________________ lttng-dev mailing list [email protected] http://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
