On Mon, Jan 26, 2015 at 7:54 PM, Wang Nan <[email protected]> wrote: > On 2015/1/27 0:25, Jérémie Galarneau wrote: >> 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 >> > > 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?
I'm not sure I agree with the "without too much work" statement since Babeltrace's CTF Writer API is not thread-safe at this moment. As Alexandre previously mentionned, the size of the resulting CTF trace could be a good indication of whether or not IO is the culprit here. We have never done any optimization work on Babeltrace and CTF Writer. We could be smarter on a number of front, notably with regards to memory allocation; there is currently no pooling of allocated objects (events, fields, types, etc.). We'll have to take a look at hard data before getting our hands dirty. However, I'm sure there are tons of optimization opportunities. 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
